ICP-MS determination of REEs in tomato plants and related products: A new analytical tool to verify traceability

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ASPND7 26(2) 41–88 (2005)ISSN 0195-5373

AtomicSpectroscopy

March/April 2005 Volume 26, No. 2

In This Issue:ICP-MS Determination of REEs in Tomato Plants and Related Products: A New Analytical Tool to Verify TraceabilityM. Bettinelli, S. Spezia, C. Baffi, G.M. Beone, R. Rocchetta, and A. Nassisi ......... 41

Distribution and ICP-MS Determination of Heavy Elements in the Surfacial Sand Along the Red Sea Coastline of Saudi ArabiaJameel Al-Hefne, Omar Al-Dyel, Didarul A. Chowdhury, and Turki Al-Ajayan ............................................................................................... 51

Determination of Major and Trace Elements in Edible Seaweeds by AAS After Ultrasound–assisted Acid LeachingNoemí Ladra–Ramos, Raquel Domínguez–González, Antonio Moreda–Piñeiro, Adela Bermejo–Barrera, and Pilar Bermejo–Barrera ............................ 59

Fast Furnace Program With End-capped Tubes for the Determination of Sub-ppb Levels of Cd in Foods and Biological SRMs Using ETAAS After Ultrasonic Probe ExtractionNoorbasha N. Meeravali, M.A. Reddy, and Sunil Jai Kumar ................................ 68

Determination of Copper in Wine by ETAAS Using Conventional and Fast Thermal Programs: Validation of Analytical MethodSofia Catarino, Inês Pimentel, and A.S. Curvelo-Garcia ...................................... 73

On the Atomization of Rb by Electrothermal Atomization, Flame Absorption, and Flame Emission Spectrometry in Uranium and Thorium MatricesParu J. Purohit, Neelam Goyal, and A.G. Page ..................................................... 79

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Atomic SpectroscopyVol. 26(2), March/April 2005

*Corresponding author.E-mail: : [email protected]

INTRODUCTION

In 2001–2004, the worldwideproduction of processed tomatoesincreased from 22.5 Mt to 34.1 Mtand is expected to reach a plateauof 29.4 Mt in 2005. The UnitedStates, particularly California, is thebiggest producer with 10.7 Mt in2004, followed by Italy with 6.3 Mt,and China with 4.4 Mt (1). Italy isthe biggest exporter in the worldtoday. China is one of Italy’s mainsuppliers of semi-manufacturedtomato products because they arevery competitively priced, but theylack the quality and nutritionalcharacteristics of the Italian prod-uct. Thus, they are often mixed or"blended" with Italian tomatoes (2).

In order to protect local produc-tion, the present work has lookedat ways of detecting if foreigntomato products, particularly richin REEs, are being mixed with localproduce on the market. For com-parison, the REEs of Italian produc-tion areas are also measured. It isfundamental to have knowledge ofthe REE concentrations of the soilwhere the plants are grown and thedistribution of the REEs in the dif-ferent parts of the plants: roots,stems, leaves, and berries.

In soil, rocks play the main roleas a source of rare earths (3,4),where the major percentage ofREEs present in the weatheredmaterials are the lightest rare earths(LREE) (5). Fertilization of soil with

Particular importance is given tosoil pH, which can influence theuptake rate of REEs in plants and isalso higher at low pH values (9).Markert (3) and Wyttembach (10),for the distribution pattern of REEsin plants, give more importance toother factors, such as changes inoxidation states and/or differencesexisting among individual plants orplant species. Wen et al. (11) recog-nize the importance of both soiland plant species, but also climateand plant growth period areregarded as important factors; littleinfluence is ascribed to the amountof fertilizer used containing REEs.

The concentration of rare earthsin plants can vary over a widerange [1–15,000 ng g–1 D.W. (dryweight)] and the distribution fol-lows the Oddo-Harkins Law withthe highest content observed for Laand the lowest for Lu. Results fromfield trials point out that rare earthaccumulation follows this order:root > leaf > stem > edible part(11–13), with a higher REE uptakerate for herbaceous than for arbo-real plant species (8).

Only in the ‘90s were the firstimportant studies on the physiolog-ical aspect and biochemical func-tion of rare earths in plant tissuespublished (14–18). In these works,REE uptake is observed to occurmainly via roots (15), sometimesvia leaves (19).

The importance of REEs in plantnutrition has not yet been proved.Positive effects of rare earths havebeen observed in the activities of

ABSTRACT

The rare earth element (REE)concentration in tomato plantsand soils of three agriculturalfarms in the Province ofPiacenza, Italy, was evaluated.The REE distribution in the differ-ent parts of the plant follows theorder: root > leaf > stem > ediblepart; the concentrations reflectthose observed in the soil. Themost reliable REE concentrationsare obtained from soil having thelowest pH and organic matter.

The analytical procedure,using inductively coupled plasmamass spectrometry (ICP-MS),proved to be suitable for thedetermination of REEs in plantsand soil, and was validated usingcertified samples. The precisionof the method resulted in betterthan 10–15% for all REEs, withrecoveries ranging from70–150%. The method detectionlimit was 10 ng/g for soil and 1 ng/g for plant material. Thistechnique seems to be promisingfor its application in tracing REEconcentrations in tomato plantsand its use in ensuring high qual-ity of typical food products.

REEs, a widespread agriculturalpractice in China since the ‘70s (6),can lead to significant amounts ofrare earths in the soil, directly orindirectly.

Regarding the distribution ofrare earths in plants, some authors(7, 8) assign a fundamental role tothe soil, its properties, and to speci-ation and immobilization processesthat take place in it.

ICP-MS Determination of REEs in Tomato Plants and Related Products:

A New Analytical Tool to Verify Traceability*M. Bettinellia, S. Speziab, C. Baffic, G.M. Beonec, R. Rocchettac, and A. Nassisid

a C.M.B. Central Laboratory, Piacenza, Italyb Laboratory of Environmental Health and Industrial Toxicology, Maugeri Foundation, Pavia, Italy

c Catholic University of the Sacred Heart Agricultural Faculty, Institute of Agricultural and Environmental Chemistry, Soil Section, Piacenza, Italy

d ARPA Regional Agency for the Environment Protection, Soil Excellency, Piacenza, Italy

42

some enzymes present in plants, inthe chlorophyll production, and intheir ability to control the absorp-tion rate of macronutrients inplants (6).

Since 1972, the use of fertilizerswith REEs, of which China has con-siderable reserves (11), has beenwidespread in China (20). The mostimportant results published regard-ing the use of some fertilizers con-taining rare earths (above all La, Ce,Pr, and Nd as nitrates) wereobtained from field and glasshousetrials where it was found thatincreased yields of about 10%(21–25) were due to low concen-trations (< 10 mg/kg) of availableREEs in the soil (26,27). At higheravailable REE values (> 20 mg/kg),no positive effects were observed.

Except for rocks, which can con-tain high amounts of rare earths,the concentration of REEs in envi-ronmental samples such as soils,sediments, and plants is generallylow, ranging from some µg/g D.W.for soil to a few ng/g D.W. forplants.

In the past, data shortage wasmainly due to a lack of accuracyand sensitivity of instrumentaldetection (3,28). Today, modernanalytical techniques such as induc-tively coupled plasma mass spec-trometry (ICP-MS) are able toprovide more detailed informationon the concentration profile of allanalytes.

In order to assess specific "mark-ers" suitable for the REE traceabilityof tomato processing products, thepresent study was carried out todetermine the distribution of REEsin different parts of tomato plantsrelative to their concentration inthe soil.

The study was carried out inthree agricultural farms in theProvince of Piacenza, Italy, in col-laboration with the Catholic Univer-sity of the Sacred Heart AgriculturalFaculty, Institute of Agricultural and

Environmental Chemistry, Soil Sec-tion, Piacenza; the Laboratory ofEnvironmental Health and Indus-trial Toxicology, Maugeri Founda-tion, Pavia; and the RegionalAgency for the Environment Protec-tion, Soil Excellency, Piacenza.

The analysis of some tomatoconcentrates of different originwere carried out to verify the possi-ble application of this analyticalmethod for the characterization ofthe REE pattern in various productson the market.

EXPERIMENTAL

Instrumentation

The instrumentation used fordigestion consisted of a closedmicrowave oven, CEM Model MDS2000 (CEM, Indian Trail, NC, U.S.)and an open microwave system,Prolabo Model Microdigest 3.6(Prolabo, Paris, France).

REE determination was made byinductively coupled plasma massspectrometry (ICP-MS) with reac-tion cell (ELAN® 6100 DRCII,PerkinElmer SCIEX Instruments,Concord, Ontario, Canada). Theinstrument was equipped with acyclonic spray chamber and a con-centric type nebulizer. The instru-mental operating conditions arelisted in Table I.

The determination of rare earthswas carried out at a flow rate of 0.4mL min–1 of NH3 in the reactioncell.

The ICP-MS spectrometer wascalibrated using certified multiele-ment solutions of 10 mg L–1 (ICP-MS calibration standard n 3, 4, MS3, MS1, CPI International, Amster-dam, NL), after dilution with watercontaining the same amount ofacids as the samples. The intensitiesof the analytes were corrected for115In, used as an internal referencestandard.

TABLE IInstrumetal Operating Conditions for ICP-MS

Using the PerkinElmer SCIEX ELAN 6100 DRCII

Power 1250 W Dwell time 25 msPlasma argon 15 L/min Sweeps/readings 2Nebulizer argon 1.0 L/min Reading for replicate 3Auxiliary argon 1.3 L/min Number of replicates 5Sample uptake 1 mL/min Sampling time 1 min 50 secNebulizer Concentric Reading delay 50 sSpray chamber Cyclonic Washing delay 45 sSampling cones Platinum Calibration mode ExternalSampling cones Platinum Conc. levels 1, 5, 10, 50,

100 µg/LScanning mode Peak hopping Regression model Linear

through zeroResolution NormalReading time 150 ms

DRC Parameters DRC Vented DRC Pressurized

Parameter q 0.25 0.65Parameter a 0 0

Gas Absent NH3 at 0.4 mL min–1

43

Vol. 26(2), March/April 2005

Reagents and StandardSolutions

The following reagents, Supra-pur® type (Merck, Darmstadt, Ger-many), were used: HCl 37% (m/v),HNO3 67% (m/v), and HF 40%(m/v).

Multielement standard solutionswere prepared by dilution with dis-tilled water, containing the samequantities of acids as the samples,starting with certified multielementsolutions of 10 mg L-1 (ICP-MS cali-bration standard n 3, 4, MS 3, MS1,CPI International, Amsterdam, NL).

Glassware was rinsed byovernight immersion in a solutionof HNO3 10% (m/v), followed by arinse with deionized water. Thehigh purity water (electrical resis-tivity >18 MΩ) used was obtainedwith a Milli-Q™ deionizing system(Millipore Bedford, MA, U.S.). Toevaluate accuracy, the followingcertified materials were used:

- CRM GBW 07308 Stream Sedi-ment (State Bureau of Metrology,The People’s Republic of China),

- CRM BCR 142 Light Sandy Soil(Community Bureau de Reference)

- CRM BCR 143 Sewage SludgeAmended Soil (Community Bureaude Reference)

- SRM NBS 1547 Peach Leaves(National Institute for Standard andTechnology)

- SRM NBS 1573 Tomato Leaves(National Institute for Standard andTechnology)

Sample Preparation

Samples of whole tomato plantsand soils were collected from threefarms in Italy, in triplicate.

The soil samples were air-driedand ground to obtain fine earth (φ< 2 mm); on this aliquot, the mainchemical-physical analysis was car-ried out. For REE determination,the soil samples were ground to 0.2mm with a planetary mill

(Pulverisette 7 FRITSCH, Oberstein,Germany) with an agate jar andballs.

Roots, stems, and leaves werecarefully hand-cleaned and oven-dried at 70°C for two days. Theplant material was then groundwith a mill (Tecator, Foss, Padova,Italy) to obtain 1-2 mm fineness.The whole berries were frozen at -18°C for 2 days, then freeze-dried at-40°C ( EDWARDS mini fast 680,Edwards, Norfolk, England) for twodays and ground with a mill toobtain a 1-2 mm fineness. The mois-ture of the soil and ground plantsamples was measured by weighingafter drying at 105°C for 16 hours.

Mineralization of the soil andplant samples was carried out witha closed microwave system (CEMMDS 2000). The tomato concen-trates were prepared by weighing2.5–2.8 g of frozen material(T=–18°C), previously refrigeratedat 4°C for 2 nights, and placingthem into the vessels of the openmicrowave system Prolabo Microdi-gest 3.6.

Analysis of Real Samples

In soil, the main chemical-physi-cal parameters were determinedaccording to the National OfficialMethods for Soil Analysis, DecretoMinisteriale, Italy (29). The determi-nation of REEs, after preliminarytrials carried out to verify the effec-tiveness of two different acid mix-tures (HCl-HNO3 – HF andHCl-HNO3), was performed as fol-lows: to 0.250 g of a 0.2-mm soilfraction, 8 mL of aqua regia and2 mL of distilled water were addedinto PTFE vessels of the microwaveoven (CEM 2000) and submitted to

a heating program (see Table II).After mineralization, the sampleswere filtered and brought to a finalvolume of 50 mL with high puritywater.

The determination of REEs intomato plants was carried out aftermineralization of about 0.500 g ofground material (φ = 1–2 mm) in amicrowave oven (CEM 2000) with6 mL of HNO3 and 4 mL of H2O2

(see Table II for operating condi-tions). The digested samples werefiltered and brought to a final vol-ume of 50 mL. Together with thesamples, some analytical blanks,prepared using the same quantitiesof acids as the samples, were ana-lyzed. The determinations were car-ried out in triplicate.

To the tomato concentrate sam-ples, 15 mL HNO3 conc. (65% m/v)was added, and the solution heatedfor 10 min at 20% of the nominalpower (Prolabo Microdigest 3.6).Then in four steps, 1, 2, 3, and 4mL H2O2, respectively, were addedand heated for 5 min at 20% of thenominal power, followed by a cool-ing step. The final solution was fil-tered and brought to a final volumeof 100 mL.

RESULTS AND DISCUSSION

The accuracy of this procedurefor the determination of REEs insoil samples was evaluated by ana-lyzing CRM GBW 07308 StreamSediment, certified for 12 rare earthelements (Y, La, Ce, Pr, Nd, Sm, Eu,Gd, Tb, Dy, Ho and Yb), CRM BCR142 Light Sandy Soil, and BCR 143Sewage Sludge Amended Soil forwhich only indicative values areavailable.

TABLE IIOperating Conditions for Microwave Dissolution

Soil Plant

Step 1 2 3 1 2 3Power (W) 250 400 600 240 360 420Time (min) 8 4 6 2 2 15

44

The results obtained for CRMGBW 07308, shown in Table III,indicate recovery values for manyof the analytes ranging from78–150%, with the exception ofthe heavy rare earth elements (Er,Tm, Yb, and Lu) for which therecoveries are lower, about50–60%. These low values can bejustified because of the absence ofHF in the acid mixture for diges-tion, which causes a partial solubi-lization of the rare earths,particularly the heaviest REEs suchas Er, Tm, Yb, and Lu. For a morecomplete recovery, Krachler et al.(30) suggest an alkaline fusion withHBF4 rather than an acid digestionwith aqua regia and HF. The pur-pose of this work was to addressthe ability to evaluate the fractionof soil available for root uptake ofagricultural crops, and in thissense, the REEs extracted withaqua regia, with respect to the totalcontent, was thought to be moresuitable for this purpose. The ana-lytical precision of three different

replicates are, on average, less than10–15%, with the exception of Yb(23%) and Tm (39%), the latterbeing present at very low concen-tration levels (0.36 µg g–1 D.W.).

As far as CRM BCR 142 and 143are concerned, the observed valuesare within the range reported byBCR for La, Nd, Sm, Eu, and Gd,and correspond to the lowestextreme for Y and Ce.

As for plant material, the fewREE values available for SRM NIST1547 and 1575, however indicative(see Table IV), enable us toexclude systematic bias in thedetermination of REEs at these con-centration levels even if they donot allow the evaluation of quanti-tative recoveries. The analyticalprecision, expressed as RSD%, isnearly always less than 10%, exceptfor Sm and Tm, which show valuesof 18% and 17%, respectively.

The method detection limits(MDL), calculated as three times

the standard deviations of threerepeated trials on different days,were slightly different for the vari-ous analytes:

- Soil : 5 ng g–1 for Y, La, Ce, andPr; 7 ng g–1 for Nd, Sm, Eu, Gd andTb; 8 ng g–1 for Dy, Ho, Er, Tm, Yband Lu.

- Plant material: 0.2 ng g–1 for Y,La, Ce, Pr and Nd; 0.5 ng g–1 forSm, Eu, Gd, Tb, and Dy; 0.7 ngg–1for Ho, Er, Tm, Yb, and Lu.

However, the method detectionlimits were set up to be equal to10 ng g–1 for soils and 1 ng g–1 forplant material. Their values weresuitable for the analytical purposesof this work, thus guaranteeing thedetermination of REEs in the inves-tigated samples.

The physical-chemical analysisof the soils sampled at the threeagricultural farms in the Provinceof Piacenza, Italy (see Table V) indi-cate that the three soils belong tothe same Great Group,

TABLE IIIREE Concentration (µg/g ) in Three Certified Reference MaterialsGBW 07308 (Stream Sediment), BCR 142 (Light Sandy Soil,) and

BCR 143 (Sewage Sludge-Amended Soil)

CRM GBW 07308 CRM BCR 142 CRM BCR 143Elements (µg/g) (µg/g) (µg/g)

Certified Found Certified Found Certified Found

Y - n.d. (30–30.7) 16.1 ± 0.9 (27–27.6) 17.3 ± 1.1 La 30 ± 5 33.9 ± 3.5 (29.5–33.3) 30.5 ± 2.6 (26.1–29.99) 28.4 ± 0.9Ce 54 ± 6 53.6 ± 2.5 (69.5–97) 63.3 ± 4.7 (65–94) 58.1 ± 2.8Pr 5.7 ± 0.5 4.5 ± 0.3 - 6.5 ± 0.5 - 6.53 ± 0.2Nd 21 ± 2 27.6 ± 1.5 (28.1) 28.2 ± 2.3 (25.17) 25.7 ± 0.7Sm 3.8 ± 0.2 5.6 ± 0.4 (6.2–7.4) 5.8 ± 0,6 - 4.89 ± 0.16Eu 0.56 ± 0.08 0.78 ± 0.12 (0.87–1.15) 1.10 ± 0.08 (0.87–1.15) 1.10 ± 0.08Gd 3.5 ± 0.6 4.2 ± 0.7 (5.7) 5.8 ± 0.5 (5.0) 5.1 ± 0.2Tb 0.54 ± 0.09 0.64 ± 0.06 - n.d. - n.d.Dy 2.6 ± 0.4 3.9 ± 0.3 (5.15) 3.5 ± 0.2 (4.3) 3.3 ± 0.1Ho (0.96) 1.07 ± 0.08 - n.d. - n.d.Er 1.8 ± 0.3 0.98 ± 0.08 (2.84) 1.68 ± 0.09 (2.47) 1.65 ± 0.13 Tm (0.36) 0.13 ± 0.05 - n.d. - n.d. Yb 2.1 ± 0.4 1.3 ± 0.3 (2.7–2.84) 1.30 ± 0.08 - 1.31 ± 0.07

Lu (0.36) 0.25 ± 0.04 - n.d. - n.d.

