Feasibility of eliminating interferences in graphite furnace atomic absorption spectrometry using analyte transfer to the permanently modified graphite tube surface

Spectrochimica Acta Part B 57 (2002) 473–484

Feasibility of eliminating interferences in graphite furnace atomic absorption spectrometry using analyte transfer to the permanently modified graphite tube surface夞 Sandra M. Maiaa, Bernhard Welzb,*, Edgard Ganzarollib, Adilson J. Curtiusb b

a ´ Instituto de Quımica, Universidade Federal do Rio Grande do Sul, 91501-970 Porto Alegre, RS, Brazil ´ ´ Departamento de Quımica, Universidade Federal de Santa Catarina, 88040-900 Florianopolis, SC, Brazil

Received 11 June 2001; accepted 7 December 2001

Abstract A procedure is proposed to avoid spectral andyor non-spectral interferences in graphite furnace atomic absorption spectrometry (GF AAS) by transferring the analyte during the pyrolysis stage from a solid sampling platform to the graphite tube wall that has been coated with a permanent modifier, e.g. by electrodeposition of a platinum-group metal. The direct determination of mercury in solid coal samples was chosen as a model to investigate the feasibility of this idea. The graphite tube surface was coated with palladium and the analyte was transferred from the solid sampling platform to the tube wall at a temperature of 500"50 8C. A characteristic mass of m0 s64 pg Hg was obtained for an atomization temperature of 1300 8C, proposing a quantitative transfer of the analyte to the tube wall. Calibration against aqueous mercury standards was not feasible as this element was lost in part already during the drying stage and could not be trapped quantitatively on the modified graphite tube surface. However, the results for all except one of the coal reference materials were within the 95% confidence interval of the certificate when the slope of a correlation curve between the integrated absorbance, normalized for 1 mg of sample, and the certified value for mercury was used for calibration. A detection limit of 0.025–0.05 mg gy1 Hg in coal, calculated from three times the standard deviation of the investigated coal samples, could be obtained with the proposed method. The spectral interference due to excessive background absorption in the direct determination of mercury in coal could be eliminated completely. It is expected that this analyte transfer can be used in a similar way to eliminate other spectral andyor non-spectral interferences in the GF AAS determination of other volatile analytes. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrothermal atomic absorption spectrometry; Analyte transfer; Graphite tube surface coating; Mercury determination; Coal analysis

夞 This paper was presented at Colloquium Spectroscopicum Internationale XXXII, held in Pretoria, South Africa, 8–13 July 2001 and is published in the special issue of Spectrochimica Acta Part B, dedicated to that conference. *Corresponding author. Tel.: q55-48-998-31344; fax: q55-48-331-9711. E-mail address: [email protected] (B. Welz).

0584-8547/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 1 . 0 0 4 0 4 - 9

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1. Introduction The presence of concomitants in the sample can cause interference, i.e. systematic errors in the measure of the signal w1x. However, interference will cause an error in the analytical result only if the interference is not adequately accounted for in the evaluation procedure w1x. A wide variety of measures have been proposed over the past decades in order to overcome the effect of interference on the analytical result. In graphite furnace atomic absorption spectrometry (GF AAS), the most widely used and most successful one to overcome spectral and non-spectral interference is the stabilized temperature platform furnace (STPF) concept, introduced by Slavin et al. w2x some 20 years ago. However, as there is no single measure that can solve all problems associated with interference, there are various other approaches described in the literature, and quite a few of them are based in one way or another on an analyte transfer. Actually, the first graphite furnaces built by L’vov w3x used an electrode on which the sample was deposited, and from which the analyte was vaporized and transferred into a preheated graphite tube. The two-step atomizer, which was pioneered and improved repeatedly by Frech et al. w4–7x, followed the same idea of separating the volatilization of the analyte spatially from its atomization, using a graphite cup in which the analyte was volatilized into a pre-heated graphite tube, in which the atomic absorption was measured. Several different designs of two-step atomizers have recently been proposed by Grinshtein et al. w8,9x, providing better sensitivity and greater freedom from interferences, compared to conventional atomizers. The greatest disadvantages of all twostep atomizers, however, are their relatively complex design, and the fact that they are not commercially available. Rettberg and Holcombe w10–13x took a much simpler approach in their ‘second surface atomization’ by inserting a cooled tantalum plug into the graphite tube of an otherwise unmodified commercial graphite atomizer. They succeeded in reducing spectral and non-spectral interferences significantly so that they could analyze solid samples directly using aqueous standards for calibra-

