High-resolution continuum source electrothermal atomic absorption spectrometry — An analytical and diagnostic tool for trace analysis

Spectrochimica Acta Part B 62 (2007) 873–883www.elsevier.com/locate/sab

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High-resolution continuum source electrothermal atomic absorptionspectrometry — An analytical and diagnostic tool for trace analysis☆

Bernhard Welz a,b,⁎, Daniel L.G. Borges b, Fábio G. Lepri b,Maria Goreti R. Vale c, Uwe Heitmann d

a Instituto de Química, Departamento de Química Analítica, Universidade Federal da Bahia, Campus Universitário de Ondina, 40170-290 Salvador — BA, Brazilb Departamento de Química, Universidade Federal de Santa Catarina, 88040-900 Florianópolis — SC, Brazil

c Instituto de Química, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, 91501-970 Porto Alegre — RS, Brazild ISAS — Institute for Analytical Sciences, Department of Interface Spectroscopy, Albert-Einstein-Str. 9, 12489 Berlin, Germany

Received 26 January 2007; received in revised form 13 March 2007; accepted 13 March 2007Available online 24 March 2007

Abstract

The literature about applications of high-resolution continuum source atomic absorption spectrometry (HR-CS AAS) with electrothermalatomization is reviewed. The historic development of HR-CS AAS is briefly summarized and the main advantages of this technique, mainly the‘visibility’ of the spectral environment around the analytical line at high resolution and the unequaled simultaneous background correction arediscussed. Simultaneous multielement CS AAS has been realized only in a very limited number of cases. The direct analysis of solid samplesappears to have gained a lot from the special features of HR-CS AAS, and the examples from the literature suggest that calibration can be carriedout against aqueous standards. Low-temperature losses of nickel and vanadyl porphyrins could be detected and avoided in the analysis of crude oildue to the superior background correction system. The visibility of the spectral environment around the analytical line revealed that the absorbancesignal measured for phosphorus at the 213.6 nm non-resonance line without a modifier is mostly due to the PO molecule, and not to atomicphosphorus. The future possibility to apply high-resolution continuum source molecular absorption for the determination of non-metals isdiscussed.© 2007 Elsevier B.V. All rights reserved.

Keywords: High-resolution continuum source atomic absorption spectrometry; Electrothermal atomization; Solid sampling analysis; Multielement determination;Crude oil analysis; Background correction

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8742. Measurement principle and special features of HR-CS AAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8753. Simultaneous multielement ET AAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8774. Direct analysis of solid samples and slurries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8775. Solution analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8796. Nickel and vanadium in crude oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8807. Determination of phosphorus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8808. Molecular absorption for the determination of non-metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8819. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 881

☆ This paper was presented at the 9th Rio Symposium on Atomic Spectrometry, held in Barquisimeto, Venezuela, 5–10 November 2006, and is published in thespecial issue of Spectrochimica Acta Part B, dedicated to that conference.⁎ Corresponding author. Departamento de Química, Universidade Federal de Santa Catarina, 88040-900 Florianópolis — SC, Brazil. Fax: +55 48 3331 6850.E-mail address: [email protected] (B. Welz).

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874 B. Welz et al. / Spectrochimica Acta Part B 62 (2007) 873–883

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 881References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 881

1. Introduction

The first experimental setups used by Kirchhoff and Bunsen[1,2] and Lockyer [3] to observe and measure atomic absorptionspectra in the 19th century were equipped with continuum lightsources, as these were the only reliable sources available at thattime, and they perfectly served the purpose. However, the use ofcontinuum light sources was for sure one of the main reasonsthat in the first half of the 20th century optical emission waspreferred over atomic absorption for spectrochemical analysis.It was obviously much easier to measure a weak emission infront of a dark background than to detect a small reduction overa narrow spectral range of a strong continuous emission —particularly when photographic detection was used, thecommon detector at that time. This situation only changed inthe early 1950s when Alan Walsh [4] proposed to use lineradiation sources with the sharpest possible emission lines foratomic absorption spectrometry (AAS). Actually, Walsh hadbeen very lucky, because the sealed-off hollow cathode lamp,the radiation source of choice for AAS, was for the first timedescribed in 1955 [5], i.e., the same year when Walsh publishedhis first manuscript about AAS as an analytical technique, and alot of research and engineering work had yet to be put into thedevelopment of this source. The use of a modulated line source(LS) and a selective amplifier tuned to the same modulationfrequency made LS AAS extremely selective and specific forthe analyte of interest. This was for sure the main reason for thesuccess of this technique over roughly half a century, although itwas practically limited to the determination of one element at-a-time.