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TABLE IVREE Concentration (µg/g ) in NIST SRM 1547 (Peach Leaves)

and NIST SRM 1573 (Tomato Leaves)

Element NIST 1547 (µg/g) NIST 1573 (µg/g)Certified Found Certified Found

Y - < 0.01 - 0.12 ± 0.06La (9) 9.8 ± 0,9 0.64 ± 0.21 0.66 ± 0.05Ce (10) 8.5 ± 0.7 (1.0) 0.64 ± 0.08Pr - 0.83 ± 0.03 - 0.072± 0.009Nd (7) 4.6 ± 0.2 - 0.26 ± 0.04Sm (1) 1.1 ± 0.2 (0.155) 0.057± 0.007Eu (0.17) 0.47 ± 0.03 - 0.017± 0.005Gd (1) 0.87 ± 0.04 - 0.046± 0.008Tb (0.1) 0.47 ± 0.05 (0.004) < 0.01Dy - 0.33 ± 0.02 - 0.024± 0.005Ho - < 0.01 - < 0.01Er - 0.34 ± 0.03 - < 0.01Tm - 0.012 ± 0.02 - < 0.01Yb (0.02) 0.088 ± 0.003 - < 0.01

Lu - 0.026 ± 0.002 (0.012) < 0.01

TABLE VMean Values of Chemical-Physical Parameters of Soils

Parameters Farm 1 Farm 2 Farm 3pH (H2O; 1: 2,5) 7.8 7.7 6.3N (Kjeldhal) (mg/kg) 0.17 0.14 0.11P2O5 available (mg/kg) 104 67 156K2O exchangeable (mg/kg) 189 251 171Organic matter (mg/kg) 2.45 2.15 1.28C.E.C. [cmol(+)/kg] 24.2 29.5 15.3

TextureSand (50–2000 µm) (g/kg) 109 76 171Silt (2–50 µm) (g/kg) 621 414 658Clay (<2 µm) (g/kg) 270 510 171

Textural Class USDA Class 8 Class 2 Class 8 Silty loam Silty clay Silty loam

ClassificationSoil Taxonomy (1994) Usix Udertic Udertic

Ustochrepts Ustochrepts Ustochrepts

FAO UNESCO (1990) Eutric Haplic Haplic Cambisols Calcisols Calcisols

Ustochrepts, even if the organicmatter values, pH, and cationexchange capacity (C.E.C.) arelower in soil Farm 3. The concen-tration pattern for REEs is similar inthe three soils (Figure 1) with sam-ple Farm 3 showing the highestcontent of rare earth elements. TheREE concentration patternobserved for tomato plants in thethree farms is similar, with a trendreflecting that of the soil (Figures2–4). The highest rare earth con-centrations were observed forplants grown in soil Farm 3, charac-terized by the lowest values of pHand organic matter.

In plant tissues, light rare earths(LREE) follow the Oddo-HarkinsLaw with decreasing concentrationsin the order: root > leaves> stem >edible part, already observed in theliterature by Wen (11), Kuang (12),and Li (13). The results of our workare in accordance with thoseobtained by Davie and Coker (9)who highlighted the importance ofpH as a factor affecting the avail-ability and absorption of REEs inplants, with an increased uptake atlow pH values.

Almost all of the concentrationsof the heavy REEs observed infreeze-dried berries were below thedetection limits (Table VI), whilefor the light rare earths (LREEs)higher concentrations wereobserved for plants grown in soilFarm 3.

The distribution pattern of thedifferent REEs in the soil, their con-centration and the physical-chemi-cal characteristics of soil (inparticular pH and organic mattercontent) can influence the contentof rare earths in different parts oftomato plants.

In order to check the applicabil-ity of this procedure for the REEtraceability of processing tomatoproducts and to guarantee greaterprotection of local food products,eight tomato concentrates of differ-

46

ent origin (3 samples from theProvince of Piacenza, Italy, and 1from Puglia, Italy; 2 Greek; 1 Span-ish; and 1 from the DominicanRepublic, a non-EU country) wereanalyzed. The values werecompared with the samples, as wasan analytical blank, obtained withthe same reagents as used for thesamples.

Figures 5–8 show the concentra-tion profiles for Ce, Gd, Nd, and Y.In spite of the low concentrationsand, in some cases, close to analyti-cal MDL values, the followingobservations were made:

(a) The three products from theProvince of Piacenza show almostequal concentrations slightly abovethat of the blank.

(b) The Spanish product showsconcentration profiles very similarto those of the products from theProvince of Piacenza.

(c) The two Greek productsshow a larger variability in the REEconcentrations with respect to thatobserved for the three productsfrom Piacenza, likely to be ascribedto a different production area(unknown).

(d) The product from theDominican Republic shows a verylow REE content, close to that ofthe blank.

(e) The Italian product fromPuglia shows the highest values forall REEs with respect to the others.The reason could likely be the addi-tion of non-local product with ahigh REEs content, which probablycaused a break in the traceabilitychain.

CONCLUSION

The use of inductively coupledplasma mass spectrometry hasenabled us to obtain importantinformation about the distributionof REEs in various parts of tomatoplants grown in soils with differentphysical-chemical characteristics.

In particular, soil pH andorganic matter content are likely toaffect the uptake of rare earths.The distribution of REEs in tomatoplants follows the order: roots >leaves > stems > berries. The REEconcentration in berries appears inrelation to the REE concentrationand some physical-chemical charac-teristics of the soil. The analyticalMDL values, 10 ng g–1 for soil and1 ng g–1 for plant material, allowsus to believe that ICP-MS is a suit-able analytical technique to investi-gate the traceability of productsfrom tomato processing plants.Knowledge of the physical-chemi-cal properties of the soil, the con-centration of REEs, and themechanisms regulating the transferof the analytes to the differentparts of the tomato plants enablesus to check if the profile and theconcentration of the analytes in theend products (berries or concen-trates) can be considered compati-ble with those in their declaredplace of origin.

The investigation is still inprogress and will analyze additionalproducts of certified origin and forwhich the soil, where they weregrown is known, in order to checkthe mechanisms of REE distributionin the different parts of the plant.This will enable us to build an up-to-date data base which will help toexpand the statistical basis for com-parisons and point to anomalies inthe traceability chain.

TABLE VIREE Concentrations in Berries

of Tomato Grown in the Three Farms of the

Piacenza Province, Italy

Ele- Farm 1 Farm 2 Farm 3ment (ng/g DW)

Y 14.8 2.4 9.4La 9.9 7.2 27.0Ce < 1 4.8 34.1Pr < 1 < 1 5.9Nd 2.5 < 1 16.4Sm 2.5 < 1 4.7Eu < 1 < 1 < 1Gd < 1 1.2 3.5Tb < 1 < 1 < 1Dy < 1 < 1 1.2Ho < 1 < 1 < 1Er < 1 < 1 < 1Tm < 1 < 1 < 1Yb < 1 < 1 < 1

Lu < 1 < 1 < 1

DW = dry weight.

47


Fig. 1. REE profile in the three soils analyzed.

Fig. 2. REE distribution in soil and plant for Farm 1.

48



49


Fig. 8. Concentration of yttrium (µg/g) in tomato concentratesof different origin.

Fig. 7. Concentration of neodinium (µg/g) in tomato concen-trates of different origin.

Fig. 6. Concentration of gadolinium (µg/g) in tomato concen-trates of different origin.

Fig. 5. Concentration of cerium (µg/g) in tomato concentratesof different origin.

50

ACKNOWLEDGMENTS

The authors express their appre-ciation to Mr. Paolo Lodigiani(Catholic University, Piacenza,Italy) for technical assistance.

Received January 2, 2005.

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51


Distribution and ICP-MS Determination of Heavy Elements in the Surfacial Sand

Along the Red Sea Coastline of Saudi Arabia*Jameel Al-Hefne, Omar Al-Dyel, Didarul A. Chowdhury, and Turki Al-Ajayan

Institute of Atomic Energy Research, King Abdulaziz City for Science and TechnologyRiyadh 11442, KSA

INTRODUCTION

Saudi Arabia is a vast desertcountry dominated by volcanic andigneous rocks on the western coastand by sedimentary formations onthe eastern side. Seas surround thecountry on both its eastern andwestern borders. Because of thelow population-to-land usage ratioand difficult topography, land-basedindustrialization proceeded at arather slow pace. On the contrary,intensive oil exploration work has

ABSTRACT

Measurement of heavy ele-ment concentrations in the sur-face soil is essential forassessment of environmental pol-lution as well as for identifyingmineral deposition due to naturalprocesses. Saudi Arabia is a vastdesert country where crude oilproduction and processing facili-ties are the major sources oftrace element emissions, andwind erosion is mainly responsi-ble for the transport ofpollutants. In this study, wereport the average concentra-tions of 28 elements found in thesurface soil of the Red Sea coastalregion of Saudi Arabia. The sandsamples, collected from both thebeach and terrestrial sites, weredigested by a mixture of HNO3,HCl, and HF and measured byinductively coupled plasma massspectrometry (ICP-MS). The con-centrations of most of the heavyelements were found to be lowerthan the average values for theearth’s crust, except for Sr, Hf,and U.

(1). Sediments integrate contami-nants over time and are in constantflux with the overlaying water col-umn. The determination of heavyelements in sediments provides arecord of the spatial and temporalhistory of pollution in a particularregion or ecosystem. Beach sands,on the other hand, are subject tocontinuous deposition of heavymetals from different sources. Butthe interaction with the sea wavesaffects the redistribution and disso-lution of the metals in seawater.The equilibrium or the residualheavy metal concentrations in thebeach sand should in principal pro-vide information about the currentstatus of pollution in the surround-ing environment. Again, the beachsands themselves originated frompast geological formations and thusincorporate the major mineralspecies present in the original rockin spite of long weathering andtransport (2).

In our previous report (3), thedistribution of heavy metals alongthe Arabian Gulf coastline of SaudiArabia was presented. The objec-tives of the present study were todetermine the distribution as wellas the levels of some heavyelements in the Red Sea coastline ofSaudi Arabia and to compare themwith the data obtained from theArabian Gulf coastline.

MATERIALS AND METHODS

Sampling Sites

A total of 98 sand samples werecollected from 18 sampling sitesalong the Red Sea coastline of SaudiArabia. These sites include Jizan,Al-Bark, South Jeddah, Al-Lith,

continued in the offshore andonshore areas for the last 40 yearsor so. In fact, Saudi Arabia has someof the largest oil deposits in theworld; the associated crude oil pro-cessing and shipping facilities arelocated in the coastal belt. Unfortu-nately, neither of its coastlines hasbeen investigated properly for thenatural or anthropogenic depositionof heavy metals, especially the toxictrace metals.

Heavy element concentrations inriver and estuarine sediments areoften studied for assessment of man-induced environmental pollution*Corresponding author.

E-mail: [email protected]

In general, the beach sandcontains lower concentrations ofheavy elements than the terres-trial sand. The observed heavyelement concentration patternsdo not indicate significant deposi-tion of typical heavy minerals atthe Red Sea coast. But substantialbuildup of heavy elements likeAg, As, Ba, Cr, Cu, Mn, Ni, Pb, Sn,Zn, Fe, La, Ce, and V is evidenton the beach near Laith, Al-Quan-fudah and Masturah. Such enrich-ment is anthropogenic in natureand probably originates from thepollutant carryover from the Jed-dah seaport.

In terrestrial sand analysis,enrichment of Cr, Cu, Mn, Mo,Se, Sn, Zn, Ti, Fe, Co, and Varound Rabegh is observed andthought to have been formed dueto emissions from industries situ-ated at the outskirts of Jeddahcity. Slight buildup of La, Ce, Sm,Hf, Th, and U around Yanbu isindicative of some nearby naturalrocks richer in these elements.

52

Al-Qunfidhah, Masturah, Rabigh, Al-Kudaimah, Tuwwal, Jahban, NorthJeddah, Al-Bahr, Al-Quarnish,Ummlajj, Haql, Duba, and Al-Wajh,respectively. The sampling hasbeen carried out from south tonorth as shown in Figure 1. Terres-trial sand samples were collectedfrom 13 offshore locations more orless parallel to the beach sand sam-pling line (200–500 m away).

Sample Collection

Sampling was done in the periodfrom June to December 2002. Usu-ally, 1.0–5.0 kg of sand was takenfrom 5–10 cm depth of the beachwith a plastic shovel and placedinto a polyethylene bag. Sand sam-ples were also collected in thesame manner. The sample bagswere closed and transported to thelaboratory for further analysis.

Sample Preparation

Sand samples were first air-driedon plastic trays and passed througha series of mechanical sieves inorder to remove dirt and particleslarger than 104 µm. The sampleswere homogenized by the methodof quartering. About 2 g was thendried in a vacuum oven (60˚C, 65kPa) for 24 h and stored in plasticbottles inside a desiccator. Accu-rately weighed portions (0.1–0.2 g)of the dried sample weretransferred to Teflon® digestiontubes (120 mL) and 7.0 mL of theacid mixture (HNO3/HF/HCl,4.5:2:0.5) was added. The tubeswere sealed and the samplesdigested with a microwave oven(Milestone ETHOS 1600) followingthe heating program as shown inTable I.

After cooling to ambient temper-ature, the tubes were opened; theinside of the lids were rinsed withdistilled and de-ionized water(DDW), and the mixture heated ona hotplate (120˚C) for 30 minutesto drive off the residual HF andHCl. The resulting digests were fil-tered in a polypropylene flask using1% HNO3 and made up to 50-mLvolume. For ICP-MS measurement,the clear digests obtained werediluted 10 times incorporating a10-µg L–1 solution of 103Rh. In gen-eral, the samples of the sand andstandard reference materials (SRM)were prepared in a batch of sixincluding a blank (HNO3/HF/HCl)digest.

Instrumentation

Measurements were carried outby means of a PerkinElmer SCIEXELAN® 6100 inductively coupledplasma mass spectrometer (ICP-MS)(PerkinElmer SCIEX, Concord,Ontario, Canada). The instrument isequipped with a quadrupole massfilter, a cross-flow nebulizer, and aScott type spray chamber. Theoperational parameters of the spec-trometer are given in Table II.

Chemicals and Reagents

High purity water (DDW) (spe-cific resistivity 18 MΩ.cm–1)obtained from an E-pure waterpurification system (Barnsted,Dubuque, IA, USA) was usedthroughout the work. HNO3, HF,and HCl used for sample digestionwere of Suprapur® grade with cer-tified impurity contents and pur-chased from Merck & Co.,Darmstadt, Germany. A multi-ele-ment standard containing 43 ele-ments was prepared fromPerkinElmer single-element ICPstandards (1000 or 10000 ppm)(PerkinElmer Life and AnalyticalSciences, Shelton, CT, USA). TheStandard Reference Material (SRM),IAEA-SOIL-7, was purchased fromthe International Atomic EnergyAgency, Vienna, Austria.


Analytical Results of Beach andTerrestrial Sand Samples

The analytical performance ofthe ICP-MS procedure used for thedetermination of various elementsin the samples under study hasbeen described earlier in terms of

TABLE IMicrowave Heating Program

Used for Dissolution of Sand,Soil, and Sediment Samples

Step 1 2 3 4Power (W) 400 0 300 400Time (min) 15 2 10 15

Temp. (˚C) 195 195 195 195

Fig. 1. Location map of the Red Sea coastline showing the sampling sites.

53


TABLE IITypical Operating Conditions for ICP-MS Measurements

Instrument ELAN 6100 Plasma TorchMass Analyzer RF frequency 40 MHzFilter type Quadrupole rod RF power 1.05 kWScan mass range 2–242 amu Outer gas flow 15.0 mL min–1

Number of points per peak 1 Auxiliary gas flow 1.0 mL min–1

Number of scan sweeps 100 Carrier gas flow 0.96 mL min–1

Dwell time per point 50 ms Lens Voltage Adjusted dailyIntegration 100 times Detector 26-segment dynode operating

in both pulse and analogue modesSample Introduction Data Acquisition Mode Peak hoppingNebulizer Cross-flow typeSpray chamber Scott-typeSample uptake 1.8 mL min–1 Instrument Control

and Data Analysis ELAN 6100 software

Wash solution 1% (v/v) HNO3 in DDW Instrument Tuning Performed using a 10-µgL–1

solution of Be, Mg, Cu, Pb, and U

TABLE IIIThe Analytical Detection Limits (DL) in µg g–1

Element Sc Ti V Cr Mn Co Ni Cu Zn As Se Sr

DL 0.29 0.06 0.08 0.81 0.07 0.01 0.05 0.02 0.05 0.04 0.19 0.01

Element Ag Cd Sn Cs Ba La Ce Sm Hf Pb Th U

DL 0.04 0.02 0.03 0.02 0.02 0.02 0.02 0.01 0.95 0.01 0.28 0.01

accuracy, precision of the measure-ments, and the concerned detec-tion limits (3). However, some newelements have been introduced inthe present analytical schemewhich include La, Sm, Ce, and Cs.In general, the concentration valueof an element was considered foraveraging if the instrumental varia-tion for three replicates agreedwithin ±20% of the mean. Thedetection limits (4), obtained underthe given digestion and measure-ment conditions, are shown inTable III. These analytical detectionlimits were calculated as the con-centrations of the element in a unitsample quantity (1 g) from theinstrumental detection limits. Theinstrumental detection limits wereestimated as the analyte concentra-tions corresponding to 3 times thestandard deviation (3σ) of the sig-nal intensities in 5 measurements

of the blank digest. The accuracy ofthe measurement given in terms ofthe percent recovery in the IAEASOIL-7 SRM for La, Sm, Ce, and Cswas 87.1, 85.9, 86.6, and 87.4,respectively.

The concentrations of the 26heavy elements found in the beachsand samples of the Red Sea alongthe Saudi Arabian border are sum-marized in Table IV. The concen-trations of the corresponding heavyelements in the terrestrial sandsamples collected from the coastalbelt are given in Table V. Atbeaches, heavy elements are oftenconcentrated in rather localizedspots in the swash zone of thewave run-up or at eroding cliffs (2).This would mean that the actualconcentration of heavy elements inthe beach sand may vary rapidlyboth in time and in location. On

the other hand, sand samples oninland locations should give moreor less uniform heavy element con-centrations in a given area definedby its geographical boundaries.

The presence of potentiallytoxic metals in surface soils of theland is of public concern (5). TheU.S. Federal and state regulationslist Ag, As, Ba, Be, Cd, Cr, Cu, Hg,Mn, Mo, Ni, Pb, Sb, S, and Zn aspotentially toxic elements (6,7).Therefore, the background concen-trations of these elements in opensea beaches or open sandy land-scapes used by humans are knownto assess human impact on theenvironment. The toxic effects ofthese elements may be severe dur-ing the summer season when winderosion forms a dusty environmentwith the finer particles of the sand(8). This situation is very common

54

TABLE IVConcentration of Heavy Metals in the Beach Sand Samples of the Red Sea Coast

Along the Saudi Arabian Border

Elemental Concentrations in µg g–1*

Sites Ag As Ba Cd Cr Cu Mn Mo Ni Pb Se Sn Zn

Haql 0.208 0.65 196 1.73 9.0 2.58 60 0.988 2.15 8.58 0.599 0.56 4.7Duba 0.137 5.26 194 2.04 24.5 17.05 326 1.179 13.05 20.31 0.371 1.22 33.7Al-wajh 0.014 1.77 54 1.67 5.1 1.65 38 0.449 1.82 1.04 0.158 0.30 2.7Ummlajj 0.106 4.22 193 BDL 14.6 5.61 207 0.719 8.33 5.47 0.444 1.63 21.0Yanbu 0.115 5.96 129 BDL 17.8 6.00 273 0.864 14.18 3.89 0.278 1.19 36.8Masturah 0.271 5.88 284 BDL 36.7 43.49 428 1.330 19.38 5.57 0.483 1.42 49.0Rabigh 0.007 1.51 37 BDL 8.2 22.27 63 0.782 2.26 1.99 0.138 0.35 9.1Al-Kudaimah BDL 6.67 49 BDL 13.2 9.37 344 0.338 12.49 4.98 0.182 0.77 55.0Tuwwal BDL 8.59 25 BDL 16.2 11.74 224 0.358 14.80 2.39 0.186 0.54 23.5Jeddah North 0.137 1.47 354 BDL 33.2 21.19 882 0.517 18.73 14.04 0.330 0.69 77.8Jahban BDL 3.09 19 BDL 5.9 1.06 43 0.542 1.92 0.18 0.246 0.12 0.3Al-Bahr 0.033 2.85 98 BDL 10.6 4.23 95 1.063 3.11 3.41 0.354 0.52 8.6AL-Quarnish 0.023 1.04 17 BDL 6.1 1.16 31 0.506 1.33 0.74 BDL 0.13 3.6South Jeddah 0.046 4.38 63 BDL 23.8 6.06 177 1.884 5.83 2.49 0.175 0.46 16.4AL-Lith 0.168 4.91 543 BDL 43.6 15.65 906 0.873 29.77 7.39 0.503 1.29 74.8Al-Qunfidhah 0.166 2.93 342 BDL 41.1 17.57 491 0.708 24.87 7.93 0.563 1.04 55.2Al-Birk 0.029 11.37 18 BDL 8.8 2.88 121 0.778 3.37 1.25 0.174 0.36 9.5

Jizan 0.171 3.34 495 BDL 32.7 10.93 358 0.758 18.42 12.83 0.175 1.06 44.9

Mean 0.091 4.21 173 0.30 19.52 11.14 281 0.81 10.88 5.80 0.298 0.76 29.3

Range LOD - 0.65 - 17 - LOD - 5.14 - 1.06 - 31 - 0.34 - 1.33 - 0.18 - LOD - 0.12 - 0.3 -0.271 11.37 543 2.04 43.59 43.49 906 1.88 9.77 20.31 0.599 1.63 77.8

* Some concentration values were below the detection limits of the given analytical method and are quoted as BDL in this table.In order to include information from those samples in the statistical evaluation, the results of those non-detected samples were replaced by one-half of the observed detection limit (15).

in desert countries like Saudi Ara-bia. As seen in Table IV, the con-centrations of enlisted elementsother than Hg and Sb are usuallymuch lower in the beach sand sam-ples studied in this work comparedto surface soils of undisturbed oruncultivated terrains (9). Such lowconcentrations can be explainedon the basis of the geographic loca-tion of the Red Sea beach.