tion. However, the recoveries were not very favorable and the authors admitted that their method would only be suitable for screening purposes. Hocquellet w14x inserted a graphite ‘vault’ that was cut from a standard graphite tube, in the upper part of the graphite tube atomizer to trap the analyte which was volatilized from the graphite tube surface. Both, the vault and the tube were coated with tantalum carbide and the author claimed that the proposed method appeared in some cases to be less affected by interferences than platform atomization. Jackson et al. w15,16x used commercially available L’vov platforms for their wall-to-platform migration studies in order to investigate interference mechanisms. They found that elements such as cadmium, lead and thallium begin to migrate at temperatures of 250–500 8C, probably through low-temperature vaporization of oxides, metals or salts of the analyte. All these transfer and migration experiments were based on the temperature difference between the tube wall and the trapping surface, i.e. the tantalum plug, the graphite vault or the platform. However, analyte migration could also be observed under ‘isothermal’ conditions in the dual-cavity platform experiments of Welz et al. w17,18x. In these experiments, analytes such as antimony and lead were pipetted into one cavity of the platform, and a nickel solution into the other one, and subject to the usual drying and pyrolysis stages. It could be shown that at pyrolysis temperatures as low as 400 8C most of the analyte had migrated into the cavity with nickel (which is known to be a chemical modifier for some analytes), as was demonstrated by a significant late shift in the appearance of the atomization signal. This created the idea that analyte migration to, and trapping at deliberately modified surfaces could be used as an analytical tool in cases where application of the STPF concept is not sufficient to remove spectral andyor non-spectral interferences, particularly in the determination of volatile analytes. The idea is even more attractive in combination with a permanent chemical modifier, a concept that has found rapidly increasing interest over the past decade, and which is summarized in a number of review articles w19–22x. The main advantages of permanent modifiers, compared to the conven-

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Table 1 Graphite furnace temperature program for the transfer and determination of mercury in solid coal samples and in aqueous solution Stage

Temperature (8C)

Ramp (8C sy1)

Hold time (s)

Internal gas flow ratea

Drying Transfer Coold Auto zero Atomize Clean

70by120c 200–650 100 100 1300 1500

10 15 35 0 1500 1500

20 60 30 5 10 5

MaxbyStopc Stop Max Stop Stop Max

a

Purge gas flow rate through, and in part around the graphite tube; maxs2.0 l miny1. 70 8C and Max gas flow rate for solid samples. c 120 8C and internal gas flow turned off for aqueous solutions. d Solid sampling platform removed in this stage. b

tional application of a modifier in solution with each injection of a sample or calibration solution, are reduced blank values w23,24x, a significant reduction in the cost per analysis w25x, often a longer tube lifetime w23x, and a great simplification of the analytical procedure, associated with a corresponding saving in time, particularly for techniques such as on-line analyte preconcentration w26x, in-tube trapping of hydrides w27–29x or direct solid sample analysis w24x. The latter aspect is also of paramount importance for the envisaged application of analyte transfer to a modified tube surface. The goal of the present work was to investigate the feasibility of vaporizing an analyte from a sample matrix that causes significant interferences in conventional GF AAS, trapping the analyte on the graphite tube wall, treated with a permanent modifier, in order to enable an interference-free determination after the matrix has been removed. The idea is based on the accepted model that analyte atoms and molecules undergo numerous collisions with other gaseous species as well as with the tube wall before they leave the graphite tube, particularly in the absence of forced gas flows w30,31x. The determination of mercury in mineral coal was chosen as a model case because we are involved in a project to develop routine procedures for the determination of potentially hazardous trace elements in coal w32x, as coal is very difficult to bring into solution w33x, particularly when mercury has to be determined, because of the extreme risk of losses and contamination