Although commercial atomic absorption spectrometers havebeen built exclusively according to the principle proposed byWalsh for more than four decades, research on the use ofcontinuum radiation sources (CS) for AAS has continued

Fig. 1. Schematic design of a high-resolution cont

throughout this period. The early publications in this fieldmainly took advantage of the instability and/or low energyoutput of the hollow cathode lamps at that time or theirunavailability for certain elements [6–9]. In the following years,several groups investigated wavelength modulation using ACscanning [10], oscillating interferometers [11,12] or a combi-nation of optical scanning and mechanical modulation [13] inorder to improve the signal-to-noise ratio (SNR) and thesensitivity of CS AAS.

A kind of turning point in this early phase of CS AAS wasthe work of Keliher and Wohlers [14] who for the first time useda high-resolution echelle grating spectrometer. This researchwas then continued over the next 25 years by the groups ofO'Haver and Harnly [15–24], who continuously improved thesystem, introducing wavelength modulation, a pulsed continu-um source and a linear photodiode array detector. They alsodescribed the first, and up to now only functional simultaneousmulti-element atomic absorption spectrometer with a continuumsource (SIMAAC) [25–27].

The final breakthrough in CS AAS however was made by thegroup of Becker-Ross in Berlin, who had started to work onechelle spectrometers in 1980. Based on their experience, theysoon discovered the weak points of the instruments used at thattime [16], i.e., the low intensity of conventional xenon arclamps in the far UV and the drawbacks of wavelengthmodulation with an oscillating quartz plate. In contrast toothers, who started from commercially available equipment andcomponents, Becker-Ross and coworkers first determined therequirements for CS AAS [28] and then they specified anddesigned the instruments according to these requirements,starting with the continuum radiation source [29,30], followedby the spectrometer [31–34] and then the detector [32–34]. Ahistoric perspective about HR-CS AAS and detector technologyis presented in a recent review article [35] and details of these

inuum source atomic absorption spectrometer.

Fig. 2. Transient absorbance signals for Tl at 276.787 nm in a marine sedimentreference material; (a) LS AAS with deuterium background correction; solidline: corrected signal; dotted line: background absorption [43]; (b) HR-CS AASafter correction for continuous background [44].

875B. Welz et al. / Spectrochimica Acta Part B 62 (2007) 873–883

prototype instruments, which were the basis for the firstcommercial HR-CS AAS equipment, are described in the bookof Welz et al. [36].

The first commercial system for electrothermal atomization,based on the graphite tube furnace design of Massmann [37]was introduced in 1970 [38], but the technique was not wellunderstood at that time, so that a multitude of interferences wereencountered. This started to change only after Slavin et al. [39],based on research by L'vov [40] introduced the StabilizedTemperature Platform Furnace (STPF) concept, which includedthe atomization from a platform inserted into the graphite tube,the use of chemical modifiers and evaluation of the area of thetransient absorption signal instead if its height, among others.This concept made it possible to control most of the notoriousnon-spectral interferences in electrothermal (ET) AAS. How-ever, background correction, i.e., the removal of spectralinterferences caused by co-volatilized matrix componentsremained the Achilles heel of ET AAS. Even Zeeman-effectbackground correction, undoubtedly the best system everdeveloped for LS AAS, has its difficulties with rapidly changingbackground signals that are typical for ET AAS, as measure-ment of total absorption (magnet off) and backgroundabsorption (magnet on) is sequential [41]. Another problemarises in the case of molecular absorption of diatomic moleculeswith rotational fine structure, when the molecule also exhibitsZeeman splitting, as in this case the background without andwith magnetic field is not the same, as will be discussed later.

Although at the point in time when this manuscript has beenwritten commercial equipment for HR-CS ET AAS was not yetavailable, it appears worthwhile to review the application workdone in this field with various prototype instruments. Theapplications published until now already show the superiority ofHR-CS AAS compared to LS AAS with respect to accuracy ofresults and freedom from interferences. As the same graphite tubeatomizers are used in both techniques, this can only be due togreater freedom from spectral interferences and a superior back-ground correction. Another aspect is the diagnostic one, i.e., thepossibility to detect the reason for interferences and artifactsobserved in LS AAS due to the visibility of the spectral envi-ronment of the analytical line in HR-CS AAS. This review willconcentrate mostly on applications published over the last decade,although some of the earlier work will be mentioned as well.