Since there is no perennial riversystem and low rainfall in SaudiArabia, the major input of toxicmetals to the beach may be theatmospheric deposition and litterfall. The beach sand on the otherhand is subjected to constantweathering due to the interactions

with the sea waves. Thus the resid-ual or the equilibrium concentra-tion of the less resistant toxicmetals as associated with lighterminerals would be lower than theterrestrial surface soils (10,11). Theconcentrations of metallic elementsassociated with heavy mineralsdeposition on the beach are alsovery low in the coastal sand sam-ples of the Red Sea (Table IV).

Recent studies by Sroor (12)of the black sand of the Mediter-ranean coast of Egypt confirmedthe presence of Sc, Cr, Fe, Zn, Cs,La, Eu, Sm, Hf, Th, and U in veryhigh concentrations. These sandshave been characterized as goodsources of monazite (12,13). In the

Egyptian black sand, Sc, Cr, Fe, andZn occur at 851 ppm, 87 ppm,89.1%, and 1.11%, respectively.The Red Sea sand samples of thepresent study show the maximumconcentrations of 28.6 ppm, 53.9ppm, 2.3%, and 104 ppm for thecorresponding metals. The elemen-tal concentrations for other rele-vant elements observed in thepresent beach sands are likewisemuch lower than expected for asubstantial deposit of heavy miner-als (11,12). Another criterion deter-mining the possibility of monazitedeposition on the beach may bedefined by the Th/U concentrationratio. Beach sands identified as themonazite sources usually have thisratio far greater than unity. For

TABLE IV cont’d. on next page

55


example, the Egyptian depositsthat happen to be more or lessclose to the present study areashowed a Th/U ratio of 12.4 to14.9. For the present samples, thisratio is less than 1 for all samples,indicating insignificant depositionof heavy minerals on the Red SeaCoast of Saudi Arabia.

The mean concentrations ofheavy elements in terrestrial sur-face samples (Table V) are signifi-cantly higher than thecorresponding beach samples,except for strontium. This isexpected because both containsandy surface layers where atmos-pheric deposition acts as the major

source of heavy element input. Theinteraction with sea waves mayaccount for the observed depletionof heavy elements from the beachsand. Concentrations for most met-als in the inland samples are in gen-eral lower than those reported forthe surface soils of other regions ofthe world (8,9,14). This may beattributed to the low clay and highsilica content which is typical ofdesert areas. A convenient way toassess trace metal buildup in a par-ticular region is to find the ratios ofthe concentration values of con-cerned elements (Mobs) to the theiraverage concentrations in theearth’s crust (Mcrust). A value of

unity or less for this ratio of a givenelement should in principle indi-cate no or insignificant natural oranthropogenic enrichment of thatelement in the studied region withrespect to the global average (4).For the present study area, a plot ofMobs/Mcrust ratio vs. isotopic massesof some important elements isshown in Figure 2. It can be seenthat the Mobs/Mcrust ratios for mostof the elements seem to clusteraround a value of 0.45, except forSr, Hf, and U. Higher abundance ofthese elements was also observedfor sand samples of the ArabianGulf Coast of Saudi Arabia (3) andthus could be attributed to the geo-chemical formation of this region.

Table IV (continued)Concentration of Heavy Metals in the Beach Sand Samples of the Red Sea Coast

Along the Saudi Arabian Border


Sites Sc Ti Fe Co Cs La Ce Sm Hf Sr V Th U

Haql 8.38 404 2443 0.52 2.41 2.66 17.69 0.692 2.53 367 6 2.89 0.75Duba 4.85 2145 9346 6.43 0.97 10.19 31.23 2.635 5.01 1541 53 9.05 2.05Al-wajh 0.85 261 914 0.70 0.42 0.17 0.18 0.052 BDL 2627 7 BDL 2.29Ummlajj 6.83 1460 6795 3.47 0.86 5.62 14.43 1.802 2.38 1644 29 1.64 1.46Yanbu 4.65 4077 9525 3.65 0.87 1.66 3.08 0.522 1.01 2872 38 BDL 3.54Masturah 7.66 3966 17472 7.99 0.84 19.56 50.80 5.859 6.40 1931 71 11.55 2.39Rabigh 2.10 331 1948 0.89 0.27 BDL BDL 0.034 BDL 2839 11 BDL 3.02Al-Kudaimah 0.41 1878 4464 5.27 0.35 0.31 0.24 0.065 0.25 1104 50 BDL 1.64Tuwwal 1.36 1531 8826 6.64 0.85 0.09 BDL 0.036 BDL 4657 42 BDL 4.74Jeddah North 6.29 3000 17133 10.43 0.54 21.56 61.37 5.717 3.65 541 90 BDL 0.62Jahban 0.14 253 1432 0.80 0.25 BDL BDL BDL BDL 6254 15 BDL 4.12Al-Bahr 4.56 453 2784 1.42 0.42 0.40 0.18 0.128 BDL 1976 20 BDL 2.18AL-Quarnish 0.20 161 1064 0.44 0.16 BDL BDL BDL BDL 4402 6 BDL 4.65South Jeddah 2.92 1072 4312 2.60 0.41 0.35 0.53 0.070 BDL 4236 30 BDL 4.04AL-Lith 8.99 4976 26151 17.13 0.48 16.99 44.84 7.164 4.57 572 130 BDL 0.64Al-Qunfidhah 8.68 3596 20740 12.38 0.62 4.78 44.17 2.440 4.65 194 102 BDL 0.64Al-Birk 0.41 541 2536 1.27 0.34 0.16 BDL 0.025 0.22 3201 16 BDL 4.56

Jizan 7.89 2907 13941 7.54 1.04 20.15 64.13 6.923 5.13 556 64 9.62 0.93

Mean 4.29 1834 8435 4.98 0.67 5.82 18.53 1.90 2.18 2306 43 1.96 2.46

Range 0.14 - 161 - 914 - 0.44 - 0.16 - LOD - LOD - LOD - LOD - 194 - 6 - LOD - 0.62 --8.99 4976 26151 17.13 2.41 21.56 64.13 7.16 6.40 6254 130 11.55 4.74

* Some concentration values were below the detection limits of the given analytical method and are quoted as BDL in this table.In order to include information from those samples in the statistical evaluation, the results of those non-detected samples were replaced by one-half of the observed detection limit (15).

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TABLE VConcentration of Heavy Metals in the Terrestrial Sand Samples of the Red Sea Coastline of Saudi Arabia


Sites Ag As Ba Cd Cr Cu Mn Mo Ni Pb Se Sn Zn

Haql 0.252 0.11 291 2.512 12.16 3.45 132 1.02 3.68 10.67 0.533 0.52 11.30Duba 0.133 3.46 260 2.048 21.11 12.43 299 0.86 12.08 5.09 0.372 0.86 33.44Al-wajh 0.020 2.68 57 1.807 10.90 4.19 111 0.92 5.43 2.17 0.269 0.51 17.71Ummlajj 0.099 1.50 251 BDL 18.63 5.68 272 1.19 10.87 6.63 0.308 0.51 27.53Yanbu 0.285 12.49 366 BDL 19.85 15.21 491 1.52 12.87 9.35 0.445 1.74 71.79Masturah 0.097 4.53 264 BDL 11.07 8.06 274 0.62 9.74 4.48 0.512 0.81 28.36Rabigh 0.123 7.23 164 BDL 83.97 26.36 1093 3.74 47.26 83.03 0.637 2.42 108.49Al-Kudaimah 0.067 4.87 311 BDL 21.68 11.22 379 1.01 11.79 3.13 0.323 0.43 22.82South Jeddah 0.045 3.08 70 BDL 12.48 15.61 125 1.32 4.96 3.14 0.181 0.48 23.86AL-Lith 0.225 0.95 367 BDL 37.41 18.83 665 0.66 27.94 8.94 0.352 1.06 65.47Al-Qunfidhah 0.182 7.84 357 BDL 29.24 10.04 708 0.45 23.28 7.03 0.333 1.21 66.37Al-Birk 0.178 13.04 146 BDL 49.37 21.48 665 1.72 37.45 5.43 0.389 1.16 63.52

Jizan 0.192 2.00 508 BDL 24.69 11.48 495 0.45 19.99 11.68 0.293 1.10 44.97

Mean 0.146 4.91 262 0.493 27.12 12.62 439 1.19 17.49 12.37 0.380 0.99 45.05

Range 0.020 - 0.11 - 57 - BDL - 10.90 - 3.45 - 111 - 0.45 - 3.68 - 2.17 - 0.181 - 0.43 - 11.30 - 0.285 13.04 508 2.512 83.97 26.36 1093 3.74 47.26 83.03 0.637 2.42 108.49

TABLE V: Continued Elemental Concentrations in µg g–1*

Sites Sc Ti Fe Co Cs La Ce Sm Hf Sr V Th U

Haql 7.38 844 4500 1.36 6.45 5.89 30.72 1.26 4.33 282 11.02 4.44 1.02Duba 4.23 1982 10945 5.98 0.96 19.54 52.62 4.64 3.00 1476 47.93 4.94 1.98Al-wajh 1.21 790 2748 2.20 1.12 0.26 0.26 0.08 BDL 2466 17.73 0.57 2.82Ummlajj 8.38 2889 9097 3.44 0.90 5.47 16.80 1.77 1.72 560 35.68 1.77 0.75Yanbu 6.96 2599 12625 5.27 3.22 40.45 96.45 7.42 8.30 948 40.86 19.12 8.30Masturah 7.07 1901 6053 4.02 0.92 2.90 6.62 0.94 1.64 1828 34.86 0.57 1.82Rabigh 8.36 8582 26264 17.04 2.46 1.92 4.15 0.64 2.54 4879 150.00 0.57 8.00Al-Kudaimah 3.14 1656 6312 5.75 0.51 4.50 22.34 1.00 2.14 661 54.36 0.57 1.19South Jeddah 2.42 778 3326 2.04 0.46 1.25 8.42 0.51 0.29 4258 22.82 0.57 3.58AL-Lith 9.75 5359 22206 13.03 0.79 3.58 13.78 1.81 5.30 348 114.55 1.14 0.69Al-Qunfidhah 8.66 4860 19388 9.42 0.76 14.73 46.25 5.06 4.53 478 94.59 3.09 0.91Al-Birk 5.00 4777 21003 15.55 1.62 29.05 79.66 6.29 3.62 1391 90.58 2.63 4.55

Jizan 9.41 3831 11008 7.03 0.90 6.54 18.85 2.74 5.90 498 71.02 3.61 1.01

Mean 6.31 3142 11960 7.09 1.62 10.47 30.53 2.63 3.34 1544 60.46 3.35 2.82

Range 1.21 - 778 - 2748 - 1.36 - 0.46 - 0.26 - 0.26 - 0.08 - LOD - 282 - 11.02 - 0.57 - 0.69 - 9.75 8582 26264 17.04 6.45 40.45 96.45 7.42 8.30 4879 150.00 19.12 8.30

* Some concentration values were below the detection limits of the given analytical method and are quoted as BDL in this table. Inorder to include information from those samples in the statistical evaluation, the results of those non-detected samples werereplaced by one-half of the observed detection limit (15).

57


The element lead is much higherfor the terrestrial samplescompared to the correspondingbeach samples. This may be due topast deposition of the metal fromvehicular and oil processing emis-sions.

Distribution Pattern of HeavyElements

The present results reveal thatalong the Red Sea Coast, the con-centrations of some elementsrelated to industrial or anthro-pogenic activities (Ag, As, Ba, Cr,Cu, Mn, Ni, Pb, Sn, Zn, Fe, La, Ce,V) (see Table IV) are much higherthan their mean values at threeadjacent sampling sites, namelyLaith, Al-Quanfudah, and Masturah,and they appear to cluster at thesethree points. Incidentally, thesethree locations are situated at therunoff region of the Jeddah interna-tional port. So, the buildup ofheavy metals around Laith, Al-Quanfudah, and Masturah maybe attributed to the effect of theseaport and needs to be studied indetail. The terrestrial samples, onthe other hand, showed highestconcentrations of elements related

to anthropogenic activities like Cr,Cu, Mn, Mo, Se, Sn, Zn, Ti, Fe, Co,and V (as seen in Table V) at thesampling site around Rabegh. Thismay be due to the influence of theindustrial installations on the out-skirts of Jeddah City. The maximumconcentrations of elements relatingto the petrogenic or geochemicalorigin of the sands (La, Ce, Sm, Hf,Th, and U) are observed at the sitesaround Yanbu and can be ascribedto some natural deposits in thatarea. The area surrounding Yanbushould be extensively studied toevaluate the spatial as well as thetemporal distribution of heavy met-als.

CONCLUSION

In this paper, the backgroundconcentrations of 26 elements inthe surface sand of the coastal andterrestrial belt along the Red Seaborder of Saudi Arabia are reportedfor the first time. Both the marineand inland samples contain lowerelemental concentrations comparedto the global average values, exceptfor Sr, Hf, and U. The mean heavyelement contents of the sand from

the Red Sea Coast are in generallower than the corresponding val-ues for the terrestrial sand. Thedepletion in the beach may be dueto the dissolution in seawater. Nosignificant deposition of typicalheavy minerals could be observedalong the Red Sea Coast of SaudiArabia. However, a buildup of cer-tain trace metals such as Ag, Ba, Cr,Cu, Mn, Ni, Pb, Se, Sn, Zn, Sc, Ti,Fe, Co, La, Ce, Sm, Hf, and V is evi-dent in three coastal areas, namelyLaith, Al-Quanfudah, and Masturah.This results from the carryover ofthe wastes and spillage from theJeddah port. Among the terrestrialsampling areas, the top layer of theland of Rabegh contains highestconcentrations of Cr, Cu, Mn, Mo,Ni, Pb, Se, Sn, Zn, Ti, Fe, Co, and V,indicating some sort of anthro-pogenic enrichment in that area.The concentrations of La, Ce, Sm,Hf, Th, and U are the highest atYanbu and may be related to somenatural sources around that area.

In order to understand the heavymetals enrichment pattern of theRed Sea Coast, the area around theJeddah port, especially Laith, Al-Quanfudah, and Masturah, shouldbe reinvestigated with the provi-sion of analyzing sediment samplesand other environmental indicators.Similarly, the terrain surroundingRabegh deserves re-examinationincluding the vertical distributionof heavy elements of pedogenicorigin.

Received October 6, 2004.

Fig. 2. Plot of Cobs/Ccrust vs. isotopic mass for various elements determined in beachand terrestrial sand samples along the Red Sea Coast of Saudi Arabia.

58

REFERENCES

1. K. Binning and D. Baird, Water SA27(4), 461 (2001).

2. R.J . De Meijer, I.R. James, P.J. Jen-nings, and J.E. Koeyers, Appl.Radiation and Isotopes 54, 535(2001).

3. J. Al-Hefne, O. Al-Dayel, D. A.Chowdhury, and T. Al-Ajyan, Proc.International Nucl. Confec. Semi-nar-III, 31 (2002).

4. R. Wei and H. Haraguchi, Anal. Sci.15, 729(1999).

5. C. Chen, X. Tian, J. Yang, M. Chen,and L.Q. Ma, Annual Meetings,American Society of Agronomy,340 (1997).

6. Florida Department of Environmen-tal Protection, FDEP, 62, 640(1995).

7. U.S. Environmental ProtectionAgency, Soil Screening Guidance,User’s Guidance, EPA 540/R, 96(1996).

8. Rao M.V. Nageshwara, S.G.Vijapurkar, S. Daga, G.S. Murthy,and G. Satyanarayana, Indian J.Environ. Protection 11(1),33(1991).

9. M.L. Berrow and G.A. Reaves, "Back-ground levels of trace elements insoils," Proc. First Internat. Confec.Environ. Contamination, CEP Con-sultants, Edinburgh, England. pp.333-340 (1984).

10. B.D. Lee, B.J. Carter, N.T. Basta,and B. Weaver, Soil Sci. Soc. Am. J.61, 218 (1997).

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13. A.M. Hassan, M. Abdel-Wahab, ANada, N. Walley-El-Dine, and A.Khazbak, Appl. Radiation and Iso-topes 48(1), 149 (1997).

14. G.R. Bradford, A.C. Chang, A.L.Page, D. Bakhtar, J.A. Framton,and H. Wright, Kearney Founda-tion Spec. Rep., Univ. of Califor-nia, Riverside, CA, USA (1996).

15. G.S. Holmgren, M.W. Meyer, R.L.Chaney, and R.B. Daniels, J. Envi-ron. Qual. 22, 335(1993).

59


Determination of Major and Trace Elements in EdibleSeaweeds by AAS After Ultrasound-assisted Acid Leaching

Noemí Ladra-Ramos, Raquel Domínguez-González, Antonio Moreda-Piñeiro, Adela Bermejo-Barrera, and *Pilar Bermejo-Barrera

Department of Analytical Chemistry, Nutrition and Bromatology, Faculty of Chemistry, University of Santiago de Compostela, Avenida das Ciencias, s/n. 15782 – Santiago de Compostela, Spain

INTRODUCTION

Seaweed has been used for yearsin western countries as a source ofalginates and as an emulsifier in thefood industry. However, thesemarine products have been con-sumed in Asia since ancient times,and they have been recognized asfoods rich in the essential elements(1). Nowadays, there is an increas-ing consumer interest in theseproducts because of their medici-nal properties, such as anti–HIVeffects, anti–tumor activity, orpotential contraceptive effects (2).In addition, the consumption ofnatural foods or health foods sincethe early 1990’s has resulted inincreased markets for seaweed.

There are some studies dealingwith major and trace elements indifferent types of edible seaweed(2–6). On the other hand, seaweedare able to concentrate inorganicspecies dissolved in solution andbecause they are an importantsource of essential elements, theyhave been known to be good indi-cators of heavy metal pollution inmarine ecosystems (7,8). Thebiosorption capacity of seaweedinvolves the formation ofcomplexes between the metal ionand certain functional groups (car-boxyl, carbonyl, amino, amido, sul-fonate, phosphate, and so forth),being the carboxyl groups of thealginate fraction (linear polysaccha-rides containing 1,4–linkedβ–D–mannuronic (M) andα–L–guluronic (G) acid residues)that play the main role in the com-plexation of heavy metals (9).Recent applications published byJordanova et al. (10) and Farías et

ABSTRACTA sample pretreatment

method based on an ultrasound-assisted acid leaching processwith diluted reagents (nitric andhydrochloric acids and hydrogenperoxide) has been developed todetermine major (Ca, K, Na, andMg) and trace (Cu, Fe, Mn, andZn) elements in edible seaweeds.Optimization has been carriedout using a Plackett–Burmandesign (PBD) as the screeningmethod to select the most signifi-cant factors affecting the ultra-sound-assisted acid leachingprocedure. The results showedthat the waterbath temperature,ultrasound frequency, nitric acidconcentration, sonication time,and extracting volume are themost statistically significant vari-ables (P = 95 %).