for this element and as a direct determination by solid sampling GF AAS proved to be impossible because of excessive background absorption. 2. Experimental 2.1. Instrumentation and procedure All measurements were carried out using an AAS5 EA atomic absorption spectrometer (Analytik Jena AG, Germany) equipped with a transversally heated graphite tube atomizer and a deuterium background correction system. A mercury hollow cathode lamp (Analytik Jena), operated at 4.5 mA was used at a wavelength of 253.7 nm and a slit setting of 0.5 nm. The graphite furnace program is shown in Table 1. Argon with ˜ Paulo, a purity of 99.996% (White Martins, Sao Brazil) was used as the internal purge and external protective gas. All experiments were carried out with special pyrolytically coated solid sampling graphite tubes without a dosing hole (Analytik Jena Part No. 078130325), modified by electrodeposition of palladium onto their inner surface (see Section 2.3). All samples were introduced into the graphite tube atomizer on pyrolytic graphite coated solid sampling graphite platforms (Analytik Jena Part No. 63-019901) using a manual solid sampling accessory (SSA5, Analytik Jena). An M2P microbal¨ ance (Sartorius, Gottingen, Germany), integrated in the software control of the AAS instrument, was used for the analysis of solid samples; solu-

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Hg were prepared freshly by serial dilution of a 1000 mg ly1 stock solution (Merck, Darmstadt, Germany) in 2% vyv nitric acid. For tube coating the following reagents were used: Na2HPO4Ø12 H2O and benzoic acid (Vetec, Rio de Janeiro, Brazil) and (NH4)2HPO4Ø12 H2O (Merck, Darmstadt, Germany). The palladium chloride solution was prepared by dissolving palladium powder (Merck) in aqua regia, evaporating the solution to dryness and dissolving the residue in warm dilute hydrochloric acid. 2.3. Electroplating of the graphite tube

Fig. 1. Schematical design of the flow system used for electroplating the graphite tubes.

tions were pipetted manually onto the solid sampling platform. Integrated absorbance was used exclusively for quantification of measurements. 2.2. Materials and solutions The following certified reference materials were investigated: SRM 1632b, trace elements in bituminous coal, SRM 1630a, trace mercury in coal (both from the National Institute of Standards and Technology, NIST, Gaithersburg, MD, USA), SARM 19 coal O.F.S., SARM 20 coal, Sasolburg (both from the South African Bureau of Standards, Pretoria, SA), BCR 40, Trace elements in coal and BCR No. 180, Gas Coal (Community Bureau of Reference, Brussels, Belgium). All samples were ground in an agate mortar to a particle size F50 mm before analysis. Analytical reagent grade nitric acid (Carlo Erba, Milan, Italy) and hydrochloric acid (Merck, Darmstadt, Germany) were further purified by subboiling distillation using a quartz subboiling still ¨ (Kurner Analysentechnik, Rosenheim, Germany). Water was purified in a Milli-Q system (Millipore, Bedford, MA, USA), resulting in a water with a resistivity of 18 MV cm. Calibration solutions with concentrations of 100, 200 and 500 mg ly1

For the electroplating of the graphite tubes, a procedure proposed by Bulska and Jedral w34x was adopted. The graphite tube was cleaned before use by heating in the atomizer to 2300 8C for 5 s, with the argon purge gas flowing at a rate of 2.0 l miny1. After this the tube was installed in the system shown schematically in Fig. 1, which included a peristaltic pump (Reglo-Ismatec, Glattburg, Switzerland) for propelling the electrolytic bath solution at a flow rate of 1.0 ml miny1. The electrolytic bath composition is shown in Table 2. The solution became pink and turbid, and a fine precipitate was forming upon mixing the components that, however, had no influence on the electrodeposition process, and the solution became clear soon after the electrodeposition was started. A platinum wire of 0.5-mm diameter and a length of 4 cm, carefully positioned in the center of the graphite tube was used as the anode, and the inner wall of the graphite tube acted as the cathode to electrodeposit the palladium. The graphite tube, which had no dosing hole, was immersed in a water bath, with the thermostat at 80 8C and an ´ amperostat (Microquımica, Santa Catarina, Brazil) Table 2 Electrolytic bath composition for palladium plating of the graphite tube Compound

