Journal of Radioanalytical and Nuclear Chemistry, Vol. 254, No. 3 (2002) 465–467

Transition metal analysis: A comparison between ICP-AESand deuteron activation

A. E. Pillay,1* J. R. Williams,1 M. O. El Mardi,2 S. M. Hassan,1 A. Al-Hamdi1

1 Department of Chemistry, College of Science, Sultan Qaboos University, P.O. Box 36, Al Khod 123, Sultanate of Oman2 Department of Agronomy, Horticulture, Entomology and Plant Pathology, College of Agriculture, Sultan Qaboos University, P.O. Box 34,

Al Khod 123, Sultanate of Oman

(Received January 2, 2002)

The analytical capabilities of ICP-AES (ICP) and deuteron activation (associated with prompt gamma-rays and delayed X-rays) for thedetermination of certain transition metals, were investigated. The sensitivities of the three methods are given, and the general applicability of eachtechnique is evaluated. In the case of ICP, numerical data were obtained from aqueous solutions containing digested plant material. Theseexperimental results were assessed against minimum detectable limits attained in relevant solid matrices that were subjected to prompt gamma-rayand delayed X-ray spectrometry using 5 MeV 2H+ ions. The analytically useful lines originated mainly from (d,p) and (d,n) reactions. Thepotential of the three techniques for routine analysis is discussed and detailed methodologies are presented.

Introduction solution preparation of fruit and leaf samples fortransition metal determination. Thirty-six date specimensand thirty-six corresponding leaf samples (Fard cultivar)were collected from different locations at the SultanQaboos University, near Muscat, in the Sultanate ofOman. Dry ashing procedure was adopted2 for preparingthe samples in solution. The reagents used for thispurpose originated from BDH Chemicals, UK. Date andleaf samples were initially dried for a few hours in anoven at 105 °C. Approximately 1 g of the dried specimenwas weighed in a porcelain crucible, transferred to amuffle furnace at 550 °C, and ashed overnight. Theashed sample was removed from the furnace and allowedto cool. It was moistened with a few drops of Milliporewater; immersed into 3 ml concentrated HCl andevaporated to dryness under low heat. The residue wasdissolved in 5 ml of 2M HNO3, filtered throughWhatman filter paper No. 41 into a 25 ml volumetricflask and diluted to volume with Millipore water. Ablank sample was prepared in an identical fashion, butwithout the plant specimen. As a precaution againstcontamination, all glassware were soaked in weakchromic acid solution prior to use.

The transition metals (especially Mn, Fe, Cu and Zn)are widely distributed and occur in almost every matrix,including plant specimens1 and archaeological artefacts.2

Various techniques, such as thermal neutron activation,PIXE and XRF have been applied to the measurement ofthese elements. Some of these methods suffer frominterferences, matrix effects and time-consuming samplepreparation.3,4 The present study evaluates the analyticalpotential of deuteron-induced prompt gamma-rays anddelayed X-rays for determining Mn, Fe, Cu and Zn andcompares these methods with ICP-AES (ICP). Thetechnique of ICP is useful for analysis of these elements.The problem is the digesting of the samples beforesubjecting them to analysis in a suitable medium. This isusually time-consuming, and in addition, there could beelemental losses via the wet chemical processes. Thispaper describes the analytical capabilities of the threemethods and presents experimental data fromappropriate samples in order to compare the potential ofeach method.

ExperimentalInstrumentation: A Perkin Elmer 3300 instrument

was used. It was computer controlled and equipped withradial and axial configurations of operation. Theinstrument was calibrated with Spectrosol atomicabsorption reference standards and appropriate softwarewas used against spectral interferences.5 The metalswere analyzed at the following wavelengths: Mn:257.610 nm; Fe: 238.204 nm; Cu: 206.200 nm; Zn:327.393 nm. The principal operating parameters of theinstrument were: argon gas flow: auxiliary, 1 l/min;nebulizer (cross-flow): 0.8 l/min; sample uptake: 60 s;configuration: radial.