These significant factors havebeen optimized by using a cen-tral composite design (CCD), andthe optimum compromise condi-tions were as follows: extractionvolume 7 mL, nitric acid concen-tration 3.7 M, sonication time 35min, ultrasound frequency 17kHz, and waterbath temperature65°C. The repeatability of theultrasound–assisted acid leachingmethod was lower than 5% forthe elements. The method wasfound to be accurate after analyz-ing different certified referencematerials (IAEA–140/TM,NIES–CRM–03 NIES–CRM–09,and BCR–61). Finally, themethod was applied to differentedible Atlantic seaweeds.

except when using X–ray fluores-cence techniques (6). However,acid digestion techniques requirethe use of acid and oxidizingreagents, such as nitric acid, at highconcentrations. The formation ofnitrous vapors when using the aciddigestion process is another draw-back. Acid leaching procedures,which use dilute acids, can be analternative to acid digestion meth-ods. These procedures do notinvolve the total matrix sampledestruction but result in the break-down of the chemical bondsbetween the elements and matrixsample constituents (12). Sinceorganic matter destruction is notrequired, low acid concentrationscan be used, which leads to a smallconsumption of mineral acids and areduction of nitrous vapors.

Ultrasound (13–16) has beenapplied to assist acid leaching pro-cedures from different biologicalmaterials. The use of ultrasound forsample preparation helps to speedup the leaching procedure becausethe induced cavitation process inthe liquid promotes an increase ofpressure and temperature, whichallows high analyte transport fromthe solid particles to the liquidphase (17). In addition, moderatetemperature conditions have beenshown to shorten the sonicationtimes (13,16).

The aim of the current work hasbeen the optimization of an acidleaching procedure assisted byultrasound energy and high temper-ature to determine major (Ca, K,Na, and Mg) and trace (Cu, Fe, Mn,and Zn) elements in seaweed sam-ples. Optimization of the variablesaffecting the sonication–high tem-perature acid leaching has been car-ried out by applying experimentaldesigns.

al. (11) discuss the analysis of sea-weeds to assess their metals conta-mination.

Sample decomposition proce-dures based on acid digestion havebeen commonly used as samplepretreatment prior to metals deter-mination in seaweed (3–5,10–11),

*Corresponding author.E-mail: [email protected]

60

EXPERIMENTAL

Instrumentation

A PerkinElmer Model 3110atomic absorption spectrometer(PerkinElmer Life and AnalyticalSciences, Shelton, CT, USA),equipped with an acetylene–airflame, was used for all determina-tions. Calcium, Fe, Mg, and ZnPerkinElmer LuminaTM hollowcathode lamps and Cu and MnCathodeon hollow cathode lamps(Cambridge, UK), operating at therecommended currents, were usedas sources of radiation. A Raypa®Model UCI–150 ultrasonic cleanerbath (frequencies of 17 and 35 kHz)from R. Espinar S.L. (Barcelona,Spain), programmable for tempera-ture and time and with fixed fre-quencies of 17 and 35 kHz,supplied the ultrasound energy andwas used to induce the acid leach-ing process. A vibrating ball mill(Retsch, Haan, Germany), equippedwith zircon cups (15 mL in size)and zircon balls (7 mm diameter),were used for pulverizing the sam-ples. A Centromix centrifuge(Selecta, Barcelona, Spain) wasused for separating the solid andliquid phases. A Samsung domesticmicrowave oven (Seoul, Korea),programmable for time andmicrowave power from 100 to 900W, was used for total sample diges-tion. The poly(tetra–fluoroethylene)(PTFE) bombs were laboratory–madewith hermetic seals and suitable forwork at low pressures. The chemo-metrics package used wasStatgraphics Plus V 5.0 for the Win-dows® OS, 1994–1999 (Manugis-tics Inc., Rockville, MD, USA).

Reagents

All chemicals were of ultrapuregrade. Milli-Q™ ultrapure water,resistance 18 MΩ cm (MilliporeCo., Bedford, MA, USA), was usedthroughout this study. Cu(NO3)2,Fe(NO3)3, and Zn(NO3)2 stock stan-dard solutions (1.000 g L-1) weresupplied by Merck (Poole, Dorset,

UK). Mn(NO3)2 stock standard solu-tion (1.000 g L–1) was from Panreac(Barcelona, Spain). Ca(NO3)2, KCl,Mg(NO3)2, and NaCl stock standardsolutions (1.000 g L–1) wereprepared from Ca(NO3)2,Mg(NO3)2, and NaCl (Merck) andKCl (Sigma–Aldrich, Steinheim,Germany). Nitric acid (70.0%) wassupplied by Merck, whilehydrochloric acid (37%) and hydro-gen peroxide (33%) were from Pan-reac. The certified referencematerials were IAEA–140/TM,Fucus – Sea Plant Homogenate(International Atomic EnergyAgency), NIES–CRM–03, Chlorella,and NIES–CRM–09, Sargasso(National Institute of EnvironmentalStudies), and BCR–61, Plant ofAquatic Origin (Commission of theEuropean Communities CommunityBureau of Reference).

Seaweed Samples

Edible seaweed samples werepurchased from a local manufac-turer as dried product (100 g) oras canned seaweed in brines(around 120 g). The samplesinclude five main Atlantic seaweedfrom the Galician coast (NorthwestSpain): Porphyra (Nori) and Pal-maria (Dulse), as red seaweed, andUndaria pinnatifida (Wakame),Himanthalia elongate (Seaspaghetti) and Laminariaochroleuca (Kombu), as brown sea-weed. Canned seaweed in brineswas lyophilized at –40°C for twoweeks (LYPH–LOCK® 6 L freeze-dry system, Model 77530 from Lab-conco Corporation, Kansas City,MO, USA). All samples were pulver-ized in a vibrating zircon ball millfor 45 minutes (power at 75%).

Microwave Acid DigestionProcedure

Powdered seaweed sampleswere subjected to an optimizedmicrowave-induced acid digestionprocedure (reference method) (18).A domestic microwave oven withlaboratory-made low pressure PTFE

bombs were used, and nitric acidand hydrogen peroxide were theacid/oxidant reagents. The aciddigests were made up to 10 mLwith ultrapure water and kept inpolyethylene vials at 4°C beforemeasurements.

Ca, Cu, K, Mg, Mn, Na, and ZnUltrasound-assisted Acid Leaching

Pulverized seaweed, about0.2 g, was directly weighed intocentrifuge tubes, and 7 mL of anacid/oxidant solution (3.7, 3.0, and3.0 M of nitric acid, hydrochloricacid, and hydrogen peroxide,respectively) was added. Afterslight homogenization by mechani-cal stirring, the tubes were placedinside the ultrasonic waterbath andsubjected to ultrasound energyremaining at 17 kHz and 65°C for35 min. The solution wascentrifuged at 3000 rpm for 10 min-utes and the acid liquid phase wasseparated by decantation. Then, thesolid residue was again subjected tothe above sonication conditions butusing 3 mL of the acid/oxidant solu-tion. After a new centrifugation at3000 rpm for 10 minutes, the acidliquid phase was combined withthe first acid leachate and the mix-ture made up to 10-mL volume.

Fe Ultrasound-assisted AcidLeaching

Because of non-quantitative Ferecoveries in acid leachates afterapplying the above optimized acidleaching procedure (Fe recoveriesaround 80%), a first sonicationstage at 17 kHz and at room tem-perature for 10 min (using 2 mL of6.0 M hydrochloric acid) was used.Then the residue was subjected tothe above ultrasound-assisted acidleaching procedure but using 1 mLof the acid/oxidant solution ratherthan 3 mL in the last step.

FAAS/FAES Determinations

Ca, Cu, Fe, Mg, Mn, and Zn weredetermined by Flame Atomic

61


Absorption Spectrometry (FAAS)and Na and K by Flame AtomicEmission Spectrometry (FAES)under optimum conditions(Table I). For Ca and Mg determina-tion, adequate volumes of LaCl3solution were added (ionizationsuppressor) to give a final concen-tration of 10.0% (m/v). The stan-dard addition method was used forall determinations. A dilution factorof 1:1000 was adopted for Na andK determinations, while dilutionfactors of 1:200, 1:50, 1:20, and 1:5were applied when determiningMg, Ca, Zn, and Fe, respectively.Cu and Mn were directlydetermined in the acid digests/acidleachates without dilution.


Variables Affecting theUltrasound-assisted Major andTrace Element Acid Leaching

The following factors were con-sidered as variables which couldaffect the ultrasound-assisted acidleaching of elements fromseaweed: (a) the concentrations ofthe three different acid and/or oxi-dant reagents (nitric acid,hydrochloric acid, and hydrogenperoxide), (b) the volume of theacid/oxidant leaching solution, (c)the exposure time to ultrasound(sonication time), (d) the tempera-ture of the ultrasonic waterbath,(e) the frequency of the ultrasoundenergy, and (f) the seaweed sampleparticle size. Additionally, a furthervariable (a dummy factor) was con-sidered. A dummy factor is an imag-inary variable for which the changefrom one level to another is notsupposed to cause any physicalchange. The use of dummy factorsallows evaluating possible system-atic errors or the importance ofunknown variables (19). To estab-lish the statistical significance ofthe above factors, a 29 3/128Planckett-Burman design (PBD),resolution III (20), involving 12experiments, was applied. Table IIlists the low (–) and high (+) levels

given to each variable, while TableIII shows the 29 3/128 PBD,which lists 12 differentexperiments. Each experiment wascarried out in duplicate and TableIII shows the response variables(analytical recovery for each ele-ment) according to the followingequation:

[ ]acid leaching

Recovery = _____________ 100[ ]acid digestion

where [ ]acid leaching is the metal con-centration obtained after the ultra-sound-assisted acid leachingprocedure (each experiment, 1 to24 in Table III, or 1 to 32 in

Table IV) and [ ]acid digestion is themetal concentration found aftermicrowave-assisted acid digestionof the seaweed sample. A recoveryclose to 100% would show quanti-tative extraction of the elements.For all these experiments, metalconcentrations in the acidleachates and acid digests weremeasured twice using the standardaddition technique and underoptimized operating conditions(Table I).

The statistical analysis of the 29

3/128 PBD at a confidence levelof 95.0% has revealed a minimumt value of 2.8 (standardized Paretocharts shown in Figure 1), obtain-

TABLE IFAAS/FAES Conditions for the Determination of Major and Trace

Elements in Acid Digests and Acid Leachates From Seafood Samples

Wavelength Slit Lamp Air C2H2Current Flow Rate Flow Rate

(nm) (nm) (mA) (L min–1) (L min–1)

Ca 422.8 0.7 15 17 2.0Cu 324.8 0.7 15 14 2.0Fe 248.3 0.2 30 12 1.5K 766.5 0.4 --a 12 1.5Mg 285.2 0.7 15 11 1.5Mn 279.5 0.2 15 12 1.5Na 589.0 0.2 -- 12 1.5

Zn 213.9 0.7 15 14 1.5

a FAES.

TABLE IIExperimental Field Definition for the Plackett–Burman Designa

Variable Symbol High Low Level (+) Level (–)

Nitric acid concentration (M) (HNO3) 3.0 0.0Hydrochloric acid concentration (M) (HCl) 3.0 0.0Hydrogen peroxide concentration (M) (H2O2) 3.0 0.0Acid leaching solution volume (mL) V 8 4Ultrasonic waterbath temperature (°C) T 60 15Sonication time (min) t 30 5Ultrasounds frequency (kHz) Pb 35 17Seaweed particle size (µm) φb 2000 < 50

Dummy factor D + 1 – 1

a The seaweed sample mass was 0.2 g for all experiments.b Non–continuous variable.

62

TABLE IIIPlackett–Burman Design Matrix for the Significant Variables Determination

Variables Recovery (%)

Run (HNO3)(HCl) (H2O2) V P t T φ D Ca Cu Fe K Mg Mn Na Zn

1 + - + - - - + + + 98.1 12.8 45.2 95.2 85.6 26.3 73.0 17.0

2 + + - + - - - + + 41.8 9.7 37.2 103.7 47.9 3.8 106.8 16.2

3 - + + - + - - - + 28.8 44.6 8.3 55.8 22.7 2.0 3.5 18.8

4 + - + + - + - - - 47.4 107.6 23.5 105.0 65.9 5.2 88.0 47.0

5 + + - + + - + - - 83.9 101.2 41.0 108.7 66.6 5.2 97.1 45.6

6 + + + - + + - + - 87.0 6.4 53.7 88.1 62.8 4.3 88.5 32.2

7 - + + + - + + - + 55.4 97.5 29.3 103.4 48.1 4.5 105.0 32.1

8 - - + + + - + + - 3.7 1.6 0.0 86.6 30.5 1.1 101.5 1.9

9 - - - + + + - + + 22.3 0.0 1.2 88.7 34.5 0.5 64.0 2.4

10 + - - - + + + - + 70.5 102.6 38.5 108.6 59.7 5.7 104.9 42.2

11 - + - - - + + + - 60.1 8.2 57.1 91.1 55.0 18.0 86.4 21.7

12 - - - - - - - - - 6.5 19.1 1.2 72.0 14.9 0.9 48.4 7.5

13 + - + - - - + + + 101.2 11.2 56.1 97.4 92.3 32.5 54.7 28.0

14 + + - + - - - + + 68.0 9.5 35.2 104.2 61.8 5.7 64.1 10.7

15 - + + - + - - - + 35.1 39.9 11.8 51.6 24.6 2.0 10.4 20.3

16 + - + + - + - - - 41.7 101.0 27.3 108.3 63.9 5.2 98.3 49.3

17 + + - + + - + - - 56.6 105.3 36.1 95.1 42.8 4.7 63.3 41.6

18 + + + - + + - + - 49.1 8.0 27.5 105.6 69.3 4.1 106.6 22.3

19 - + + + - + + - + 55.2 100.7 34.5 86.9 47.6 4.5 71.9 40.7

20 - - + + + - + + - 6.9 3.1 3.5 90.1 29.7 1.3 107.4 4.2

21 - - - + + + - + + 16.3 0.0 0.0 104.3 17.9 1.7 108.2 1.4

22 + - - - + + + - + 56.2 106.0 38.7 109.1 51.9 5.2 102.3 46.1

23 - + - - - + + + - 58.8 7.9 63.4 92.7 49.8 19.4 86.9 26.6

24 - - - - - - - - - 6.5 18.8 1.2 64.6 12.2 1.1 77.4 8.8

ing as statistically significant vari-ables those with a t value higherthan ± 2.8. It can be seen that thenitric acid concentration,ultrasound waterbath temperature,sonication time, and volume of theacid/oxidant leaching solution arethe most significant variables formost of the elements. The effect ofthese four variables, except the vol-ume of the acid/oxidant leachingsolution, is positive, meaning that

an increase in the variable magni-tude from the low to high levelsinvolves an increase in the percent-age of leached element. The vari-ables of hydrochloric acidconcentration and hydrogen perox-ide concentration are statisticallysignificant with a positive effectonly for Fe and Mg. The variables ofparticle size and frequency of theultrasound energy are not signifi-cant for most of the elements, and

when these variables are significant(particle size for Cu and Zn, andfrequency of ultrasound energy forFe, Mg, and Mn), they present anegative effect. Finally, the variableof the dummy factor was not statis-tically significant, and it couldtherefore be said that there are nosystematic errors and/oruncontrolled variables affecting theprocess.

63


and sonication time wereoptimized by applying a centralcomposite design (CCD).

Central Composite Design Optimization

A central 23 + star, orthogonalcomposite design with six degreesof freedom and involving 16 newexperiments in duplicate was per-formed to simultaneously optimizethe nitric acid concentration, thevolume of the acid/oxidant leach-ing solution, and the sonicationtime. Table IV shows the CCDmatrix together with the responsevariables. It can be seen that thelow and high values for the nitricacid concentration were 1.5 and4.0 M, respectively, while the lowand high values for the volume ofthe acid extraction solution were 4and 8 mL. Similarly, 15 and 45 minwere the low and high values forthe sonication time. After studyingthe estimated response surfaces(some of them plotted in Figure 2),volumes of the acid extracting solu-tion between 6.0 and 8.0 mL were

found as optimum for the acidleaching of Ca, Fe, K, and Zn, whilevolumes within 5.0–8.0 mL wereadequate to extract Mn and Na. Theacid leaching of Mg was only possi-ble by using volumes of the acidextracting solution between 7.0and 8.0 mL. Similarly, nitric acidconcentrations between 3.5 and 4.0M were found as optimum values toextract Ca, Fe, K, Mg, and Zn,while values between 2.0 and 3.5 Mwere needed for Cu, Mn, and Na.Finally, sonication times within the20-35 min range offer optimum Ca,Fe, K, Mg, and Zn recoveries, whileCu, Mn, and Na can be efficientlyleached from seaweed after sonica-tion from 15 and 45 min. Sincethese ranges compromise optimumvalues of 3.7 M for the nitric acidconcentration, 7.0 mL for theacid/oxidant leaching solution plus35 min of sonication time werechosen. The optimum sonicationtime is slightly larger than for othertissues such as mussel or humanhair (13,16). As indicated above,high sonication times together withthe use of high sonication tempera-

Because the variable of frequencyof ultrasound energy can onlyadopt two values (high of 37 kHzand medium of 17 kHz) and sincehigher recoveries for someelements were reached when usingthe low frequency, this variablewas fixed at 17 kHz as the optimumvalue. Similarly, the particle sizewhen being significant led to lowerrecoveries when using the high val-ues (non–pulverized seaweed), andwas fixed to the low value (pulver-ized seaweed, mean particle sizelower than 50 µm). On the otherhand, the variable of temperatureof the waterbath was significant forall cases with a positive effect. Themaximum allowed temperature ofthe waterbath was chosen as theoptimum value (65°C). Thevariables of hydrochloric acid con-centration and hydrogen peroxideconcentration were only significantfor some elements with positivesigns, and they were fixed at thehigh values: 3.0 M for eachelement. Finally, the variables ofnitric acid concentration, volume ofthe acid/oxidant leaching solution,

Fig. 1. Standardized main effects of Pareto charts after Plackett-Burman design.

64

TABLE IVCentral 23 + star Orthogonal Composite Design for the Set [HNO3]/V/t in the Acid Leaching

of Major and Trace Elements From Seaweed

Variables Recovery (%)

Run V (HNO3) t Ca Cu Fe K Mg Mn Na Zn(mL) (M) (min)

1 6.0 2.75 30.0 52.2 97.8 91.2 108.4 102.5 94.3 105.2 93.7

2 4.0 1.5 15.0 42.7 88.4 72.6 86.9 86.7 85.5 91.7 86.7

3 8.0 1.5 15.0 10.7 96.3 76.2 92.4 84.0 84.4 88.5 101.1

4 4.0 4.0 15.0 89.6 85.5 74.5 91.7 93.1 77.2 72.0 84.7

5 4.0 4.0 15.0 62.9 100.1 90.0 104.1 86.4 85.4 77.9 104.3

6 8.0 1.5 45.0 50.6 89.6 69.9 95.8 67.4 87.1 51.3 90.3

7 4.0 1.5 45.0 46.9 87.4 77.5 100.3 107.4 89.6 71.7 98.3

8 8.0 4.0 45.0 83.8 87.6 72.7 90.0 88.2 82.1 87.4 77.7

9 2.64 4.0 45.0 75.2 75.6 81.1 108.5 105.2 85.8 106.6 88.0

10 9.36 2.75 30.0 50.3 40.3 65.5 90.8 87.4 72.7 24.5 65.8

11 6.0 2.75 30.0 63.0 45.8 74.8 106.3 101.8 85.3 81.7 84.6

12 6.0 0.65 30.0 87.9 62.0 74.4 100.7 67.4 84.9 63.8 69.7

13 6.0 4.85 30.0 75.8 77.8 87.2 106.5 95.8 80.7 67.9 99.4

14 6.0 2.75 4.8 55.4 61.2 64.2 99.7 97.1 76.2 67.6 72.2

15 6.0 2.75 55.2 75.4 79.7 75.5 98.9 63.8 83.8 48.5 73.8

16 6.0 2.75 30.0 74.2 89.6 79.6 105.2 98.0 84.7 75.6 107.2

17 6.0 2.75 30.0 52.0 89.1 80.7 86.1 85.2 90.1 104.7 91.9

18 4.0 1.5 15.0 37.2 90.5 71.4 80.5 83.2 82.8 95.5 88.3

19 8.0 1.5 15.0 16.2 99.1 82.4 106.0 82.6 88.6 105.6 88.5

20 4.0 4.0 15.0 67.4 86.9 76.3 102.2 80.0 86.3 74.8 80.3

21 4.0 4.0 15.0 80.1 96.2 86.6 108.5 67.1 84.8 60.6 101.7

22 8.0 1.5 45.0 53.8 85.2 65.2 78.2 66.4 86.0 38.1 88.4

23 4.0 1.5 45.0 80.2 88.5 76.8 95.4 100.1 89.4 95.1 91.3

24 8.0 4.0 45.0 79.6 89.4 84.8 105.8 105.1 86.5 66.8 77.5

25 2.64 4.0 45.0 64.9 76.3 78.8 104.8 109.1 81.2 90.5 71.9

26 9.36 2.75 30.0 44.4 39.2 54.7 67.4 82.1 68.1 34.7 57.3

27 6.0 2.75 30.0 69.5 52.7 81.2 104.1 106.2 86.4 59.5 88.3

28 6.0 0.65 30.0 76.1 66.8 83.7 105.3 93.2 87.7 49.6 76.6

29 6.0 4.85 30.0 76.9 70.5 78.8 106.3 103.0 78.7 73.8 81.9

30 6.0 2.75 4.8 31.5 63.0 63.9 97.3 57.5 77.6 67.1 38.4

31 6.0 2.75 55.2 86.8 80.6 67.4 92.7 85.3 83.7 55.5 75.1

32 6.0 2.75 30.0 64.4 86.2 76.8 106.7 69.9 85.7 66.6 78.2

65


tures means a stronger structure ofseaweed than other environmen-tal/biological samples.