Amount, g 100 mly1

PdCl2 Na2HPO4Ø12 H2O (NH4)2HPO4Ø12 H2O Benzoic acid

1.0 20 5 0.2

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was used to maintain the current at a constant value during the entire electroplating process. The total volume of the electroplating solution was 2.0 ml and the total time for electroplating was 60 min. After the electroplating cycle was finished, the tube was washed with deionized water and dried at room temperature. Then it was inserted into the atomizer, dried carefully by heating it three times to 80 8C with a temperature ramp of 5 8C sy1 and a hold time of 60 s, and finally it was heated to 2000 8C for 5 s, using a temperature ramp of 1000 8C sy1. 2.4. Procedure The solid samples, typically approximately 1 mg, were placed into the solid sampling platforms, weighed on the micro-balance and after the sample weight has been acquired by the software of the instrument, the sample was transferred to the furnace using the manual solid sampling tool, a pre-adjusted pair of tweezers. In stage 1 (Table 1), the sample was dried shortly, then in stage 2, the internal purge gas was stopped and the temperature was increased to transfer the mercury from the sample to the treated tube wall (at varying temperatures). In stage 3, the solid sampling platform with the sample residues was removed from the tube and then the program was continued automatically through stage 6. The coal residue was removed mechanically from the platform using a tissue and it was made sure by regular blank measurements that no analyte was retained on the platform. When aqueous calibration solutions were applied, a drying temperature of 120 8C was applied in stage 1 with the internal gas flow stopped in order to reduce mercury losses in this stage to a minimum; the rest of the program was the same. 3. Results and discussion 3.1. Direct solid sample analysis The direct determination of mercury in a variety of environmental materials, such as ash or sediment samples, using direct solid sample analysis,

477

proved to be fairly easy, as mercury was retained in such samples without losses up to a pyrolysis temperature of 300 8C, even without the use of a modifier and the background signal during atomization was negligible w35x. The application of various modifiers had very little effect on the overall situation, however, calibration remained a problem, as it turned out that it was very difficult to stabilize mercury in an aqueous solution using a permanent modifier, such as iridium w35x. In contrast to this experience, the determination of mercury in coal using direct solid sample analysis proved to be impossible, particularly due to the tremendous background signal produced by the coal matrix during atomization. The problem was obviously in the pyrolysis stage, where mercury could not be stabilized to a high enough temperature to remove the sample matrix without loosing the analyte completely at the same time. The application of various stabilizing agents, applied either in solution or as permanent modifiers, could not solve the problem. 3.2. Surface coating Hence, the idea was borne that mercury, which is apparently lost from coal samples at temperatures -400 8C, as has also been shown in experiments using electrothermal vaporization inductively coupled plasma mass spectrometry w36x, could probably be trapped at a ‘second surface’, treated with a modifier and retained for later atomization. It is well established in the meantime that (i) analytes can migrate within the graphite tube very easily at relatively low temperatures, (ii) mercury is the most mobile metallic element in the periodic table, and (iii) a lot of analytes can be trapped on graphite surfaces that have been modified with carbide-forming or platinum-group elements. At the beginning, a soaking procedure, as originally proposed by Ortner and Kantuscher w37x, was investigated, which, however, resulted in only an insufficient deposition of modifier on the graphite tube surface, probably because of the relative impermeability of modern pyrolytic graphite coatings. Therefore, the electrodeposition of modifiers on the graphite tube surface, as proposed by Bulska and Jerdal w34x, was

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Fig. 2. Influence of the transfer temperature on the integrated absorbance signal, normalized for 1 mg of sample (normalized integrated absorbance), for mercury in SARM 20 coal, obtained at an atomization temperature of 1300 8C, using a transfer time of 60 s. Error bars indicate the standard deviation of three independent measurements.

adopted as it was considered the best approach for our purpose. Palladium was chosen as the modifier for this feasibility study as modifiers with a higher melting point, such as iridium, have been found to have an inferior stabilizing power for mercury w35,38x. Although the mass of palladium that was deposited on the inner tube surface was not determined, it could be assumed that most of the 20 mg of Pd that were contained in the 2-ml of solution have been transferred to the graphite tube in the electroplating process. The analytical lifetime of the coating, i.e. the number of atomization cycles that could be carried out until the sensitivity for mercury started to decrease, was approximately 70–80 firings, using the temperature program shown in Table 1, which is in good agreement with the numbers found by Bulska and Jedral w34x. 3.3. Analyte transfer and figures of merit The South African reference coal was chosen as a model coal because tively high mercury content of 0.25 order to investigate the feasibility of