ICP study

Sample preparation: The analysis of solutions byICP necessitated a sample preparation procedureinvolving digested material to account for factors such ascontamination and sample losses associated with the wetchemical process. The simple use of aqueous referencestandards would not provide adequately accurate resultsif these factors are not taken into account. For thisparticular aspect of the study, plant material wasanalyzed and, therefore, this part of the work involved

* E-mail: avin@squ.edu.om

0236–5731/2002/USD 17.00 Akadémiai Kiadó, Budapest© 2002 Akadémiai Kiadó, Budapest Kluwer Academic Publishers, Dordrecht

A. E. PILLAY et al.: TRANSITION METAL ANALYSIS

Table 1. Nuclear reactions leading to delayed X-ray emission

Target Reaction Product X-ray DetectionNuclide Abundance, % Nuclide Half-life limit, �g/g

57Fe 2.2 (d, n) 58mCo 8.94 h Co 1063Cu 69.17 (d, p) 64Cu 12.7 h Ni 165Cu 30.83 (d, p) 66Cu 5.1 m Cu 3068Zn 18.8 (d, p) 69mZn 13.8 h Zn 10

2H+ study This has the advantage of confirming that the overalldetection method is efficient, and proclaims towardsproblems such as memory effects in the ICPmeasurements. To achieve this, individual leafspecimens, originating from the same bulk sample, wereseparately processed and the recorded data produced arelative standard deviation for Fe of 5.3%, indicatingthat good recoveries were achieved.

The feasibility of applying this particular techniqueto the analysis of solid biological specimens wasexamined by evaluating minimum detection limits inpure solid matrices. To achieve this, the samples usedconsisted of discs of 13 mm diameter and about 2 mmthick, which were cut from sheets of pure metal. Thesesamples were irradiated on a remote-controlled verticalladder that fitted into a multi-purpose scatteringchamber.6 Beams of 5 MeV 2H+ ions were employedfrom the 6 MV Van de Graaff accelerator at the NationalAccelerator Centre, Faure, South Africa. The beamdiameter was 4 mm and currents were of 5–10 nA. Theprompt gamma-rays were measured with an 80 cm3

Ge(Li) detector, and the data were saved and processedoff-line.

The experimental data for the analyzed samplesappear in Table 2. For the four elements, levels between0.25–155 �g/g were obtained for the fruit, and for theleaves, concentrations between 0.10–143 �g/g wererecorded. The levels for Cu varied the most. However,the sufficiency range for transition metals in plantmaterial is relatively broad and the values in Table 2 are,therefore, within acceptable limits but are expected tovary with the phases of the fruiting season and withdifferent locations.7 The data indicate that the techniquecould be implemented for measuring these elements inthe sub-�g/g range in biological specimens of this nature.

In the case of the delayed X-rays each sample wasirradiated for between 30–120 minutes, with a beamcurrent of 300 nA, commensurate with expected half-lives of the radioactive products and natural abundances(Table 1). The total integrated charge was recorded ineach case. The irradiated sample was measured with astandard Si(Li) detector placed 4 cm from the samplesurface. These measurements were repeated over aperiod of 3 days to acquire suitable decay data. Accuraterecords of the duration of irradiation, delays and counttimes were essential. Manganese was excluded from thisset of measurements.

2H+ measurements

The potentially useful delayed X-rays and promptgamma-rays are listed in Tables 1 and 3, respectively.The assignments in Table 3 are labeled according to theChemists’ Convention.8 In general, the limit of detectionwas equivalent to three standard deviations of thebackground and was normalized to a specificaccumulated charge. In the prompt gamma-ray work datawere normalized to 100 mC. For comparable yieldvalues, in the case of the delayed X-rays, the standardthat was adopted was the calculated yield from apure element (in its natural isotopic composition) aftera 2-hour irradiation with a 1 �A beam of 4 mmdiameter, with measurement by a Si(Li) detectorplaced 4 cm away from the surface of the sample.

Results and discussion

ICP application

In validating the methodology, we considered twoimportant points: (1) that of elemental losses (throughvolatility or wet-chemical processes); and (2) that ofcontamination. The question of contamination wasseriously examined. Subjecting blanks to the identicalprocedure as the samples confirmed that contaminationwas minimal. Measurement of the blanks by ICPrevealed that any form of contamination was negligible(zero readings were obtained).

Table 2. ICP data (in �g/g) for dates and corresponding leaf samples*

Sample Mn Fe Cu Zn

Dates 5.3 155 0.25 6.0Leaf 8.8 143 0.10 4.3To monitor any elemental losses and establish that

our technique worked successfully, we specificallyevaluated the levels of Fe from replicate analyses.