After applying the optimizedultrasound-assisted acid leachingconditions shown above, Fe recov-eries of around 85% were achieved.Non–quantitative recoveries havebeen reported when extractinginorganic As from seaweed (18,21)and Fe from vegetables (15). There-fore, a first step, involving the useof 2 mL of concentrated hydrochlo-ric acid (≈ 12.0 M) and 3.0 and 6.0M hydrochloric acid solution, wastested using ultrasound. The use ofhydrochloric acid has beenproposed by some authors in orderto disrupt the cell wall and breakdown the sulphur bonds in richpolysaccharide matrices (18,21).Results have shown Fe quantitativerecoveries when using hydrochlo-ric acid solutions at 6.0 and 12.0 M(recoveries of 99.5 ± 2.3% and104.0 ± 1.7%, respectively), a firstsonication step with 6.0 Mhydrochloric acid at room tempera-ture, and 17 kHz was applied for10 min to leach Fe.

Analytical Performances

Aqueous calibration andstandard addition graphs were com-pared in order to observe matrixeffects. After statistical comparison(using the Cochran and Bartletttests at 95.0% to compare variancesand the ANOVA test to comparethe means) of the slopes of aqueouscalibration and the standard addi-tion graphs, it can be said thatthere are matrix effects in thedetermination of Cu, K, and Mn.The standard addition techniquemust be applied to determine thesethree elements. Table V lists themean slopes, expressed as mean ±S.D. for the aqueous calibration andthe standard addition graphs, whileTable VI shows parameters such asthe limit of detection, limit of quan-tification, and the sensitivity of themethods. The repeatability of theoverall procedure, assessed by ana-lyzing the same seaweed sampleeleven times after the optimizedultrasound-assisted acid leachingprocess, is listed in Table VI as%RSD values. It can be seen that

adequate limits of detection andgood precision of the methods areachieved.

The accuracy of the method wasassessed by analyzing four certifiedreference materials (IAEA–140/TM,NIES–CRM–03, NIES–CRM–09 andBCR–61). Each reference materialwas subjected to the optimizedultrasound-assisted acid leachingprocedure five times, and the ele-ments were measured twice ineach acid leachate. Table VII showsthe results obtained after analyzingthe acid leachates, where goodagreement with the certified/infor-mative concentrations after statisti-cal comparisons (t-test) wasachieved.

Applications

The proposed method wasapplied to eight edible seaweedsamples including Nori, Dulse,Wakame, Sea Spaghetti, and Kombutypes. Portions of around 0.2 g ofpulverized sample were subjectedto the optimized ultrasound-assistedacid leaching procedure in triplicate.

Fig. 2. Some estimated response surfaces after central composite design.

66

TABLE VMean Slopes for Calibration and Standard Addition

Calibration Standard Addition

Mean Slope ± S.D.a Mean Slope ± S.D.b(L mg–1) (L mg–1)

Ca 0.081 ± 0.008 0.085 ± 0.007Cu 0.125 ± 0.007 0.137 ± 0.011Fe 0.077 ± 0.005 0.079 ± 0.005K 0.205 ± 0.016 0.098 ± 0.015Mg 1.565 ± 0.113 1.432 ± 0.103Mn 0.161 ± 0.012 0.169 ± 0.015Na 0.942 ± 0.054 0.894 ± 0.065

Zn 0.569 ± 0.022 0.552 ± 0.025

a n = 6; b n =12.

TABLE VISensitivity, LOD, LOQ, and Repeatability

of the Methods

LODa LOQb Sensitivityc RSDd

(µg g–1) (µg g–1) (mg L–1) (%)

Ca 4.75 15.8 0.0430 2.8Cu 0.04 0.10 0.0300 3.0Fe 0.24 0.79 0.0580 0.9K 0.5 17.0 0.0450 2.8Mg 0.24 0.82 0.0030 0.4Mn 0.02 0.06 0.0300 1.1Na 2.37 7.85 0.0045 2.0

Zn 0.50 1.67 0.0080 2.8

3 SD 10 SD a LOD = ______ and b LOQ _______

m mwhere SD is the standard deviation of 11measurements of ablank and m is the slope of the aqueous calibration or stan-dard addition graph.c Sensitivity is the analyte concentration which gives anabsorbance/emission value of 0.0044 units.d Repeatability of the overall procedure for n = 11.

TABLE VIIAnalysis of Certified Reference Materials After Ultrasound-assisted Acid Leaching

IAEA–140/TM NIES–CRM–09 NIES–CRM–03 BCR–61

Certified Found Certified Found Certified Found Certified Found Value Value Value Value Value Value Value Value

(µg g–1) (µg g–1) (µg g–1) (µg g–1) (µg g–1) (µg g–1) (µg g–1) (µg g–1)

Ca 12730 ± 1782 12659 ± 443 13400 ± 536 13449 ± 250 4900 ± 294 4959 ± 54 ----a ----

Cu 5.05 ± 0.30 5.04 ± 0.13 4.9 ± 0.2 4.9 ± 0.1 3.5 ± 0.3 3.6 ± 0.1 720 ± 31 709 ± 14

Fe 1256 ± 38 1290 ± 21 187 ± 6 184 ± 2 1850 ± 92 1807 ± 19 ----a ----

K 31100 ± 2488 31055 ± 232 61000 ± 1830 61667 ± 1060 12400± 620 12898 ± 407 ----a ----

Mg 9070 ± 907 9325 ± 220 6500 ± 325 6321 ± 82 3300 ± 198 3360 ± 36 ----a ----

Mn 56.1 ± 2.2 55.1 ± 0.6 21.2 ± 1.1 21.4 ± 0.1 69 ± 5 68 ± 1 3771 ± 78 3774 ± 29

Na 32000 ± 6720 31317 ± 1068 17000 ± 850 17071 ± 394 ----a ---- ----a ----

Zn 47.3 ± 1.9 47.7 ± 1.4 15.6 ± 1.2 15.8 ± 0.7 20.5 ± 1.0 20.3 ± 0.5 556 ± 13 563 ± 5

a Not certified.

67


Each acid leachate and acid digestwas analyzed twice, so the meanconcentration reported for eachsample is related to six indepen-dent determinations. K concentra-tions between 3.9% (m/m) (Nori)and 16.6% (m/m) (Kombu), and Naconcentrations within 0.7% (m/m)(Dulse) and 5.9% (m/m) (Wakame)were obtained. The Ca concentra-tions were between 0.2% (m/m)(Dulse) and 1.0% (m/m) (Seaspaghetti) and between 0.1%(m/m) (Dulse) and 0.9% (m/m)(Wakame) for Mg. Elements such asCu, Fe, Mn, and Zn can be consid-ered as trace elements. The Cu con-centrations were between 1.73 µgg–1 (Wakame) and 5.96 µg g–1

(Nori), Fe concentrations between28.53 µg g–1 (Sea Spaghetti) and152.60 µg g–1 (Nori), Mn concentra-tions between 3.89 µg g–1 (Kombu)and 69.40 µg g–1 (Sea Spaghetti),and Zn concentrations between13.71 µg g–1 (Wakame) and 156.85µg g–1 (Dulse).

CONCLUSION

Different variables affectingultrasound-assisted acid leachingwere simultaneously optimizedafter 28 experiments by applyingexperimental designs. Variablessuch as nitric acid concentration,volume of the acid/oxidant leach-ing solution, sonication time, andwaterbath temperature were themost significant variables. Quantita-tive extractions were found afterapplying the optimized ultrasound-assisted acid leaching process,although an additional step such asusing an ultrasound treatment with6.0 M hydrochloric acid wasneeded to leach Fe. The results arecomparable to those obtained afterapplying microwave-assisted aciddigestion. However, since dilutedacids are used and nitrous vaporformation is reduced, the methodoffers advantages such as lowreagent cost, low waste, and is anenvironmentally friendly methodol-ogy.

ACKNOWLEDGMENTS

The authors are grateful to "Sec-retaría Xeral de Investigación eDesenvolvemento – Xunta de Gali-cia" for financial support (researchprojects PGIDIT02RMA20301PRand PGIDIT02PXIB20901PR) andto ALGAMAR (Redondela, Ponteve-dra, Spain) for providing the edibleseaweed samples. The authorswould also like to thank Miss ElenaCantó–Mesías, student of Chem-istry (University of Santiago deCompostela), for her collaborationin this study.

Received November 8, 2004.

REFERENCES

1. V.J. Chapman and D.J. Chapman, SeaVegetables (algae as food for man),Chapman & Hall, London, UK(1980).

2. C. van Netten, S.A. Hoption Cann,D.R. Morley, and J.P. van Netten,Sci. Total Environ. 255, 169(2000).

3. M.A. Munilla, I. Gómez–Pinilla, S.Rodenas, and M.T. Larrea, Analusis23, 463 (1995).

4. M.H. Norziah, and C.Y. Ching, FoodChem. 68, 69 (2000).

5. L. Campanella, G. Crescentini, P.Avino, and A. Moauro, Analusis 27,533 (1999).

6. M.L. Carvalho, J.G. Ferreira, P.Amorim, M.I.M. Marques, and M.T.Ramos, Environ. Toxicol. Chem.16, 807 (1997).

7. P. Foster, Environ. Pollut. 10, 45(1976).

8. D.J.H. Phillips, Environ. Pollut. 13,281(1977).

9. E. Fourest, and B. Volesky, Appl.Biochem. Biotechnol. 67, 215(1997).

10. A. Jordanova, A. Strezov, M.Ayranov, N Petkov, and T.Stoilova, Wat. Sci. Tech. 39, 207(1999).

11. S. Farías, S. Pérez–Arisnabarreta, C.Vodopivez, and P. Smichowski,Spectrochim. Acta B 57, 2133(2002).

12. P. Bermejo–Barrera, A. Moreda–Piñeiro, and A. Bermejo–Barrera,Talanta 57, 969 (2001).

13. P. Bermejo–Barrera, A. Moreda–Piñeiro, and A. Bermejo–Barrera, J.Anal. At. Spectrom. 15, 121(2000).

14. A.V. Filgueiras, J.L. Capelo, I. Lav-illa, and C. Bendicho, Talanta 53,433 (2000).

15. C.C. Nascentes, M. Korn, andM.A.Z. Arruda, Microchem. J. 69,37(2001).

16. P. Bermejo–Barrera, O. Muñiz–Naveiro, A. Moreda–Piñeiro, andA. Bermejo–Barrera, Anal. Chim.Acta 439, 211(2001).

17. R.E. Majors, LC–GC 17(6S), S8(1999).

18. O. Muñiz–Naveiro, A.Moreda–Piñeiro, A. Bermejo–Barrera, and P. Bermejo–Barrera,At. Spectrosc. 25, 79 (2004).

19. W.P. Gardiner and G. Gettinby,Experimental Design Techniquesin Statistical Practice: A PracticalSoftware–based Approach, Hor-wood Publishing Limited, WestSussex, UK (1998).

20. Statgraphics Plus V. 5.0, ReferenceManual, Manugistics Inc,Rockville, MD, USA (1992).

21. O. Muñoz, D. Vélez, and R. Mon-toro, Analyst 124, 601(1999).

68



Fast Furnace Program With End-capped Tubes for the Determination of Sub-ppb Levels of Cd

in Foods and Biological SRMs Using ETAAS After Ultrasonic Probe Extraction

Noorbasha N. Meeravali, M.A. Reddy, and *Sunil Jai KumarNational Center for Compositional Characterization of Materials

Bhabha Atomic Research Center, ECIL-postHyderabad 500 062, India

INTRODUCTION

Ultrasonic-assisted extraction incombination with electrothermalatomic absorption spectrometry(ETAAS) has become an efficientand fast method for the determina-tion of trace metals in biologicalsolid matrices (1). Powdered sam-ples in the form of slurries are sus-pended in a liquid (usually water,containing appropriate amounts ofacid) and then subjected to ultra-sonic irradiation either in an ultra-sonic bath or through a probe(2–8). The extent of extractiondepends on a number of parame-ters such as particle size, type ofacid used, and the nature andstrength of the different metalspecies binding to the solid phase.

There are numerous reports ofcadmium being extracted from bio-logical samples and referencematerials by ultrasonic extractionand determination by ETAAS(1,8–12). A Placket-Burman experi-mental design was used to evaluatethe extraction of Cd along withother elements from hair and thesamples determined by ETAASusing Pd(NO3)2-Mg(NO3)2 modifier(10). Extractions in the range of65–89% have been reported in cab-bage leaves and roots using aprobe in 0.05–5% nitric acid (11).Using an ultrasonic bath, a nearly100% recovery has been reportedin different matrices such as bio-logical reference materials (12–14)(UE-CRM 176 ash, UK-135 WCRsewage sludge, MURST-ISSA 2

ABSTRACT

A simple and rapid methodfor the determination of Cd atlow ppb levels in 10 differenttypes of food and biologicalstandard reference materials isdescribed (Non-fat milk powderNIST SRM 1549, Total diet NISTSRM 1548, Rice flour NBS SRM1568a, Algae IAEA 391 and 392,Bovine muscular powder NBSRM 8414, Sea plant homogenateIAEA 140/TM, Tobacco leavesCTV-OTL-1, and CTV-VTL-2 and 4samples). Ultrasonic probeextraction and a fast furnace tem-perature program (withelectrothermal atomic absorptionspectrometry and an end-cappedtransversely heated atomizer)was used. Ultrasonic extractionconditions were optimized usingsample amounts as high as 200mg. No modifier was used andthe ashing step in the tempera-ture program was omitted. Theduration of one furnace cyclewas only 45 s. The student t-testat the 95% confidence levels indi-cated good agreement with thecertified values. The characteris-tic mass (m0) of Cd was 0.4 pg,while the limit of detection was0.1 ng/g. The precision of theresults obtained was between3–6%.

tion. A method for the multi-ele-ment determination of cadmiumand lead in urine was proposed bysimultaneous electrothermalatomic absorption spectrometry(SIMAA) using ammonium hydro-gen phosphate as the modifier withan end-capped transversely heatedgraphite atomizer (EC-THGA) (17).

Fast furnace programs have ear-lier been used with various modi-fiers (18,19) for the determinationof Cd. However, using the fast fur-nace program with end-cappedtubes without a modifier has, tobest of our knowledge, not beenreported. The aim of the presentwork is to develop a method forthe determination of Cd up to alevel of 0.5 ppb. Ten differenttypes of reference materials andfour real samples having Cd con-centrations in the range 0.5–1500ppb were analyzed.

EXPERIMENTAL

Instrumentation

A PerkinElmer Model 4100ZLspectrometer, equipped with atransversely heated graphite atom-izer (THGA), was used(PerkinElmer Life and AnalyticalSciences, Shelton, CT, USA). Back-ground correction was performedusing longitudinal Zeeman effect.The measurements were made atthe analytical wavelength of Cd(228.8 nm, slit 0.7 nm) using a Var-ian® hollow cathode lamp oper-ated at 8 mA. The optimized fastfurnace program is given in Table I.Argon gas was used as a protectivegas throughout the program. The

antarctic Krill), environmental ref-erence materials (15), and seafood(16), but only a 30% recovery isreported for human hair (12). Thecontradiction in some of the above-reported results are due to differentexperimental conditions and a vari-ation in particulate size, whichplays an important role in extrac-

69


instrument was operated in manualmode. Particle size distribution wasobtained using a laser diffractionparticle size analyzer (Horiba, LA-500, R. I. Version, Kyoto, Japan).An ultrasonic probe of 130 Wpower, 20 kHz frequency (Cole-Parmer, Vernon Hills, IL, USA,Model CP130PB-1, equipped with a3-mm titanium probe, was used.

Reagents and StandardSolutions

High purity de-ionized water(resistivity >18 MΩ cm) obtainedfrom a Milli-Q™ (Millipore,Bedford, MA, USA) water purifica-tion system was used throughout.Nitric acid (analytical grade) wasobtained from Merck (Darmstadt,Germany). Stock standard of Cdwas prepared from 99.99% puremetal dissolved in nitric acid andworking standards were preparedby subsequent dilution.Micropipettes with disposablepoly(propylene) tips were used forpreparing the samples. PFA (perflu-oroalkoxy) tubes of 30-mL capacitywere used for ultrasonic extraction.All containers were cleaned bysoaking in 10% (v/v) sub-boilednitric acid for 1 h at 60ºC and thenwashed with high purity water.

Procedure for Ultrasonic Extraction

Between 0.05–0.2 g of the abovematerials were weighed accuratelyinto clean 30-mL PFA centrifugetubes with screw caps, after which2 mL of 5% nitric acid was added.These solutions were sonicated

using ultrasonic probes equippedwith a 3-mm titanium probe for5 min at 40% amplitude.Subsequently, the liquid phase wasseparated after centrifugation at2000 revolutions per minute for10 min, and the supernatant wastransferred to sampler cups. A 10-µL aliquot of the supernatant wasinjected manually into theatomizer.


Optimization of TemperatureProgram

The effect of drying temperatureon the integrated absorbance signalof Cd was studied without anymodifier using end-capped tubes.The drying temperatures were opti-mized for NIST 1548 Total Diet andNBS RM 8414 Bovine MuscularPowder after ultrasonic extraction.The drying temperature curves areshown in Figure 1. It can be seenthat when the drying temperatureis increased from 150 to 350ºC,there is a fast reduction inbackground signal from 2.5 to lessthan 0.1 s. It was also observed thatmaximum integrated atomicabsorbance for both these matriceswas between 300–400ºC. Different

ramp and hold times were alsostudied at this temperature, and theoptimum ramp and hold timeswere 15 and 25 s, respectively.Under these conditions we did notobserve boiling or splattering of thesample solution for the matricesstudied; however, if boiling orsplattering is observed, the ramptime should be increased further.The optimum atomization tempera-ture of 1300ºC (not shown in theFigure 1) was used. The total timerequired to complete one atomiza-tion cycle (furnace cycle) was 45 s.

Recovery Obtained Using FastFurnace Program

In order to ascertain the maxi-mum limit at which Cd can bedetermined using a fast furnace pro-gram (without any interferencefrom the residual matrix co-extracted with the analytes duringultrasonic extraction), knownamounts of Cd were added to thesupernatant solution after ultrasoni-cation; then centrifugation andrecovery were determined. Theresults, given in Table II, are for dif-ferent reference matrices used. Itcan be seen that better than 95%recovery was obtained even usinga 200-mg sample in a 2-mL volume

TABLE IThe Optimized Temperature Program for the

Determination of Cd Using End-capped ETAAS

Stage Temp. Ramp Hold Gas Flow(ºC) (ºC) (s) (mL/min)

Drying 300 15 25 250

Atomizationa 1300 0 3 0

Cleaning 2200 1 1 250

a Read.