2. Measurement principle and special features ofHR-CSAAS

In this chapter we only want to review briefly themeasurement principle of the HR-CS AAS prototypes built bythe group of Becker-Ross in Berlin, Germany, and two featuresthat are most important for ET AAS analysis, namely thevisibility of the spectral environment around the analytical lineat high resolution and the different modes of backgroundcorrection. All other features of this technique might be found inthe literature [36].

The basic design of the equipment that has been used for mostof the analytical work cited in this review is shown in Fig. 1. Itconsists of a high-pressure xenon short-arc lamp, operating in ahot-spot mode, a high-resolution double monochromator, and a

linear CCD array detector [33]. The continuous radiation is pre-dispersed in a prismmonochromator, and the prism is positionedin a way that the part of the spectrum that contains the analyticalline can pass through an intermediate slit and enter the echellegrating monochromator, where it is highly resolved. The secondmonochromator does not have an exit slit, so that the entiresection of the spectrum that passes the intermediate slit, whichcovers a spectral interval of about 0.3–3 nm, depending on thewavelength range, reaches the linear CCD array detector with512pixels, 200 of which are used for analytical purposes. Thisdesign does not allow simultaneous multielement determinationunless more than one analyte line passes the intermediate slit;however, it is the basis of other features that are extremelyimportant in ET AAS.

Firstly, the entire spectral environment of the analytical linebecomes ‘visible’, i.e., instead of getting information aboutabsorbance over time only, as is usual in LS AAS, in HR-CSAAS we get three-dimensional information about absorbanceover time and over wavelength. This obviously greatly helps torecognize and to avoid spectral interferences and to facilitatemethod development in general. Secondly, as typically only


about three pixels of the CCD array are used for signalevaluation [42], all the others of the 200pixels are available forall kind of correction purposes. And as all pixels are illuminatedand read out simultaneously, all these corrections are simulta-neous and not sequential as in LS AAS. There is for example anautomatic correction for any kind of intensity changes thatappear in the same way on all pixels. This means that anychange in the emission intensity of the lamp, which obviouslyaffects all pixels in the same way, is corrected automatically, i.e.,the equipment works like a simultaneous double beam system,providing an extremely stable baseline.

Even more important is the automatic correction for anyreduction in intensity that affects all pixels, i.e., any kind ofbackground absorption that is ‘continuous’ over the spectralinterval under consideration. And this background correction isalso simultaneous, which means that even the fastest back-ground does not cause any artifacts as it does in LS AAS. Afterthis correction for spectrally continuous events all spectrallydiscontinuous events remain, which is any atomic absorptiondue to the analyte or a concomitant element and molecularabsorption due to diatomic molecules that exhibit rotational finestructure. This kind of background absorption cannot behandled at all by deuterium-lamp background correction, asthis technique is based on the theory that background absorptionis continuous over the spectral range transmitted by the exit slitof the monochromator. It also cannot be treated accurately bybackground correction systems based on high-current pulsing(Smith–Hieftje background correction), as with this system thebackground is measured on both sides of the analytical line,which inevitably causes errors.

A typical example for artifacts that might be observed usingLS AAS and deuterium lamp background correction is shown inFig. 2a for the transient atomization signal of Tl in a marine

Fig. 3. Highly resolved absorbance spectrum of the NH4H2PO4 modifier with and widifference of the two absorption spectra (lower part) (from Ref. [33]).

sediment reference material (MESS-2) [43]. An overcorrection(deviation of the analyte signal to absorbance values b0) appearsabout 2 s after the start of the atomization cycle although thebackground absorption only reaches values of about 0.1, whichshould not cause any problems for the correction system. Thereis another distortion visible in the rising part of the absorptionsignal that also seems to be related to a background signal thatappears at the same point in time. Obviously, there is no way inLS AAS to find out the real source for these artifacts, as only theabsorbance signal over time is visible within the spectral rangethat corresponds to the width of the analytical line emitted by theradiation source. Fig. 2b shows essentially the same thing, i.e.the absorbance spectrum observed for a similar marine sedimentreference material (PACS-2) in the vicinity of the Tl resonanceline using HR-CS ET AAS [44]. From this time- andwavelength-resolved absorbance spectrum it becomes veryobvious that the overcorrection observed with LS AAS is dueto a molecular absorption with a pronounced fine structure, asituation that cannot be handled by a deuterium backgroundcorrection system. What is not visible in Fig. 2b (but which canbe made visible if necessary) is the very fast continuousbackground signal that is preceding the Tl absorption signal,which obviously caused the distortion of the rising part of theabsorption signal. The speed of the appearance of a backgroundsignal is of no importance in HR-CS AAS, as measurement andcorrection of continuous background absorption is simultaneouswith the measurement of the total absorption. The structuredmolecular background absorption could be completely elimi-nated subtracting a reference spectrum generated by ‘atomizing’KHSO4, although this correction was actually not necessary, asthe analyte signal was separated in time andwavelength from themolecular absorption spectrum. HR-CS ET AAS could in thiscase serve as a diagnostic tool to detect the source of the artifacts

thout magnetic field in the vicinity of the Cd line at 228.802 nm (upper part) and