* Average of 36 sample each.

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A. E. PILLAY et al.: TRANSITION METAL ANALYSIS

Table 3. Minimum detection limits for some promptgamma-rays induced by 5 MeV deuterons

particular area of sample preparation that ICP tends toreveal its major weakness. There is the problem ofcontamination from various sources and also possibleelemental loss due to volatility and the process ofdigestion. The actual determination itself by ICP takesonly a couple of minutes, and the technique is well suitedto analyses in the sub-�g/g range but considerable care isneeded to ensure that no serious errors occur in thesample preparation stage.

Element E�, keV Identity8 Detectionlimit,* �g/g

Mn 847 55Mn n (1,0) 0.111238 55Mn n (2,1) 0.23

Fe 352 56Fe p (3,1) 0.351224 56Fe n (1,0) 0.601378 56Fe n (2,0) 0.39

Cu 992 63Cu n (1,0) 0.201039 65Cu n (1,0) 0.32

Zn 359 66Zn n (2,0) 1.38752 64Zn n (6,2) 1.50911 64Zn n (7,0) 1.56

Conclusions

The potential for analysis of certain transition metalsby ICP, prompt gamma-rays and delayed X-rays excitedby 5 MeV 2H+ ions has been evaluated. Each methodpossesses its own special problems. In the case of ICP,errors could arise in sample preparation, whereas in theactivation analysis the achievement of comparabledetection limits can only be possible with the use of highenough beam currents, which could damage biologicalsamples by overheating them. The analysis by ICP iswell suited, provided contamination and elemental lossesare minimized in sample preparation. However, ifminimal sample preparation is desired and if adequatesample cooling can be achieved, the use of deuteron-induced prompt gamma-rays could be suitable.

* Normalized to 100 mC.

The lines shown in Tables 1 and 3 originated mainlyfrom (d,p) and (d,n) reactions.3,4 These reactions arelikely ones and, therefore, could be useful in analyticalapplications. Generally speaking, levels between1–10 �g/g are achievable with the method of delayedX-rays. The short-lived product of Cu is an alternativeand produces a detection limit of 30 �g/g, but therelatively long-lived one is more useful for analyticalpurposes. In the case of the prompt gamma-rays, sub-�g/g levels can be attained if the total accumulatedcharge is high enough. This implies that if high enoughbeam currents can be used, without damaging theirradiated sample, sensitivities comparable with ICP canbe achieved.

*

The authors thank Sultan Qaboos University, Muscat, Oman forfinancial assistance with the ICP study. The National AcceleratorCentre, Faure, South Africa is acknowledged for the use of itsfacilities associated with the activation analysis work.

Comparative analytical capabilities

ReferencesThe principal difficulty with the prompt gamma-raywork is the length of time necessary to achieve thedetection limits shown in Table 3. The detection limitslisted in Table 3 are unrealistic if impractically longtimes are taken to achieve them. High beam currentsaccompanied by sample cooling could be employed forthis purpose, but if biological specimens are to beirradiated some sample damage due to overheating isbound to occur. The problem of sample damage is ofconsiderable concern in the delayed X-ray work becauseimproved detection limits can only be attained if thebeam currents are high enough. On the other hand, theactivation analysis is capable of measuring isotopes andrequires minimal sample preparation. It is in this

1. C. PUNYADEERA, MSc Thesis, University of the Witwatersrand,Johannesburg, South Africa, 1996.

2. J. B. JONES, Plant Analysis Handbook, Micro-Macro Publishers,Georgia, USA, 1991.

3. M. PEISACH, C. A. PINEDA, A. E. PILLAY, J. Radioanal. Nucl.Chem., 188 (1994) 43.

4. M. PEISACH, C. A. PINEDA, A. E. PILLAY, Nucl. Instr. Meth., B85(1994) 142.

5. R. I. BOTTO, Anal. Chem., 54 (1982) 1654.6. I. S. GILES, PhD Thesis, University of Cape Town, Cape Town,

South Africa, 1978.7. S. M. HASSAN, MSc Thesis, Sultan Qaboos University, Oman,

2001.8. M. PEISACH, J. Radioanal. Nucl. Chem., 12 (1972) 251.

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