Fig. 1. Drying temperature (ºC) for NBS RM 8414 BovineMuscular Powder, 14 (100 mg/mL) [atomic (8414), background(8414(b)], and SRM NIST 1548 Total Diet (50 mg/mL) [atomic(1548), background (1548(b)], integrated absorbance using end-capped tube with fast furnace program.

70

of the liquid solution. These resultsclearly indicate that there is nomatrix interference inside the fur-nace using the fast temperatureprogram even in the absence ofany matrix modifier.

Optimization of ExtractionParameters

The parameters such as ampli-tude, duration of extraction, acidconcentration, and maximumamount of matrix that can beextracted were optimized usingultrasonic probes for two matrices(NIST 1548 Total Diet and NBS RM8414 Bovine Muscular Powder) asillustrated in Figure 2. It can beseen that the amount of matrix andconcentration of acids are veryimportant. Nearly 100% extractionwas observed for amounts up to200 mg (in a 2-mL volume). Byincreasing the amount beyond 200mg, the extraction reducesabruptly. Similarly, with a nitricacid concentration between 4–10%(v/v), an extraction from 90-100%

was obtained, but this reducesgradually with an acid concentra-tion below 3% (v/v). It was alsofound that an ultrasonic amplitudebetween 40–60% was needed forcomplete extraction; but even witha 20% amplitude, a 90% extractionwas observed. It was found that anextraction duration of 4–10 min-utes was sufficient for completerecovery.

Particle size of the powderedsample appears to be important inultrasonic extractions. Wemeasured the particle size distribu-tion of these reference materialsusing a laser particle size analyzer.In most of these materials, 90–95%of the particles were below the120-µm size. The median particlesize was 65, 47, and 36 µm for NBSRM 8414 Bovine Muscular Powder,NIST SRM 1549 Non-fat Milk Pow-der, and NIST SRM 1548 Total Diet,respectively. Therefore, even withhigh quantities of these matrices,complete extraction was possible.

TABLE IIRecovery of Cd From Ultrasonic Probe-extracted

Solutions Using a Fast Furnace Program With End-capped ETAAS

(A 10-µL volume of extracted solution was injected into the atomizer without modifier.)

Amount of Amount ofMatrix Matrix Cd Added Recovery

(mg in 2 mL) (ng/mL) (%)

Non-fat Milk Powder 100 0.5 100±4NIST SRM 1549 200 1 100±5

Total Diet 50 0.5 100±4NIST SRM 1548 100 1 95±5

Rice Flour 5 0.5 100±2NBS SRM 1568a 150 1 98±3

Algae 50 0.5 100±2IAEA-392 100 1 99±3

Bovine Muscular Powder 100 0.5 100±3NBS RM 8414 200 1 95±5

Musscel Tissue 5 1 100±2NIST SRM 2976 10 1 99±4

Fig. 2. Optimization of ultrasonicprobe extraction conditions for NIST1548 Total Diet (TD) and NBS RM8414 Bovine Muscular Powder (BMP).

71


Analytical Figures of Merits

Sensitivity and Limit of DetectionWith end-capped THGA, gener-

ally an enhanced sensitivity ofabout 2–3 times higher is observedas compared to standard THGA,depending on the dimensions ofthe orifice. As a result, an improve-ment in characteristic mass of0.4±0.1 pg was obtained.

The limit of detection (LOD) andlimit of quantification (LOQ) is alsogiven in Table III, along with valuesreported in the literature using theZeeman-effect background correc-tion systems. Using a fast furnaceprogram in combination with end-capped tubes, a LOD of 0.1 ng/g(3 s/m) and a LOQ (10 s/m) of 0.33ng/g were obtained, which is animprovement over the existing pro-cedure. Where, "s" is the standarddeviation of 10 blank measure-ments and "m" is the slope of thecalibration curve.

RESULTS

Table IV shows the resultsobtained for different certified ref-erence materials using the normaland standard addition calibrationcurve. These results were obtainedby using optimized conditions ofNIST 1548 Total Diet and NBS RM8414 Bovine Muscular Powder.Most of the values agree well withthe certified values. The results fortwo spinach, one skim milk pow-der, and one tuna fish samples arealso reported in Table IV and thevalues are in good agreement withslurry atomization (20). The valuesobtained by standard addition areclose to those obtained by normalcalibration, which shows thatmatrix interferences are negligibleat these levels. The results are pre-sented as an average±confidenceinterval at 95% confidence levelusing a t-test, student t=2.571.

CONCLUSION

Low ppb levels of Cd can beextracted from food and biologicalstandard reference materials andreal samples by using an ultrasonicprobe under optimum conditions.Quantification at these levels ispossible by ETAAS with end-capped THGA tubes and using thefast furnace program, without anymodifier. By using this method, acharacteristic mass of 0.4 pg and aLOD of 0.1 ng/g were obtained.This method reduces the timerequired for sample preparation tojust 15 minutes and the determina-tions are completed in 45 seconds,thus increasing the overall samplethroughput.

ACKNOWLEDGMENTS

The authors are thankful toDr. J. Arunachalam, Head, for hisconstant encouragement and sup-port during the course of the work.Thanks also to Mr. R.B. Yadav, Man-ager Control Lab, NFC, for the dataon particle size distribution.

Received October 11, 2004.

REFERENCES

1. E.C. Lima, F. Barbosa, Jr., F.J. Krug,M.M. Silva, and M.G.R. Vale, J.Anal. At. Spectrom. 15, 995(2000).

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14. H. El. Azouzi, M.L. Cervera, and M.de la Guardia, J. Anal. At. Spec-trom. 13, 533 (1998).

15. K. Ashley, R.N. Andrews, L. Cava-zos, and M. Demange, J. Anal. At.Spectrom. 16, 1147 (2001).

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72

TABLE IIIComparison of LOD and Characteristic Mass for Cd Obtained by the Present Method With the Values Reported in the Literature Using Ultrasonic-assisted Extraction and

Determination by ETAAS with Zeeman-effect Background Correction System

Matrix LOD LOQ Characteristic Instrument/Atomizer/ Atomization/(ng/g) (ng/g) Mass m0 (pg) Tubes/Background Modifiers (Ref.)

Correction

Biological SRMs 3 10 1.1 4100 ZL, Zeeman Conventional (1), Standard THGA Std. THGA tube with

W-Rh coating

Soil SRM 40 134 1.1 AAnalyst™ 800 Conventional (9)End-capped THGA NH4H2PO4+Mg(NO3)2

Zeeman

Food and 0.1 0.33 0.4 4100 ZL, Present methodBiological SRMs End-capped THGA Fast furnace program

Zeeman No modifier

TABLE IVResults Obtained by Ultrasonic-assisted Extraction of Cd in Different SRMs Using End-capped THGA Tube

Without Modifier and Fast Furnace Program(Values by standard addition and normal calibration are also compared. All values are in ng/g.)

SRM/Samples Ultrasonic-assisted Extactiona CertifiedX ± ts/√N Values

Standard Normal Addition Calibration

Non-fat Milk Powder, NIST SRM 1549 0.6±0.1 0.5±0.1 0.5±0.2Total Diet, NIST SRM 1548 30±2 27±3 28±4Rice Flour, NBS SRM 1568a 21±2 22±3 22±2

Algae, IAEA-391 7±1 6±1 6.6b

Algae, IAEA-392 18±1 19±1 17.9b

Sea Plant Homogenate, IAEA 140/TM 530±30 535±30 537±37

Tobacco Leaves, CTA-OTL-1 1100±50 1100±40 1120±120Tobacco L,eaves CTA-VTL-2 1500±50 1530±60 1520±170Bovine Muscle Powder, NBS RM 8414 10±2 9±2 13±11Mussel Tissue NIST SRM 2976 800±30 810±35 820±160Sample Skim Milk Powder 540±20 550±30 560±30c

Spinach-1 230±20 240±30 220±20c

Spinach-2 280±15 280±20 270±30c

Tuna Fish 10±1 10±1 <20c

aAverage value ± confidence interval ( P=0.05; N=6, t=2.571). b Values are only indicative.cSlurry sampling method was used for quantification (20).

73



Determination of Copper in Wine by ETAAS Using Conventional and Fast Thermal Programs:

Validation of Analytical Method*Sofia Catarino, Inês Pimentel, and A.S. Curvelo-Garcia

Estação Vitivinícola Nacional, Instituto Nacional de Investigação Agrária e das Pescas, 2565-191 Dois Portos, Portugal

INTRODUCTION

The content of metals at differ-ent stages of the winemakingprocess is of great concern becauseof legal and wine quality require-ments (1,2).

From an ecological point ofview, copper is a trace element andits determination is importantbecause of its toxicity, which canalso lead to wine spoilage throughhaze formation (2–5). In addition,Cu2+ presents inhibitory effects onmalolactic fermentation, enhancedby higher concentrations of alcoholand other factors such as low pHand higher concentrations of SO2

(6).

Part of the Cu found in wine isof endogenous origin due to thenature of the grapes themselves.The major sources of Cu contami-nation are pesticides, namely cop-per sulphate applied to preventmildew (nowadays, this practice isno longer in use), blending, andcontact with copper, tin, or bronzematerials during winemaking andstorage processes (2,3,7).

For musts, a range of acceptableCu levels (1–7 mg L–1) has been ref-erenced (8). During the fermenta-tion process, most of the Cu iseliminated since it precipitatesmainly as copper sulphide. Somepublished data indicate Cu wineconcentrations of 4 mg L–1, butmost of the wines show concentra-tions below 0.2 mg L–1 (1,3,8–12).In reduction conditions, wine con-taining more than 0.2–0.4 mg L–1

may suffer from copper cloudiness,

ABSTRACT

A study involvingconventional and fast thermalprograms, with and withoutmatrix modifiers, for the determi-nation of Cu in wine by ETAAS ispresented. With the conventionalthermal program, better resultswere obtained when usingmatrix modifiers [PdNO3 andMg(NO3)2]. This program pre-sents better sensitivity and conse-quently it was the programselected for method validation.

The fast thermal programallows a higher throughput andcan be a useful alternative to theconventional program. Themethod involves reduced risk ofsample contamination eliminat-ing prior treatment other thandilution (1:5).

White and red wines withseveral Cu concentrations wereused for method validation. Thedetermination was performed inthe linear range of 1–50 µg L–1;the detection limit in undilutedsamples was 5 µg L–1; the recov-eries were between 93% and100%; the repeatability (n=10),expressed as %RSD, was lowerthan 3%. Comparable values ofconcentrations were obtained byETAAS and ICP-MS (differences<15%). The advantages of themethod (practicability, sensitiv-ity, precision, and accuracy)make it useful for routine deter-mination of this metal in wine,when compared to flame AAS,which inevitably requires eithersample pre-concentration or theuse of the standard additionsmethod.

a phenomenon well known whichaffects stability and commercialacceptability.

Nowadays, the Office Interna-tional de la Vigne et du Vin (OIV),Paris, France, prescribes amaximum limit of 1 mg L–1 for Cu(13).

The most frequently usedmethod for Cu determination inwine is flame atomic absorptionspectrometry (FAAS) (1,11,14,15).This method is employed by theOIV and is also the officialPortuguese and European Unionmethod for Cu determination(13,16). However, theseprocedures present problems dueto the complexity of the matrix inwine samples and the sensitivity ofFAAS, which is too low for the Cucontent in the majority of wines.The standard additions method,used in these situations, is not agood enough alternative since itpresents low precision for the low-est Cu concentrations (15). On theother hand, the alternative of sam-ple pre-concentration decreases thepracticability of the method.

The determination of low Cuconcentrations is feasible with elec-trothermal atomization atomicspectrometry (ETAAS) (5,17),potentiometric stripping analysis(18), polarography (19), inductivelycoupled plasma optical emissionspectrometry (ICP-OES) (20,21),and inductively coupled plasmamass spectrometry (ICP-MS)(12,22,23).

The conventional thermal pro-gram of the ETAAS technique usu-ally includes the following steps:drying, pyrolysis, atomization, andcleanout. The pyrolysis step is used

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to remove as many matrix compo-nents as possible. A matrix modifiercan be used to stabilize the analyteor aid in removing matrix compo-nents (24,25,26).

The most time-consuming stepsof thermal programs are drying andpyrolysis. Analysts have long soughtsome means to reduce or eliminatethe time required for these pre-treatment stages. The combinationof two techniques, Zeeman-effectbackground correction and Stabi-lized Thermal Platform Furnace(STPF), was the key to providingfaster analyses.

In fast furnace analysis, thepyrolysis step and matrix modifica-tion are usually eliminated. Usingthese procedures, considerabletime saving is achieved without sac-rificing analytical precision or accu-racy. This technique may not becompatible with all instrumentation(the instrumentation used mustprovide Zeeman background cor-rection) and with all sample types.Very complex matrices may stillrequire at least a short pyrolysisstep and the use of a matrix modi-fier for optimum results (25,27).

The main aim of the presentwork was to compare conventionaland fast thermal programs, withand without matrix modifiers, inorder to improve the analytical per-formance of Cu determination inwine, in terms of higher through-put and lower cost, without sacri-ficing analytical sensitivity,precision, or accuracy. In addition,a method using the selected ther-mal program was validated.

EXPERIMENTAL

Instrumentation and AnalyticalConditions

The ETAAS instrument used wasa PerkinElmer Model 4110 ZLgraphite furnace atomic absorptionspectrometer (PerkinElmer Life andAnalytical Sciences, Shelton, CT,USA), using Zeeman-effect

background correction with aModel AS-72 autosampler, and thePerkinElmer AAWinLab™ software,version 2.5. Argon N50 (purity>99,999%) was used to protect andpurge the graphite tubes with aninternal flow rate of 250 mL min–1.A PerkinElmer Lumina™ hollowcathode lamp was used. The spec-trometer settings for pyrolyticallycoated graphite tubes with endcaps and L’vov platforms are givenin Table I. The measurement modewas integrated absorbance. Theautosampler was programmed topipette 20 µL of the sample (Custandard solution or wine) and 5 µLof each matrix modifier onto theplatform.

ICP-MS measurements were car-ried out with an ELAN® 9000 ICP-MS (PerkinElmer SCIEX, Concord,Ontario, Canada), equipped with across-flow nebulizer, a Scott-typespray chamber made of Ryton®material, nickel cones, and a peri-staltic sample delivery pump. APerkinElmer autosampler AS-93Plus,protected by a laminar-flow-laminarclean room class 100 (Reinraum-

technik Max Petek, Germany) wasused. The water was deionizedwith a Seralpur Pro 90 CN purifier(Seral, Ransbach-Baumbach, Ger-many).

Materials

To eliminate possible contamina-tion, all glassware and polyethylenematerial (volumetric flasks,micropipette tips, and autosamplercups) were immersed for 24 hoursin freshly prepared 20% (v/v) HNO3

and then rinsed thoroughly withdeionized water before use. Anexhaustive cleaning of theglassware and the use of plasticmaterials are very important. Forthe ICP-MS determinations, onlypolyethylene material was used.

Reagents and Calibration

Standard solutions (10, 20, 30,40, and 50 µg L–1) were prepareddaily from a 1000 mg L–1 solution(CertiPUR, Merck) of Cu in HNO3

(0.2% v/v) and C2H6O (2% v/v) withdeionized water (conductivity <0.1µS cm–1). Palladium nitrate andmagnesium nitrate, both from

TABLE IFurnace Thermal Program

Step Temp. Ramp Time Hold Time (ºC) (s) (s)

Dry 1 110 1 30Dry 2 140 10 25Pyrolysisa 1100 10 30Atomizationb 2300 0 5Clean-out 2450 2 5

Wavelength (λ) 324.8 nmSlit width 0.7 nmHollow cathode lamp current 15 mABackground correction Zeeman-effectIntegration time 5 sInjection volume 20 µL

Total time 118 s (conventional thermal program); 78 s (fast thermal program)

a Step eliminated at fast thermal program.b Stop argon flow.

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Merck, were tested as chemicalmodifiers in the standard solutionand wine samples [5 µg PdNO3 and3 µg Mg(NO3)2]. The wine sampleswere diluted (1:5) with HNO3

(0.2% v/v). Immediately afterpreparation, the standard and sam-ple solutions were transferred topolyethylene material. Standardsand samples were analyzed in tripli-cate.

For the ICP-MS determinations, amulti-element standard solution of10 µg L–1 (PerkinElmer) was usedfor external calibration; Rh and Rewere used as the internal standardsin 10 µg L–1 concentrations (Merck)and ultrapure HNO3 (J.T. Baker).


Thermal Programs

Different studies involving con-ventional and fast thermalprograms with and without matrixmodifiers were developed. Theoptimized thermal program wasdirected not only at obtaining amore sensitive and reproduciblemethod, but also for optimizing therespective costs and analytical timein order to allow its routine use.This work was carried out with asolution of Cu at a concentration of

about 20 µg L–1 in parallel withdiluted samples of a white and ared wine.

The dry step was established bythe visual control of the samplebehavior during the heating stageand by the consistency of instru-mental output. A two-stage proce-dure for the dry step provedconvenient. During the first stage(110ºC), the alcohol volatilizeswithout spattering and the dryingof the sample is completed duringthe second stage (140ºC).

For the pyrolysis and atomiza-tion steps, the variation of the inte-grated absorbance withtemperature was studied for thedetermination of Cu. The influenceof different temperatures, ramp,and hold times were also assessed.

The results using the conven-tional program with and withoutmatrix modifiers are shown in Fig-ures 1A and 1B, respectively. Thepyrolysis curves (atomization tem-perature 2000ºC) show that thecombination of PdNO3 andMg(NO3)2 matrix modifiers wasable to thermally stabilize Cu up to1100ºC. Without matrix modifier,Cu losses by volatilization began at1100ºC. Although high absorbance

signals were obtained at 2000ºC,with and without matrix modifiers,it was observed that some winespresented unsatisfactory absorp-tion profiles: the analyte peakshows a "tail" (no return to base-line). The increment of the temper-ature did produce appreciableeffects; however, at 2300ºC theabsorption profiles were accept-able. The use of matrix modifiersled to a higher absorbance signal(about 20% higher), except for thestandard solution.

An additional study was devel-oped varying the amount of bothmatrix modifiers deposited in thegraphite tube. Better results wereobtained with the initialconditions: 5 µg PdNO3 and 3 µgMg(NO3)2.

The results using the fast pro-gram with and without matrixmodifiers are shown in Figures 2Aand 2B, respectively. For bothmodalities, the optimum atomiza-tion temperature is 2100ºC. Thewhite wine shows a particularbehavior: when using matrix modi-fiers, the atomization temperaturecould be 2000ºC. However, at2300ºC the absorption profileswere preferable. The use of matrix

Fig. 1A. Conventional thermal program: pyrolysis and atom-ization curves in white and red wines (diluted 1:5) and Custandard solution (20 µg L–1), with matrix modifiers [5 µgPdNO3 and 3 µg Mg(NO3)2 ].

Fig. 1B. Conventional thermal program: pyrolysis and atom-ization curves in white and red wines (diluted 1:5) and Custandard solution (20 µg L–1), without matrix modifiers.

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modifiers did not lead to a signifi-cant variation in the absorbancesignal.

The overall optimized furnaceprograms are summarized in Table I.

In conclusion, fast and conven-tional programs (with or withoutmatrix modifiers) can be used fordetermining Cu in wines. However,the conventional program withmatrix modifiers allowed a highersensitivity and consequently it wasthe program selected for methodvalidation.

Validation

Algorithm Calibration Curve,Selectivity, and Accuracy

Calibration against acidified stan-dard solutions was carried out. Twowhite wines and two red wineswere spiked with four concentra-tion levels of Cu, never exceedingthe total concentration of 50 µg L–1.The linearity (y=a+bx) of theresponse curves was investigated.

The results presented in Table IIconfirm that the ∆b/b coefficientsare lower than 5%, meaning thatthe curves are satisfactory. The vari-ance homogeneity was investigatedand confirmed by the Cochran test.