Table 1Publications about direct solid sampling analysis using HR-CS ET AAS

Analyte Matrix Modifier Calibration LODa/mg kg−1 Reference

Cd Coal Ir permanent Aqu. std.b 0.002 [52]Pb Coal None Aqu. std.b 0.008 [53]Tl Coal None Aqu. std.b 0.01 [54]Ag Biol. mater. None Aqu. std.b 0.0006 [55]Co Biol. mater. None Aqu. std.b 0.005 [56]Hg Boil. mater. None Aqu. std.b 0.1 [57]

+KMnO4

Pb Biol. mater. Ru permanent Aqu. std.b 0.01 [58]a Limit of detection, based on the ‘zero mass response’ [59]; n=10.b Aqueous standards, acidified with nitric acid.


observed with LS AAS, but it also is an analytical tool, as thedetermination of Tl in marine sediment could be carried outwithout any interference, even without the addition of anymodifier using aqueous standards for calibration [44].

Zeeman-effect background correction is obviously muchmore suited to control spectral interferences in ET AAS,particularly when the latest developments in this technique areconsidered [45]. This technique can correct for structuredbackground absorption if the magnet is at the furnace (inverseZeeman effect) and provided that the background is exactly thesame without and with magnetic field, i.e. that the moleculeunder consideration does not exhibit any Zeeman splitting.However, Heitmann et al. [33] have shown that this prerequisiteis not necessarily fulfilled. They observed significant over-correction in the determination of cadmium at 228.8 nm in thepresence of the ammonium phosphate modifier using Zeeman-effect background correction. As shown in Fig. 3, thisovercorrection is caused by the fact that themolecular absorptionspectrum of PO, which is generated by the phosphate modifier, isaffected by the magnetic field in a way that the backgroundabsorption at the Cd line increases when the magnetic field is on.

In the above-described HR-CS AAS equipment this kind ofstructured background can be eliminated measuring a referencespectrum that is subtracted from the sample spectrum using aleast-squares algorithm, similar to the principle that is commonuse in optical emission spectroscopy. However, this least-squares background correction (LSBC) has to be applied only ifthe structured background is overlapping with the absorptionsignal of the analyte and cannot be separated in time, which isfrequently possible in ET AAS.

3. Simultaneous multielement ET AAS

One of the driving forces for research in CS AAS has beenthe expectation to overcome the limitation to determine oneelement at-a-time only, particularly in ET AAS, where typicalmeasurement times are of the order of a few minutes. However,only one real simultaneous multielement atomic absorptionspectrometer with a continuum source (SIMAAC) has beenbuilt [25,26] and used for ET AAS. Harnly et al. [46]investigated different forms of heating and signal evaluationand obtained accurate results for the simultaneous determina-tion of Ca, Cu, Fe, Mn and Zn in National Bureau of StandardsStandard Reference Materials Bovine Liver and Rice Flour andfor the simultaneous determination of Al, Co, Cr, Fe, Mn, Mo,Ni, Pb, V and Zn in Acidified Waters. Lundberg et al. [47]investigated the performance of three furnace systems withrespect to interference effects and carry-over contaminationwhen operated with the SIMAAC, but they did not analyze realsamples.

The only other ‘simultaneous’ CS AAS instrumentsdescribed in the literature for ET AAS used photodiode arraydetectors that covered a spectral range of 2.5 nm [48], 10 nm[49] and 4 nm [50], respectively, and only elements that hadabsorption lines falling within this narrow spectral windowcould be detected simultaneously. In their first work the authorsinvestigated a 19-component mixture as well as a standard

bronze and a seawater sample [48] without carrying out realdeterminations. In their second publication [49] the authors alsoconcentrated mostly on detection limits without analyzing realsamples. Only in their third publication, in which they used atungsten coil atomizer [50], the authors determined Al, Ni and Vin spiked engine oil samples, Cd, Co and Zn in urine and Cd,Co, Sr and Yb in water samples. Similar equipment has alsobeen used for atomic emission spectrometry using the sametungsten coil atomizer [50].