SARM 20 of its relamg gy1 in the idea to

make a direct determination of mercury in solid coal possible by a transfer of the analyte to a modified graphite tube surface during the pyrolysis stage. The coal was placed onto the solid sampling platform, introduced into the graphite atomizer, heated to a preset temperature with the internal purge gas turned off (Gas Stop) and then removed before the atomization stage. This introduction and removal of the platform could be very easily managed using the adjustable pair of tweezers of the solid sampling accessory without interrupting the graphite furnace program. The influence of the transfer temperature on the atomization signal obtained after trapping is shown in Fig. 2. There is a clear plateau between 450 and 500 8C, where the maximum sensitivity for mercury is obtained. At temperatures -450 8C, mercury is apparently not completely released from the coal, at least not within the time period chosen (1 min), whereas at temperatures )500 8C, mercury cannot be retained any more quantitatively on the Pd-coated tube surface. It is remarkable that mercury is retained by palladium as a modifier up to a temperature of 500 8C without losses,

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which might be due to a better stabilization of mercury from the gas phase than from solution. A similar stabilizing power was reported by Welz et al. w39x only when palladium was used as a permanent modifier in addition to the conventional modifier, mixed with the solution to be analyzed, whereas a maximum pyrolysis temperature of only 300 8C could be used with the permanent modifier alone. A heating time of 1 min at 450–500 8C was sufficient for the analyte transfer from the coal to the tube wall, at least for this coal, ground to a particle size of F50 mm. Longer heating times did not increase the mercury signal, but shorter heating times (30 s) resulted in a lower sensitivity. No attempt was made to investigate if mercury could be trapped more efficiently at a lower transfer temperature using a longer transfer time. However, a clear influence of the particle size was observed in a separate experiment with BCR 180 coal, which was analyzed ‘as received’, i.e. with a particle size of 63–212 mm, and ground to F50 mm. A 60% lower integrated absorbance, normalized for a sample mass of 1 mg, was obtained in the former case, compared to the latter, using a transfer time of 60 s at 500 8C. Again, no attempt was made to investigate if a higher recovery could be obtained for the coarser particle size using a longer transfer time. The characteristic mass calculated from the integrated absorbance, normalized for 1 mg of sample (normalized integrated absorbance), of 0.018 s in Fig. 2 for the SARM 20 coal reference material is approximately 60 pg Hg. This value is very close to the theoretical value of 69 pg calculated and also measured by L’vov w40x, considering the dimensions of the graphite tube and the atomization temperature. This excellent agreement proposes that the analyte transfer from the coal to the modified graphite tube surface, and the analyte release from the palladium-coated surface are quantitative. A typical absorption pulse for mercury is shown in Fig. 3. Both, the peak half width of more than 1 s and the total peak width, including the tail, of more than 5 s, are more than a factor of 2 greater than typically observed for this element w38x, and very unusual for volatile elements such as mercury in transversely heated graphite

479

Fig. 3. Absorption pulse for mercury in coal (BCR No. 40), using the proposed method. Solid line is atomic absorption; dotted line is background absorption.

tubes. This can only be explained by kind of a ‘chromatographic effect’, i.e. the mercury atoms are sticking to the palladium-coated tube much longer upon collision and are hence leaving the tube much more slowly than they would diffuse out of a pyrolytic graphite coated tube. This phenomenon is obviously further supported by the fact that solid sampling graphite tubes without a dosing hole were used for these experiments and all atoms had to diffuse out through the tube ends, increasing the probability of repeated collisions with the palladium coated tube wall significantly. 3.4. Analysis of coal reference materials After it had been demonstrated in principle that mercury can be volatilized from a solid coal sample at a temperature of 450–500 8C, and be trapped on the graphite tube wall coated with palladium, a variety of other coal reference materials from Europe, North America and South Africa were investigated in a similar way. The normalized integrated absorbance plotted against the transfer temperature for all five materials investigated are shown in Fig. 4. It is obvious that, although there are some minor differences in the individual temperature behavior, the general appearance of all curves is very similar. This means that the conditions established for one coal can be used for a variety of other coal samples without modification. Although a slightly higher sensitivity was obtained for two coal samples at a transfer temperature of 550 8C, a temperature where losses were already

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Fig. 4. Influence of the transfer temperature on the normalized integrated absorbance for mercury in five different coal reference materials.