For each curve, a high percentageof the total variance is explained bythe regression, which has a calcu-lated F (F1) higher than the Fisher’sF at 99.5%. The residual variancedue to the adjustment error is sig-nificant in all cases (F2), except forRed Wine 2. Although the residualvariability is very low, since thepure error and adjustment error arenot of the same magnitude, the testis significant. This may result fromthe small variability within eachgroup because the replicates arenot independent.

The slope of the curves plottedby wine standard additions (wineCu concentration as a function ofCu addition) are not different of 1,except for Red Wine 1, indicatingthat there are no matrix effects (Fig-ure 3). Thus, the method presents avery satisfactory selectivity.

Accuracy was determined by thetechnique of standard additions todetermine recovery of the spikedanalyte. This approach was usedbecause we could not find refer-ence materials for this parameter.Spiking wine samples with aqueousCu gave recoveries between 93%and 100%: 94±7%; 97±6%; 93±3%;100±2%, for White Wine 1, White

Wine 2, Red Wine 1, and Red Wine 2,respectively. The accuracy of themethod is satisfactory and in agree-ment with the required quality ofthe analytical results.

One white wine and one redwine were also analyzed by ICP-MS(semi-quantitative method). Theanalytical results obtained using thesemi-quantitative mode of the ICP-MS technique presented an accu-racy of ±20%. Thus, the resultsobtained by the two techniques arecomparable (differences lower than15%).

Analytical Limits

The detection limit was calcu-lated as the mean concentration,plus three standard deviations ofthe blank, obtained with 20 deter-minations. The quantification limitwas calculated as the mean concen-tration, plus 10 standard deviationsof the blank. The experimental con-ditions used enabled to detect 1 µgL–1 and to quantify 3 µg L–1. Thequantification limit permitted thedetermination of Cu in undilutedwines with 15 µg L–1. In fact, theCu concentration of wines is gener-ally higher than 15 µg L–1.

Fig. 2A. Fast thermal program: pyrolysis and atomizationcurves in white and red wines (diluted 1:5) and Custandard solution (20 µg L–1), with matrix modifiers [5 µgPdNO3 and 3 µg Mg(NO3)2 ].

Fig. 2B. Fast thermal program: pyrolysis and atomizationcurves in white and red wines (diluted 1:5) and Custandard solution (20 µg L–1), without matrix modifiers.

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Precision

Replicates of four white winesand four red wines, at several con-centration levels, gave relative stan-dard deviations between 1% and 3%(n = 10), indicating that the analyti-cal method is of high within-runprecision (repeatability). The rela-tive standard deviations ofbetween-run precision(reproducibility) are from 5% to 8%(N = 3). The higher values werefound in wines with lower concen-trations.

CONCLUSION

The method presented for thedirect analysis by ETAAS can beused to measure the copper con-tent in wines on a routine basis,especially in samples containingvery low concentrations of thismetal. The characteristics and fig-ures of merit of the methoddescribed (practicability, reducedrisk of sample contamination, sensi-tivity, selectivity, analytical limits,

Fig. 3. Curves obtained for white and red wines by standard additions method.

TABLE II Calibration and Wine Response Curves, Characteristic Parameters

(Variance analysis of the linear regressions.)

Calibration Curve White Wine 1 White Wine 2 Red Wine 1 Red Wine 2

y = a + bx y=(0.007±0.003)+ y=(16.7±0.8)+ y=(32.5 ± 0.5)+ y=(10.6±0.3)+ y=(27.9±0.3)+(0.0078±0.0001)x (1.02±0.03)x (10.2±0.04)x (0.95±0.01)x (1.02±0.02)x

r2 0.999 0.997 0.995 0.999 0.999

∆b/b 1.5% 3.2% 4.3% 1.5% 2.1%

Cochran (C) 0.44b 0.47b 0.31b 0.43b 0.53b

C(0.05; 6.2) = 0.62

C(0.05; 5.2) = 0.68

(F1) 18864.9a 4445.5a 2563.2a 21419.1a 10460.7a

F(0.005; 1.16) =10.6

F(0.005; 1.13) =11.4

(F2) 110.2a 46.9a 12.1a 21.9a 5.8b

F(0.005; 4.12) =6.52

F(0.005; 3.10) =8.08

a Significant difference. b Without significant difference.The calculated values should be lower than the theoretic values, except for F1.

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accuracy, and precision) make ituseful, in contrast to flame AASwhich inevitably requires samplepre-concentration or the use of themethod of standard additions. Bothfast and conventional thermal pro-grams (either with or withoutmatrix modifiers) can be used; thefast program allowed a higherthroughput; however, the conven-tional program with matrix modi-fiers allowed a higher sensitivityand was consequently validated.

ACKNOWLEDGMENT

The authors would like tothank the scientific collaborationof Professor Ana Mota, from theInstituto Superior Técnico, Lisbon,Portugal.

Received December 20, 2004.

REFERENCES

1. A.S. Curvelo-Garcia, Controlo deQualidade dos Vinhos, QuímicaEnológica, Métodos Analíticos, 420p, Instituto da Vinha e do Vinho,Lisboa, Portugal (1988).

2. A.S. Curvelo-Garcia and S. Catarino,Ciência e Técnica Vitivinícola 13(1-2): 49 (1998).

3. J. Ribéreau-Gayon, E. Peynaud, P.Sudraud, and P. Ribéreau-Gayon,Sciences et Techniques du Vin.Tome I – Analyse et contrôle desvins, 645 p., Dunod, Paris, France(1982).

4. M.T. Vidal, P. Poblet Montserrat, M.Constanti, and A. Bordons, Ameri-can Journal of Enology and Viticul-ture 52(3), 223 (2001).

5. M.T. Vasconcelos and M. Azenha,Feuillet Vert de l’OIV, 1135(2001).

6. M.T. Vidal, M. Constanti, and A.Bordons, Vignevini Bologna26(7–8), 50 (1999).

7. J-P. Quinche, Revue Suisse de Viticul-ture, Arboriculture, Horticulture,Nyon, 17, 341 (1985).

8. H. Lay and W. Lieb, Wein-Wissenschaft 43, 107 (1988).

9. C.S. Ough and M.A. Amerine, Meth-ods for Analysis of Musts andWines, 377 pp., John Wiley &Sons, New York (1988).

10. H. Lay, Deutsches Weinbau-Jahrbuch 44, 255 (1993).

11. P. Sudraud, B. Médina, and J.P.Grenon, Feuillet Vert de l’OIV, 984(1995).

12. I. Rodushkin, F. Odman, and P.K.Appelblad, Journal of Food Com-position and Analysis 12, 243(1999).

13. OIV, Recueil des Méthodes Interna-tionales d’Analyse des Vins, OfficeInternational de la Vigne et du Vin,Paris, France (1990).

14. M. Netzer and F. Bandion, FeuilletVert de l’OIV, 1024 (1996).

15. S. Catarino, D. Pinto, and A.S.Curvelo-Garcia, Ciência e TécnicaVitivinícola 18(2), 65 (2003).

16. CT83, Norma Portuguesa NP 2442,Instituto Português da Qualidade,Lisboa, Portugal (1988).

17. A.A. Almeida, M.I. Cardoso, andJ.F.C. Lima, At. Spectrosc. 15(2),73 (1994).

18. A.M. Green, A.C. Clark and G.R.Scollary, Fresenius’ Journal of Ana-lytical Chemistry, 358 (6): 711-717(1997).

19. J.B. Fournier, M. El Hourch, andG.J. Martin, Journal Internationaldes Sciences de la Vigne et du Vin32(1), 45 (1998).

20. G. Thiel and K. Danzer, Fresenius’Journal of Anal. Chem.357(5), 553(1997).

21. G. Nicoli, R. Larcher, P. Pangrazzi,and L. Bontempo, Vitis 43(1), 41(2004).

22. J.P. Greenough, H.P. Longerich,and S.E. Jackson, Australian Jour-nal of Grape and Wine Research, 3(2) (1997).

23. S. Catarino, J. Soares, A.S. Curvelo-Garcia, and R. Bruno de Sousa,Ciência e Técnica Vitivinícola19(1), 29 (2004).

24. PerkinElmer, The THGA GraphiteFurnace, Techniques and Recom-mended Conditions, PerkinElmer(1995).

25. R. Beaty and J.D. Kerber, Concepts,Instrumentation and Techniquesin Atomic Absorption Spectrome-try, PerkinElmer (1993).

26. G. Schlemmer and B. Radziuk, Ana-lytical Graphite Absorption Spec-trometry – A Laboratory Guide,Birkhauser Verlag, Basel, Switzer-land (1999).

27. S. Catarino, J.L.Capelo, A.S.Curvelo-Garcia, and M. Vaião,Journal of AOAC (in press).

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*Corresponding author.E-mail: : [email protected]

rium matrices by flame emissionand atomic absorption spectrome-try (AAS) using the flame and elec-trothermal atomization (ETA)modes. Though ETA-AAS is wellknown for its high atomization effi-ciency, the atom population in ETA-AAS is affected by various factors(5–8) in the presence of theserefractory matrices. To understandthe matrix effect, the reactionmechanism for analyte atom forma-tion has also been investigated inthe presence of these matrices.

EXPERIMENTAL

Instrumentation

A Model GBC-906 atomic absorp-tion spectrometer (GBC, Australia),equipped with flame and graphitefurnace modes of atomization andD2 lamp background correction,was used for studying the flameemission/absorption and ETA-AASmeasurement of rubidium. Thewavelength of the primary analyti-cal line of Rb used was 780.0 nm.Since D2 continuum has a sharp fallin intensity beyond 350.0 nm, theD2 background correction facilitywas not used. The GF-3000 powersupply provides the stepwise heat-ing program with a heating rate ashigh as 2000ºC/s for the analyteatomization in ETA-AAS. A single-element Rb hollow cathode lampwas used to obtain the absorbancesignal at the 780.0-nm analyticalline. An Eppendorff 5 µL fixed vol-ume pipette with a certified preci-sion value of 2–3% RSD was usedfor dispensing the sample/standardaliquots into the graphite furnace,while other Eppendorff variable-volume pipettes were used in thepreparation of these solutions.

ABSTRACT

A comprehensive study forthe determination of Rb in thepresence of uranium and thoriummatrices by flame emission, flameatomic absorption, and electro-thermal atomization atomicabsorption spectrometry (ETA-AAS) is described here. Low tol-erance of dissolved solidsrestricts the matrix concentrationup to 2 mg/mL in flametechniques whereas in spite oflarge suppressive effects in ETA-AAS, the matrix concentrationcould be tolerated up to 20mg/mL. The detection limitsobtained by ETA-AAS in uraniumand thorium matrices are bythree orders of magnitude betterthan those obtained by flametechniques.

These studies have beenextended to examine the atom-ization mechanism of Rb by ETA-AAS in uranium and thoriummatrices. The effect of the pres-ence of matrix and its accumula-tion in the furnace reveals thatthe absorbance is suppressed inthe presence of these matricesand can be correlated with therelease of oxygen during matrixdisproportionation. The reactionmechanism for rubidium, how-ever, remains the same in theaqueous and in the presence ofuranium matrix while it changesin thorium matrix, probably dueto the use of hydrofluoric acid inthe dissolution of the thoriummatrix.

cation of AAS techniques to com-plex matrices such as uranium andthorium.

In the present work, a compre-hensive approach is described forthe direct determination of Rb inthe presence of uranium and tho-

On the Atomization of Rb by ElectrothermalAtomization, Flame Absorption, and Flame Emission

Spectrometry in Uranium and Thorium Matrices*Paru J. Purohit, Neelam Goyal, and A.G. Page

Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India

INTRODUCTION

Rubidium plays a significant rolein the production of vacuum cellsand photocells, and is also used todetermine the age of a mineral.Rubidium (Rb) is one of the fissionproducts formed in the nuclearreactor which plays an importantrole in fuel-clad chemical reactions(1). Thus, the determination oftrace concentrations of Rb in ura-nium and thorium fuel materials isalso of importance in the context ofthe nuclear fuel technology.

Inductively coupled plasmaatomic emission spectrometry (ICP-AES) is by far the most widely usedanalytical technique for the deter-mination of trace metal constituents.Even though it offers very high sen-sitivity and a large dynamic rangefor most of the elements, it has itsdrawbacks with regard to a pooranalytical response for elementssuch as Rb and Cs. In the presentcontext, it is all the more difficultto determine Rb in the presenceof refractory matrices such as ura-nium and thorium with ICP-AES.Literature reports are available forthe determination of Rb inrelatively simple matrices such aswell and mineral water by flameemission spectrometry (2), in clini-cal samples using both flame andelectrothermal atomic absorptionspectrometry (3), and riverinewater, estuarine water, and seawater by wavelength-modulationdiode laser absorption spectrome-try using graphite furnace (4).There are no reports on the appli-

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Preparation of Standards andSamples

Suprapur® HNO3, HCl, and HF(E. Merck, Darmstadt, Germany)were used in the preparation of allstandard solutions. Stock solutionsof Rb and other elemental solutionsat 2-mg/mL concentration and ura-nium matrix solutions at 200-mg/mL concentration wereprepared by dissolving appropriateamounts of high purity compoundsin HCl/HNO3 and finally convertingthe same to nitrate. The thoriummatrix solution (200 mg/mL) wasprepared by dissolving ThO2 inconcentrated HNO3 with smallamounts of HF by subsequentremoval of the untreated fluorideduring repetitive evaporation withHNO3. All solutions were finallybrought into O.1 M HNO3 usingdoubly distilled water obtainedwith a quartz apparatus.

Series of solutions, using a mid-range concentrations of Rb (2 µg/mLfor flame and 0.01 µg/mL for ETA-AAS) with amounts of uranium andthorium varying in the range of0–100 mg/mL, were prepared tostudy the matrix effects on the ana-lyte signal when aspirated into theflame or injected into the graphitefurnace. A number of solutionswere also prepared with varyingamounts of Li, Na, K, and Cs inorder to study their effects as ion-ization suppressor.

Working standards wereprepared with each containinggraded amounts of Rb and fixedamounts of 2 mg/mL of U and Thfor the flame emission/absorptionstudies and 20 mg/mL of thesematrices for the ETA-AAS studies.These standards contained 21 othermetallic elements of interest in thefuel specification (i.e, Ag, Al, B, Ca,Cd, Ce, Co, Cr, Cu, Dy, Er, Eu, Fe,Gd, Mg, Mn, Mo, Na, Ni, Sm, andZn) in the concentration range ofRb. In the absence of certified ref-erence materials for uranium andthorium, three synthetic samples at

different analyte concentrationswere prepared by adding appropri-ate amounts of uranium and tho-rium to the E. Merck standardsolution for their analysis. One sam-ple was prepared with 10 timeshigher concentrations of the othermetallic elements to study theireffect on analyte absorbance.

Experimental Procedure

Upon stabilization of the flame,mid-range concentration solutionsof rubidium (2 µg/mL) in theabsence and presence of uraniumand thorium matrices were fed intothe air-acetylene flame to measuretheir emission/absorption signals.The effect of uranium and thoriummatrices was studied by varyingtheir concentration in the 0–20mg/mL range. The flame parame-ters were optimized separately inthe aqueous, uranium, and thoriummatrices.

For the ETA-AAS studies, a 5-µLsample aliquot was placed into thegraphite furnace; the best possiblesignal was obtained after optimiz-ing the heating cycle. The effectsdue to the presence of U and Thmatrices, their varying concentra-tions in the range of 0–100 mg/mL(0–500 µg), and the matrix accumu-lation inside the graphite furnacedue to the repetitive use of thesame atomizer for a number ofatomization cycles were investigated.Zero concentration signifies amatrix-free solution. The reactionmechanism in the graphite furnacewas studied by calculating the acti-vation energies using the Arrheniusplots for the aqueous, uranium, andthorium matrices. The measuredappearance temperature and thecalculated activation energies werethen compared with the data avail-able in the literature to establishthe mode of atomization for thethree matrices.

The effects of the 21 elements(Ag, Al, B, Ca, Cd, Ce, Co, Cr, Cu,Dy, Er, Eu, Fe, Gd, Mg, Mn, Mo, Na,Ni, Sm, and Zn) at concomitant

concentration levels and of the lowionizing alkali elements (Li, Na, K,and Cs as matrix modifiers) werestudied in detail in order toimprove and ensure the interfer-ence-free signal by both the flameand ETA-AAS techniques.


Due to the low first ionizationpotential of Rb, followed by thehigh excitation energy required forexcitation of the Rb+ ion with anelectronic configuration of Kr-aninert structure, the argon ICP doesnot prove to be an effective spec-tral excitation source in AES. Itcannot, therefore, be applied fortrace level determination of Rb.The presence of U/Th matrix dueto its line-rich spectrum makes itdifficult to determine any analyteby ICP-AES in the presence of thesematrices. Neutron activation analy-sis (NAA), though highly sensitiveand preferred for trace metal analy-sis of nuclear fuel materials, is notsuitable since the fission productsgenerated due to irradiation ofU/Th would interfere with analytedeterminations. Atomic absorptionspectrometry known for its reason-ably good sensitivity is thereforeexamined in the analysis of thesematerials for the determination ofRb. The low excitation energyrequired for the determination ofRb makes it prudent to examineflame emission spectrometry aswell. This method has thereforebeen studied for the direct determi-nation of Rb in uranium and tho-rium matrices using air-acetyleneflame for analyte excitation.

Flame Emission

The flow rates for oxidant-fuelcombination and burner heightwere optimized to obtain the bestpossible emission signal for Rb. Theoptimum flow rate for air and C2H2

was 10.3 and 1.5 L/min, respectively.The analytical range obtained byflame emission spectrometry was0.2–9.0 µg/mL.

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Effect of Other Alkali Elements

Due to the low ionization poten-tial of Rb, it gets ionized even atflame temperatures which leads topoor sensitivity. With the view toimproving its analyticalperformance, the effect of otheralkali metals (Li, Na, K, and Cs) asionization suppressor was studied.The schematic representation ofthe effect observed due to thepresence of these alkali metals at10-µg/mL and 100-µg/mL concen-trations on both 2 µg/mL and 4µg/mL of Rb concentrations isshown in Figure 1. As can be seen,the addition of K and Cssignificantly improved the Rb sig-nal, whereas a marginal increase inits signal intensity was observed inthe presence of Li and Na.

It is interesting to note from Fig-ure1 that the increase in emissionsignal in the presence of K is muchhigher than that obtained with Cs.

This observation can be related tothe ionization potentials (I.P.) andatomic mass of the alkali elementsused. The ionization potentialdecreases from Li to Cs at the sametime as the atomic mass increases.The I.P. for Cs is relatively lowcompared to that of Rb; however,due to its high atomic mass, theeffective atom density for Cs is less.Potassium, on the other hand, hasno appreciable change in ionizationpotential, but its sufficiently lowatomic mass provides nearly doublethe atom density. The effectiveatom density for K is thus morethan that of Cs. Hence, the flameenergy used up in the ionization ofpotassium is much more than Cs,thereby preventing the ionizationof Rb. These points can be wellunderstood with the help of Table Iwhich shows the ionization poten-tial, atomic mass, number ofatoms/gram calculated using theAvagadro’s number, and the

amounts used for the alkalielements each producing theatom density equivalent to that ofRb (2 µg/mL) providing 140x104

atoms/mL. As can be seen fromTable I, the concentration of K andCs is equivalent to 2 µg/mL of Rb,resulting in an atom density of 0.91and 3.1 µg/mL, respectively.

The effect of Cs and K studiedwith the amounts producing a 10to 100 times higher atom densitycompared to the Rb atom densityis shown in Figure 2. It can be seenthat the observed emission inten-sity is higher with Cs than thatobtained in the presence of K.Even though the atom density forboth Cs and K is the same in thesolution, Cs ionizes more easily.Hence, the flame temperature iscontrolled in the presence of Cs,preventing the ionization of Rb andproducing higher signal intensity.The increase in the emission signal

Fig. 1. Influence of 10 and 100 µg/mL of Li, Na, K, and Cs on (a) 2 µg/mL of Rb and (b) 4 µg/mL of Rb

82

with a 100 times higher atom den-sity is relatively low as comparedto the 10 times higher atom den-sity. This is due to the excessivelowering of the flame temperaturein the former case. Thus, an opti-mum analytical signal for Rb can beobtained with a Cs atom density10 times higher than for Rb.