Becker-Ross et al. [51] described an echelle spectrographwith internal order separation in tetrahedral mounting, whichallows the simultaneous recording of the spectral range from200 to 465 nm. However, a measurement time of severalminutes is necessary with this system in order to obtain goodsignal-to-noise ratios, so that this spectrograph is not suitablefor the fast transient signals of ET AAS. However, thisequipment has been an extremely useful diagnostic tool forstudies of structured background absorption of diatomicmolecules, many of which can also be observed in graphitefurnaces, such as PO, NO, CS, SiO and others. The majority ofthe molecular spectra measured with this equipment, includingtheir exact classification can be found in Ref. [36].

4. Direct analysis of solid samples and slurries

The direct analysis of solid samples is routine in opticalemission spectrometry using arcs and sparks, in X-rayfluorescence and in a number of other spectrometric techniques.Nevertheless, direct analysis of solid samples using ETAAS hasbeen considered kind of a weird approach by the vast majorityof scientists working in atomic spectroscopy and has not foundbroad application up to now. The major arguments against directsolid sampling (SS) ET AAS have been (i) the difficulty tointroduce a solid sample into the graphite tube; (ii) the relativelyhigh relative standard deviation (RSD) obtained with thistechnique, compared with solution analysis due to the naturalinhomogeneity of real samples; and (iii) the need of certifiedreference materials (CRM) with matrix composition and analytecontent as close as possible to the sample composition forcalibration. Problem (i) has been solved years ago with theintroduction of commercial equipment for manual and auto-matic SS-ET AAS. Problem (ii) is well known and fullyaccepted in all techniques that are based on direct solid sample

Fig. 5. Time and wavelength-resolved absorbance spectrum for DORM-1Dogfish Muscle CRM in the vicinity of the mercury line at 253.652 nm usingdirect SS-HR-CS ET AAS; atomization temperature: 1100 °C; no pyrolysisstage; no modifier (from Ref. [57]).

Fig. 4. Absorbance over time for lead in NIST SRM 1632b coal measured at thecenter pixel only (217.001 nm) with (black signal) and without (gray signal)correction for continuous background absorption; (a) pyrolysis temperature400 °C; (b) pyrolysis temperature 700 °C (from Ref. [53]).


analysis. However, at this point the question arises, what is moreimportant: (a) an RSD of 1% of a result that might be affected bysystematic errors due to analyte loss or contamination becauseof a multi-stage sample preparation, or (b) an RSD of 10% of anaccurate result, because no or only a minimum of samplepreparation has been involved. Problem (iii) is kind of derivedfrom arc and spark emission and X-ray fluorescence, where theuse of CRM for calibration is kind of routine practice. However,this does not necessarily mean that the same is valid for SS-ETAAS, as the need of CRM for calibration means that there is asignificant influence of the sample matrix on the analyte signal.It has been shown decades ago that non-spectral interferences inETAAS can usually be controlled applying the STPF concept ina strict manner [39]. The weak point remained backgroundcorrection in LS AAS, and background absorption might be thepredominant feature in SS-ET AAS, as very large amounts ofmatrix are usually introduced with this technique. Hence, itremains to be investigated how HR-CS ETAAS with its superiorbackground correction capabilities can cope with this problem.

The publications about direct solid sample analysis usingHR-CS ETAAS are summarized in Table 1. Aqueous standardshave been used in all cases for calibration, as can be seen in Table1, which means that the determination was free of interferences.In the analysis of coal a very high and fast continuousbackground appeared for all analytes at the beginning of theatomization stage that reached values of A=2–4, as shown inFig. 4a, when pyrolysis temperatures of up to 600 °C were used.

Nevertheless, this background absorption only caused anincrease in the baseline noise as essentially no radiation reachedthe detector, and the atomization signal could be evaluatedwithout problems starting signal integration right after the noisehad ceased. The strong background absorption essentiallydisappeared when the pyrolysis temperature was increased to700 °C, as can be seen in Fig. 4b, a pyrolysis temperature thatcould be applied for the determination of lead [53] and thallium[54] without using a modifier, whereas iridium was applied aspermanent chemical modifier for the determination of cadmium[52]. A structured background absorption appeared for allelements after the analyte signal, which could however beseparated in time from the atomic absorption by optimizing theatomization temperature, so that no additional backgroundcorrection using LSBC had to be applied.

A very similar behavior was found for the biologicalmaterials, i.e., a strong continuous background absorptionappeared at the beginning of the atomization stage for pyrolysistemperatures up to 600 °C, which essentially disappeared whenthe pyrolysis temperature was increased to 700 °C. Cobalt [56]and silver [55] were determined without a modifier, whereas theauthors preferred to use ruthenium as permanent modifier forthe determination of lead [58] in order to be able to apply apyrolysis temperature of 900 °C. Similar to the coal matrix astructured background appeared in the case of cobalt before[56], and in the case of silver [55] and lead [58] after theatomization signal. In all cases the structured background couldbe separated in time from the atomic absorption by optimizingthe atomization temperature. In the case of lead the authorsidentified the molecule PO as the source of the structuredbackground by subtracting a reference spectrum generated by‘atomizing’ (NH4)2HPO4 using the same temperature programand subtracting this spectrum using LSBC [58].


It is worth mentioning that the resonance line at 217.001 nmwas used for the determination of lead both in coal [53] and inbiological materials [58]. This line is known to be about twice assensitive as the line at 283.306 nm, but the signal-to-noise ratio(SNR) is clearly inferior in LS AAS, so that the latter one isusually preferred. In HR-CS AAS, in contrast, the former oneclearly exhibits not only higher sensitivity, bur also a betterSNR and better LOD.

The determination of mercury in biological materials [57]merits separate discussion, as it is obviously not possible,because of the volatility of this analyte, to use a pyrolysistemperature of 700 °C, even in the presence of a modifier, inorder to get rid of the continuous background absorption. Theauthors actually found that the use of palladium as permanentmodifier shifted the atomization signal for mercury in time toappear together with the excessive continuous backgroundabsorption, making a determination impossible. The authorsalso observed that without a modifier mercury was lost alreadyat temperatures around 100 °C from fish samples, most likely asmethyl mercury, whereas in meat and hair samples it was stableat least up to 200 °C. The authors therefore used a very shorttemperature program without any pyrolysis stage and only 3 sdrying at 100 °C, followed by an atomization at 1100 °C. Underthese conditions the mercury signal appeared before thecontinuous background and could be separated in time fromthe excessive noise caused by the background absorption, asshown in Fig. 5. Calibration was carried out against aqueousstandards, which had to be stabilized with potassium perman-ganate in order to avoid mercury losses during the drying stage,as shown by the same authors in earlier work [60].

In contrast to direct solid sampling, slurry analysis has beenaccepted much earlier as sampling technique for ETAAS, mostlikely because of the commercial availability of an automatedslurry sampler [61], and it was considered to ‘combine both theadvantages of liquid and solid sampling’ [62]. The mosttroublesome aspect of the slurry technique, when the aboveautosampler – which has been withdrawn from the market – isnot available, is the stabilization of homogeneous slurry duringthe time required to carry out the determination by ET AAS[62]. This might explain why only a few applications of thistechnique with HR-CS ET AAS have been published.

Welz et al. [44] succeeded to determine thallium in marinesediment reference materials without the addition of a modifierand with calibration against aqueous standards using slurrysampling HR-CS ET AAS. This determination could not becarried out at all using LS AAS with deuterium lampbackground correction, and with Zeeman-effect backgroundcorrection only using a combination of ruthenium as permanentmodifier and ammonium nitrate added in solution [43,63].Pedroso et al. [64] determined nickel in sewage sludge samplesat two secondary nickel lines at 231.096 nm and 231.235 nm,which were both within the spectral window used for thedetermination. A pyrolysis temperature of 1400 °C could beused and no modifier was necessary for this determination.Pronounced structured background absorption was observed inthe vicinity of both analytical lines, which was due to theelectron excitation spectrum of the SiO molecule. The

background could be eliminated using a reference spectrumgenerated by ‘atomizing’ an aqueous stock standard solution ofSi and LSBC. No overlap with the molecular structures wasobserved at 231.096 nm, whereas significant positive errorswere found at the 231.235 nm line without the use of LSBC.

Probably the most interesting and surprising result has beenobtained by Bianchin et al. [65] for the determination ofcadmium in coal using slurry sampling HR-CS ETAAS. Whilethis determination could be carried out without any problemsusing direct SS-HR-CS ET AAS, the results of slurry samplingwere not convincing although the same equipment has beenused and a variety of modifiers has been investigated. Mostmodifiers could stabilize cadmium only to pyrolysis tempera-tures of 500–550 °C, and an analysis of the supernatantconfirmed that either the acid concentration or some matrixcomponent that has been leaching out of the samples wasresponsible for the low stabilizing power of the modifiers. Onlya mixture of 300 μg Wand 200 μg Ir could stabilize cadmium atleast to a pyrolysis temperature of 600 °C, which however wasnot sufficient to reduce the background to a manageable level inmost of the investigated samples. This is probably the firstreport about an application that could not be carried out reliablyusing slurry sampling, but worked without problems usingdirect solid sample analysis.