Flame Absorption

As an independent check on theresults obtained by the flame emis-sion technique, the experimentalparameters were freshly optimizedfor flame atomic absorption spec-trometry using a Rb hollow cathodelamp and measuring the absorbancein the air-acetylene flame. The opti-mized flow rates for oxidant andfuel were 10.7 and 1.92 L/min,respectively, and the analyticalrange obtained was 0.2–12.0 µg/mL.

Effect of Uranium and ThoriumMatrix

The effect due to the presenceof these matrices was studied usingboth flame emission and flameabsorption by aspirating a fixedconcentration of 2 µg/mL Rb withvarying concentrations of uraniumand thorium ranging from 0–20mg/mL, i.e., 0, 1, 2, 5, 10, and 20mg/mL. The analyte signalmeasured reveals that the signalintensity for Rb gets suppressedwith increasing matrix concentra-tions. Use of high matrix concentra-

tion leads to high memory effects,making it difficult to obtain a zerobase line, and resulting in cloggingof the nebulizer and burner. Thisnecessitates the aspiration of a con-centrated acid to clean the burnercausing reduced memory effect. Toavoid this effect, the matrix concen-tration was restricted to 2 mg/mL.The analytical range obtained inuranium and thorium matriceswere 2–40 µg/mL. The slope of theanalytical curves obtained for Rb inthe presence of 2 mg/mL of U andTh was significantly reduced com-pared to the matrix-free solutiondue to the reduced sensitivity inthe presence of the matrix. Thedetermination of Rb in thepresence of uranium and thoriummatrices using the flame techniqueshas therefore limited application inthe nuclear industry; however,these techniques are the mostimportant approach in other areasof application.

ETA-AAS Technique

Relative detection limitsobtained by flame AAS are signifi-cantly poor due to its low tolerancefor uranium and thorium matrices.This has led to the development ofthe ETA-AAS method for the directdetermination of Rb in thepresence of these matrices. Despitethe poor reproducibility and inter-ference effects due to analyte-car-bon and matrix-carbon interactions

in ETA-AAS, its high sensitivityallows the method to be used fortrace metal determinations. Theexperimental parameters wereoptimized using a mid-range con-centration solution for the aqueous,uranium, and thorium matrices.Five µL sample volumes wereinjected, dried, then ashed, andfinally atomized using a heatingrate of 2000ºC/sec. The ash/pre-atomization stage is introduced forthe matrix decomposition and ahigher atomization temperature isrequired to atomize Rb from thesematrices. The optimized tempera-tures for the dry, ash, and atomiza-tion stages are given in Table II.

TABLE IIonization Potentials, Atomic Mass, and Number of Atoms

Calculated for Alkali Elements

Element First I.P. Atomic Mass No. of Atoms/g Amounta

(e.V.) (g) (µg)

Li 5.39 6.94 867.87 x1020 0.162

Na 5.14 22.99 261.98 x1020 0.535

K 4.34 39.1 154.02 x1020 0.91

Rb 4.18 85.47 70.45 x1020 2.0

Cs 3.89 132.91 45.31 x1020 3.1

a Amounts equivalent to 2 µg/mL of Rb.

Fig. 2. Influence of the presence of Kand Cs on Rb at 10 and 100 timeshigher atom densities.

83


Effects Due to the Presence ofMatrix

Matrix-dependent studies for theanalyte absorbance at the 0.05-ngconcentration in both the uraniumand thorium matrices have showna reduction in the absorbance sig-nal. The effect was studied by vary-ing the concentration of thesematrices separately in the 0–100mg/mL range, i.e., maximum of500 µg per aliquot of 5 µL solution.It was observed that when matrixconcentration is increased up to 20mg/mL, the analyte absorbance isreduced to ~50% as compared toits value obtained in a matrix-freesolution. Moreover, at concentra-tions higher than 20 mg/mL,smooth evaporation of the samplealiquot does not take place andleads to irreproducible observations.The matrix concentration wastherefore optimized at 100 µg peraliquot for further studies.

These matrices, when heated inthe graphite furnace, are eventuallyconverted to ThO and UO2 at thetemperatures of 1673 K and 1500 K,respectively, with the release ofoxygen (9,10). The matrix amountused and associated oxygenreleased in the presence of matrixis much higher, at least by threeorders of magnitude as comparedto the matrix-free solution sincethe analyte is only in nanogram tosub-nanogram concentrations. Asdiscussed by L’vov (11), anincrease in partial pressure ofoxygen at or below the signalappearance temperature (Tapp),

changes the reaction equilibrium.The appearance temperature forthe present studies was 1758 K and1861 K for the uranium and tho-rium matrices, respectively, whichis higher than the temperaturesat which oxygen is released.This leads to a reduction insignal intensity.

Effects Due to Matrix Accumula-tion Inside the Furnace

The effect of accumulatedmatrix inside the atomizer wasstudied since the same atomizerwas used for a number of atomiza-tion cycles. For the initial 30–35atomization cycles, the signal wassuppressed and reduction was ashigh as 50%. Thereafter, itproduced stable reproducible sig-nals up to ~150 atomization cycles.The reduction in the absorbancesignal with consecutive loading ofuranium is due to the formation ofa uranium carbide layer on theinner surface of the graphite fur-nace. Initially, the oxygen liberateddue to the formation of UC/UC2

reacts with graphite and escapes asCO/CO2. As a result, a large atomdensity is available for the AA sig-nal. On successive loading, a uni-form layer of carbide is formed atthe loading surface and the avail-ability of active carbon sitesreduces. Thus, most of the oxygenreleased on decomposition of thematrix remains in the furnace andslows down the atomizationprocess. The significantly largenumber of atomization cyclesrequired to stabilize the analyte sig-

nal in the presence of the uraniummatrix can be understood by thefollowing reaction:

570K 1500K 1973K

Uranyl nitrate → UO3 → UO2 → UC2/UC

Sample aliquot

As seen from the reaction, withsubsequent atomization, uraniumcarbide partially dissolves (12) inthe dilute HNO3-based samplealiquot loaded. An earlier reporton uranium buildup effects ongraphite filament atomizer (13) hasrevealed that the uranium carbideformed inside the atomizer istotally hydrolyzed in the presenceof nitric acid with a release ofhydrogen, ethylene, ethane, anduranyl salt, with no evidence ofUC/UC2 types of carbides. In thepresent studies (see Figure 3),there is clear evidence of the pres-ence of UC and UC2 obtained withthe STOE X-ray powder diffractionsystem for the residue collectedafter the atomization cycle. How-ever, the formation and partial dis-solution of the carbide layer areboth simultaneous reactions, thusrequiring a significant number ofatomization cycles to form a stableuniform layer of carbide. After30–35 atomization cycles, an equi-librium results and the signal stabi-lizes for a given concentration.

The formation of U and Th car-bides and their stability at the tem-peratures used in the graphite tubeatomizer was also confirmed by theX-ray diffraction studies carried outindependently by us on the residuecollected from the graphite tubeover a large number of atomizationcycles. Furthermore, the stability ofuranium and thorium carbides (14)is the same for the temperaturesused in the present studies.

In the presence of Th, the signalstabilizes within 4–5 atomizationcycles. In the initial few atomizationcycles, a carbide layer is formed inthe furnace where the sample is incontact with the atomizer surface.

TABLE IIHeating Program for Graphite Furnace Atomizer

Temperature Settings for Graphite Furnace

Dry Ash AtomizeTemp./Time Temp./Time Temp./Time

(K/sec) (K/sec) (K/sec)

Aqueous 573/10 – 2273/0.2

Uranium 573/10 873/25 2573/0.2

Thorium 573/10 873/25 2673/0.2

84

(evaluated from the analyticalcurve) are given in Table III. Inspite of the high suppressiveeffects, the lowest amount of Rbdetermined in uranium andthorium by ETA-AAS is better bya factor of three than when usingthe flame emission and absorptiontechniques; and the relative detec-tion limits in ppm calculated on amatrix basis are better by fourorders of magnitude in ETA-AAS.The relative detection limit in ppmis the analyte concentration in µg/gof matrix and will depend on thematrix concentration used peraliquot per atomization cycle. Inthe present studies, each 5-µL sam-ple aliquot atomized in the furnacecontained 100 µg of matrix, but2 mg/mL of matrix for the flametechnique.

The detection limits obtained forRb in the literature by wavelengthmodulation diode laser absorptionspectrometry in a graphite furnace(WLMDLA-GF) (4), laser-enhancedionization (LEI) (15) spectrometrywithout pre-concentration, and byflame AAS (16) in aqueous solution

ThC2 thus formed is highly stableand is not affected by the loadingand atomizing of the subsequentsample aliquot; thus, the stabilizedtube conditions are achieved faster.

Effects of Other MetallicElements

The presence of Cs was foundto enhance the Rb emission signalto a great extent. With a view toexamining such an effect in ETA-AAS, the absorbance signals weremeasured with Cs at 2, 5, 10, and100 times higher atom density thanthat for Rb at 0.05 ng. The presenceof Cs as high as 100 times higherconcentrations did not affect therubidium absorbance. Controlledheating with optimized tempera-tures in the graphite furnace atom-izer is probably responsible for Csnot showing any beneficial effectson the Rb determination by ETA-AAS.

The effects due to the presenceof the 21 common metallicelements on the Rb signal and gen-erally required to be determined in

nuclear fuel (Ag, Al, B, Ca, Cd, Ce,Co, Cr, Cu, Dy, Er, Eu, Fe, Gd, Mg,Mn, Mo, Na, Ni, Sm, and Zn) werestudied using flame and ETA-AAS.The presence of these elements atconcomitant levels as well as at 10times higher concentrations didnot show any significant effects onthe absorbance or emission signalintensity.

ANALYTICAL RESULTS

In light of these studies, an ana-lytical procedure was developedfor the direct determination of Rbin uranium and thorium matrices.It was observed that the matrixamount that could be tolerated inETA-AAS is 20 mg/mL (10 timeshigher than for the flametechniques) resulting in better lim-its of detection. The relative perfor-mance of the analytical techniquessuch as linear analytical range, low-est amount determined, sensitivity(evaluated from the slope of theanalytical curves and the character-istic concentration), the concentra-tion producing 0.0044 absorbance

Fig. 3. STOE powder diffraction results for residue collected for uranium after the atomization cycles.

85


are shown in the Table IV. Thedetection limits obtained byWLMDLA-GF and LEI are distinctlybetter than even those obtained byETA-AAS in the present studies, butthese methods are difficult to applyfor the matrices studied here.Instrumental neutron activationanalysis (INAA) and X-ray fluores-cence (XRF) have also been appliedfor the determination of Rb in geo-logical materials with detection lim-its of 5 ppm and 20 ppm,respectively (17).

Using the standardized proce-dure, three synthetic samples wereanalyzed in aqueous solution and inthe presence of 20 mg/mL (100-µg/aliquot) of each of the uranium andthorium matrices. The estimatedvalues for these samples are inclose agreement with the addedamounts and are shown in Table V.The precision in the determinationof Rb at different concentrations,calculated from 10 repetitive peakabsorbance measurements, wasfound to be 5% in matrix-free solu-tions; whereas in the presence ofmatrix it was better than 10% RSD.The amount of sample estimated inthe presence of 10 times higherconcentrations of the above-men-tioned elements also indicates closeagreement with the expectedamounts. Hence, the presence ofthese elements does not affect theRb determination.

Atomization From GF Atomizer

The presence of uranium andthorium significantly suppressesthe Rb absorbance. Studies werecarried out to understand the reac-tion mechanism involved in theatomization of Rb in the presenceof these matrices. Absorbance wasmeasured as a function of tempera-ture in aqueous, uranium, and tho-rium matrices. Signal appearancetemperatures (Tapp) were measuredand activation energies (Ea) werecalculated for Rb using the Arrhe-nius plot; the results are shown inTable VI.

TABLE IIIComparison of Relative Performance of Three Techniques

Flame Emission Flame Absorption ETA-AASSensitivity Signal / Abs./ Abs./

(µg/mL) (µg/mL) ngAqueous 0.119 0.076 4.18Uranium 0.018 0.034 1.46Thorium 0.012 0.029 1.12

Characteristic Concentrationa (µg/mL) (µg/mL) (pg)

Aqueous 0.038 0.058 1.05Uranium 0.238 0.129 2.95Thorium 0.347 0.153 3.60

Analytical Range (µg/mL) (µg/mL) (ng)Aqueous 0.2 – 9.0 0.2 – 12 0.005 – 0.25Uranium 2.0 – 40 2.0- 40 0.01 – 0.35

Thorium 2.0- 40 2.0- 40 0.01 – 0.35

Relative DetectionLimitb (ppm) (ppm) (ppm)

Aqueous 0.2 0.2 0.001 Uranium 1000 1000 0.1Thorium 1000 1000 0.1

Absolute Detection Limit (g) (g) (g)

Aqueous 2.0 x10–7 2.0 x10–7 5.0 x10–12

Uranium 2.0 x10–6 2.0 x10–6 1.0 x10–11

Thorium 2.0 x10–6 2.0 x10–6 1.0 x10–11

a Concentration producing 0.0044 absorbance. b ppm values calculated on the basis of matrix amount used.

TABLE IVDetection Limits for Rb Reported in Aqueous Solutions

Reference Method Adopted Detection Limits(g /mL)

4 Wavelength Modulation Diode Laser Absorption Spectrometry in Graphite Furnace (WLMDLA-GF) 1x10–15

15 Laser-Enhanced Ionization Spectrometry (LEI) 2x10–12

16 Flame AAS 2.5x10–5

Present Work Flame Emission 2x10–7

Present Work Flame Absorption 2x10–7

Present Work ETA-AAS 1x10–9

(Absolute Amount5x10–12 g)

86

Ea value obtained in the presentstudies (gas stop mode) comparesreasonably well with the reported(11) value for the reaction:RbOH(l) → Rb(g) + OH.

The Ea value calculated in thepresence of uranium remains nearlythe same as for the aqueous solu-tion. Hence, the mode of atomiza-tion responsible for the formationof free Rb atoms is not affected inthe presence of uranium matrix;however, the appearance tempera-ture is increased by 200 K.

The activation energy calculatedfor the atomization of Rb inthorium is 99 Kcal/mole, which issignificantly higher compared tothat obtained in aqueous and ura-nium matrices. The Ea valueobtained for the thorium matrix iscloser to the bond dissociationenergy of RbF(g), which is reported(10) to be 111 Kcal/mole. Use ofHF in the dissolution of the Thmatrix probably leads to the forma-tion of RbF with its melting pointof 1050 K and boiling point of 1680K, both being less than the signalappearance temperature for the Rbin Th matrix. Due to its low boilingtemperature, RbF must have rea-sonably high vapor pressure at sig-nal appearance temperatures.Atomization of Rb in the Th matrixcan therefore be correlated withthe dissociation of gas phase mole-cules of rubidium fluoride.

As seen from Table V, theappearance temperature measuredfor the analyte has increased by200–400 K in the presence of theuranium and thorium matrices ascompared to the aqueous medium.A shift in appearance temperaturetowards the higher value wasobserved (11) when the concentra-tion of oxygen in the sheath gaswas increased. In the present studies,the oxygen concentration could beincreased due to the partial decom-position of the matrix itself,thereby causing an upward shift inthe signal appearance temperature.

In the present studies, an activa-tion energy of 64 Kcal/mole wasobtained for Rb in the aqueousmedium. While correlating thisvalue with the thermodynamic dataavailable in the literature (10,11),it was observed that the Ea valueobtained corresponds to two differ-ent processes: the bond dissocia-tion energy of RbO(g) and thedissociation energy of RbOH(l)

which have nearly the same Ea

value (60 Kcal/mole). In view ofthe closeness of the Ea valuesreported, it was difficult to inferabout the mode of atomization. Todecide which one of the two mech-anisms is operating under the con-ditions of ETA-AAS measurements,physical parameters for the possi-ble intermediate compoundsformed were taken into considera-tion. The analyte in the form ofnitrate when loaded in the graphitefurnace is converted to Rb2O,which is the most commonly occur-ring oxide of Rb. Rb2O decomposesat 673 K. Hence, at Tapp it must be

in the form of metallic Rb. Theatomization energy (10) for Rb(s) is19 Kcal/mol; the theoretically pre-dicted (11) Ea value for the atomiza-tion of Rb2O as stated by L’vov is 50Kcal/mol, both of which are sub-stantially smaller than those foundexperimentally in the present stud-ies. Thus, the atomization of Rb bydissociation of the metal oxide can-not be the mode of atomization.

Hydroxides of alkali metals areeasily formed at low temperatureseven in the presence of smallamounts of water. It was alsoreported (11) that argon gas con-tains water at ~1Pa; thus, Rb2O pre-sent in the graphite furnace reactswith water and forms an hydrox-ide. Since the melting point of Rbhydroxide is 655 K, it is expectedthat at the time of atomization Rbshould be in the form of RbOH(l).The experimental Ea value reported(11) for Rb both in gas flow (Arflow 300 cm3/min) and also in gasstop mode are nearly the same. The

TABLE VResults for Synthetic Samples in

Aqueous, Uranium, and Thorium Matrices

Amounts Added/Determined Amounts Added/Determinedb

Aqueous Uranium Thorium

Sample Added Det. RSD Added Det. RSD Det. RSDNo. (ng) (ng) (%) (ng) (ng) (%) (ng) (%)

1 0.01 0.0095 0.8 0.01 0.0099 8 0.009 10

2 0.05 0.0555 4.3 0.05 0.048 3.7 0.06 5.5

3 0.2 0.2045 4.4 0.25 0.2365 5.5 0.27 3.9

4 0.05+0.5a 0.06 5.3 0.05+0.5a 0.055 7.0 0.049 8

a Concentration of concomitant elements.b Added amounts for U and Th, though studied separately, are the same.

TABLE VISignal Appearance Temperature and Activation Energies

in Three Matrices

Aqueous/(Ref.) Uranium Thorium

Appearance Obtained 1574 1758 1861Temperature ( K) Reported 1300 (11)/1580 (18) – –

Activation Energy Obtained 64± 5 59± 6 99±6( Kcal/mole) Reported 60 – –

87


CONCLUSION

A comprehensive analyticalapproach for the determination ofRb in uranium and thorium matri-ces has been described. The ETA-AAS technique, even though itsuffers from significant matrix inter-ferences in the presence ofuranium and thorium matrices, pro-vides a detection sensitivity by overfour orders of magnitude higher incomparison to the flame emissionand flame absorption techniques.Reduction of the absorbance signalin U/Th matrices and an increase inthe appearance temperature is cor-related to the release of oxygen dueto the partial dissociation of thematrices. The large number ofatomization cycles required to sta-bilize the signal in the uraniummatrix could be due to theuranium-carbon interaction and thepartial destruction of the uraniumcarbide layer in the subsequentatomization cycles. Atomizationstudies carried out have shown thatin spite of significant signalsuppression in the presence of theuranium matrix, the mode of atom-ization remains unaffected. The sig-nificantly high Ea value obtained inTh matrix is correlated to the disso-ciation of the RbF formed duringthe dissolution of ThO2 with hydro-fluoric acid.

Received November 23, 2003.Revision received January 31, 2005.

REFERENCES

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9. J.C Bailar, H.J Emeleus, RonaldNyholm, and A.F. Trotman-Dicken-son, Comprehensive InorganicChemistry; Pergamon Press Ltd.,Vol.5, pp. 221 (1973).

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There, that’s all the training you need.Walk up to the AAnalyst 200 and let the touch screen guide you through everythingfrom setup to analysis. It practically tells you what to do—and in your own lan-guage. All instrument controls are right there on the screen, available at your fin-gertips. Even troubleshooting and repairs are easier, with quick-change parts yousimply snap out and snap in. No service visit, no down time. As rugged and reli-able as ever, our newest AAnalyst is a better way to do AA. Experience it for your-self. Talk to a PerkinElmer inorganic analysis specialist today.

© 2005 PerkinElmer, Inc.

Atomic Absorption

Justtouch

and go.

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Documents

ICP-MS determination of REEs in tomato plants and related products: A new analytical tool to verify traceability