5. Solution analysis

A nitrate and phosphate containing solution was one of thefirst samples investigated using HR-CS ET AAS with the goalto study the spectral interferences observed in the determinationof cadmium using Zeeman-effect background correction [33]. Itcould be shown that the interference is due to the molecularabsorption of the PO molecule, which is influenced by themagnetic field as has been shown in Fig. 3. In later work,Becker-Ross et al. [66] have shown that similar problems canalso be observed in the determination of arsenic and selenium inhuman urine. In the case of arsenic molecular absorption due toPO and NaCl was observed, both with pronounced finestructure, and in the case of selenium determination the finestructured background was caused by PO and NO molecules.For both elements significant spectral interference was observedusing Zeeman-effect background correction, but with HR-CSET AAS all interferences could be removed using LSBC.

Ribeiro et al. [56] determined cobalt in a variety of biologicalreference materials after solubilization in tetramethylammo-nium hydroxide (TMAH) in good agreement with certifiedvalues. The structured background due to the electron excitationspectrum of PO could be separated from the atomic absorptionsignal by optimizing the atomization temperature. Torres et al.[67] used the same solubilization in TMAH for the determina-tion of mercury in biological materials after cold vaporgeneration and trapping of the mercury vapor in a gold-coatedgraphite tube. The method was free from interference, and thedetection limit was 0.02 mg kg−1.

Salomon et al. [68] used HR-CS ETAAS as a diagnostic toolto investigate systematic errors due to spectral interferences inthe determination of aluminum in seawater that occurred when

Fig. 7. Time- and wavelength-resolved absorbance spectrum for 10 μg P asNH4H2PO4 in the vicinity of the 213.618 nm non-resonance phosphorus linewithout the addition of a modifier (from Ref. [74]).


Zeeman-effect background correction was used. The authorscould show that the interference was mainly due to the calciumand magnesium matrix in seawater, which is difficult toeliminate. Another problem identified by these authors is thedifficulty to obtain Al-free blank of seawater for correctionpurposes. A similar problem has been addressed by Bohrer et al.[69] in an attempt to determine aluminum in pharmaceuticalproducts containing high concentrations of iron and sugar. Thiskind of solutions could not be analyzed by LS AAS withZeeman-effect background correction due to severe spectralinterference. It was found that a secondary iron line at309.278 nm, which is located in between the two lines of thealuminum doublet at 309.271 nm and 309.284 nm is responsiblefor the spectral interference at this wavelength. Using HR-CSET AAS and the center pixel only for signal evaluation madepossible interference-free determination of aluminum in thepresence of high iron concentration. An alternative is to carryout the determination at the 396.152 nm aluminum line, whichdoes not have an iron line close enough to cause spectralinterference, however the high sugar concentration causescontinuum and structured background. One of the majorproblems in this determination was that aluminum is aubiquitous element and it is almost impossible to find reagentsthat are free of this contaminant.

6. Nickel and vanadium in crude oil

The analysis of crude oil is undoubtedly one of the moredifficult tasks for ET AAS because of the large amount oforganic matrix that is introduced into the graphite tube and thatis not easy to remove prior to the atomization of the analyte.Usually measurements are not possible using pyrolysistemperatures of less than 600 °C due to the extremely highbackground absorption caused by the petroleum matrix.However, nickel and vanadium are considered thermally stableelements, so that relatively high pyrolysis temperatures areusually applied in order to remove the organic matrix. However,

Fig. 6. Pyrolysis curve for vanadium in a crude oil sample using HR-CS ETAAS(from Ref. [72]).

using HR-CS ET AAS with its significantly better backgroundcorrection capability, which allowed quantitative measurementsusing a pyrolysis temperature of only 300 °C, Vale et al. [70]discovered that a very significant part of nickel might be lostalready at pyrolysis temperatures above 400 °C. The sameproblem was discovered soon after also for vanadium, andpalladium was proposed as chemical modifier to avoid theselow-temperature losses [71].

These losses have not been reported previously for ETAAS,and they are obviously due to the volatile nickel and vanadylporphyrin complexes, whereas the polar non-porphyrins arestable up to relatively high temperatures. Lepri et al. [72]proposed a procedure for speciation analysis of volatile andnon-volatile vanadium compounds in crude oil using HR-CSET AAS. The total content of vanadium (and nickel) could bedetermined using a pyrolysis temperature of 400 °C, where nolosses occur, whereas the content of thermally stable analyte isdetermined using a pyrolysis temperature N800 °C, as shown inFig. 6. The content of volatile analyte, i.e., the content ofporphyrin complexes can be determined by difference. Theauthors point to the fact that this kind of speciation analysis is ofsignificance, as it is for sure the volatile porphyrin complexesthat are transported preferentially to low-boiling fractions in thedistillation process and act as catalyst poison, whereas the non-porphyrins remain preferentially with the asphaltene fraction.

7. Determination of phosphorus

Phosphorus is certainly not one of the elements that aretypically determined by ET AAS, as its resonance lines aresituated between 167.16 nm and 178.77 nm in the vacuum UVrange of the spectrum, which is not accessible with conventionalAAS equipment. L'vov and Khartsyzov [73] were the first ones


to propose the non-resonance doublet at 213.5/213.6 nm asalternate lines for the determination of phosphorus. Lepri et al.[74] investigated the phosphorus atomization using HR-CS ETAAS and found that without a chemical modifier the mainabsorption peak is not due to atomic phosphorus, but to POmolecular absorption, as shown in Fig. 7. Phosphorus atomicabsorption only appears as kind of a broad tail, suggestingstrong interaction between phosphorus and graphite. Theauthors also investigated a number of modifiers for the deter-mination of phosphorus and found that palladium-based mod-ifiers produce the highest concentration of phosphorus atoms.

8. Molecular absorption for the determination of non-metals

The use of a continuum source instead of line sources in HR-CS ET AAS offers the possibility that not only atomic lines areused for absorption measurement, but also molecular bands thatare within the spectral range of the instrument. Huang et al. havepublished a whole series of papers on the determination ofphosphorus [75], sulfur [76,77], fluorine [78] and chlorine [79]using sharp molecular absorption ‘lines’ selected from the finestructure of electron transition spectra produced in an air–acetylene flame. The advantage of flame in this case is that itsstoichiometry can bemodified in order tomake it oxidizing for theproduction of an oxide, such as PO [75], or reducing in order toproduce a carbide, such as CS [76,77]. The buffer effect of theflame gases can hence be used to produce the analyte molecule ofchoice, a possibility that does not exist to the same extent in agraphite furnace. Nevertheless, Heitmann et al. [80] showed in arecent publication that this kind of molecular absorptionspectrometry is also feasible in a graphite furnace and determinedchlorine via the AlCl molecule, fluorine via GaF, phosphorus viaPO and sulfur via the CS molecule. This obviously opens acompletely new field of application for ETAAS.

9. Conclusion

The development of HR-CS AAS equipment for ET AAShas introduced a new quality of analysis that provides theanalyst much more confidence in the results due to the visibilityof the spectral environment of the analytical line. Thesimultaneous correction of continuous background also avoidsartifacts that are quite common in LS ETAAS and that result inpeak distortion and overcorrection. Direct solid samplinganalysis appears to benefit a lot from these features, as theamount of matrix that is introduced with this technique issignificantly greater than in solution analysis. But othercomplex samples, such as crude oil or highly concentratedpharmaceutical solutions also demonstrate the superiority ofHR-CS ET AAS over conventional LS AAS. One of theexciting new fields that has yet to be investigated in detail is thedetermination of non-metals using HR-CS molecular absorptionspectrometry, which will obviously increase the attractivenessof this technique. Moreover, at present the feasibility of anextension of HR-CS AAS for truly simultaneous multielementdetermination, i.e. the registration of the entire wavelengthrange in a single measurement is studied at the ISAS Berlin.

Acknowledgements

The authors are grateful to Conselho Nacional de Desenvolvi-mento Científico e Tecnológico (CNPq) and to Fundação deAmparo à Pesquisa do Estado da Bahia (FAPESB) for financialsupport. D.L.G.B., F.G.L. and M.G.R.V have scholarships fromCNPq and B.W. has a research scholarship from FAPESB. Thefinancial support by the Senatsverwaltung für Wissenschaft,Forschung und Kultur des Landes Berlin and the Bundesminister-ium für Bildung und Forschung is also gratefully acknowledged.

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High-resolution continuum source electrothermal atomic absorption spectrometry — An analytical and diagnostic tool for trace analysis