observed for the other coal samples, a transfer temperature of 500 8C was chosen for all future experiments. In order to quantify the obtained results, the normalized integrated absorbance values obtained for the different coal samples at a transfer temperature of 500 8C and an atomization temperature of

1300 8C were plotted against the certified values for mercury, and the correlation is shown in Fig. 5. The 95% confidence interval of the certified values is indicated by the horizontal error bars (SRM 1632b has only a recommended value of 0.1 mg gy1 Hg) and the standard deviation of at least five determinations is shown in the vertical error bars. Although the value found for SRM 1630a was outside of the 95% confidence interval, the correlation was considered quite reasonable for a feasibility study and no attempt was made to improve the results. The slope of the correlation curve could be used for calibration purposes, e.g. for a fast screening analysis of a larger number of coal samples, reducing the potential influence of different coal matrix compositions on the mercury signal. The characteristic mass of m0s64 pg Hg, calculated from the slope of the correlation curve is very close to the theoretical value calculated by L’vov w40x, as discussed in Section 3.3. The standard deviation of five replicate determinations of mercury, except for BCR No. 40, was between 0.008 and 0.016 mg gy1 Hg, suggesting that a detection limit (calculated as three times the stan-

Fig. 5. Correlation curve for the normalized integrated absorbance obtained for five coal reference materials and the certified mercury content for five coal reference materials, using a transfer temperature of 500 8C and an atomization temperature of 1300 8C. 1s SRM 1630a; 2sSRM 1632b; 3sBCR 180; 4sSARM 20; 5sBCR 40. For details see text.

S.M. Maia et al. / Spectrochimica Acta Part B 57 (2002) 473–484

Fig. 6. Relative integrated absorbance of an aqueous mercury solution in dilute nitric acid (10 ml of a 250 mg ly1 Hg solutions2.5 ng Hg) against the transfer temperature. Error bars indicate the standard deviation of three independent determinations.

dard deviation of the analyzed reference materials) between 0.025 and 0.05 mg gy1 Hg could be obtained in coal samples without difficulty using this method. 3.5. Aqueous standards Calibration against solid standards, such as certified reference materials, is common practice in the direct analysis of solid samples in GF AAS, although it may not necessarily be the ideal case w41x. Calibration against the slope of a correlation curve, established with several reference materials of different analyte content may reduce some of the limitations associated with the use of only one reference material w24x. The ideal case, however, would be if aqueous standards could be used for calibration, because they are very easy to handle, and they provide the best precision and lowest uncertainty. For this reason, aqueous standards were investigated as well in this analyte transfer experiment. The solution was pipetted manually onto the solid sampling platform, and a drying step at 120 8C was introduced, as shown in the alternate temperature program of Table 1. This drying was carried out with the internal purge gas turned off (gas stop conditions) in order to decrease the risk that mercury was carried out of

481

the graphite tube together with the water vapor, as no modifier was applied in this case. In Fig. 6 the relative integrated absorbance obtained for an aqueous mercury standard in dilute nitric acid is plotted against the transfer temperature. The same optimum transfer temperature of 500 8C was obtained in this experiment as previously found for the coal samples, indicating that this temperature is independent of the coal matrix. However, the rest of the curve was significantly different, as 80% of the maximum sensitivity was already obtained at a transfer temperature of 200 8C, demonstrating the well-known high volatility of mercury from aqueous solutions. Moreover, the characteristic mass obtained for aqueous mercury solutions was only approximately 100 pg Hg, and hence, significantly inferior compared to the characteristic mass obtained for the coal samples. This indicates that mercury was probably, in part, carried out of the graphite tube together with the rapidly expanding water vapor already during the drying stage and could not be trapped quantitatively by the palladium coating. This loss of mercury from aqueous solutions in the graphite furnace in the absence of a modifier has already been observed by Ediger w42x back in 1975. In an attempt to avoid these losses and to stabilize the mercury in aqueous solution, a set of standards was prepared in a mixture of 6% vyv hydrochloric acid and 4% vyv hydrogen peroxide. However, the characteristic mass deteriorated further to values of approximately 150 pg Hg. It may be assumed that the additional gas that is developing upon the decomposition of H2O2 supported the carrier effect and increased the losses. The calibration curves obtained for mercury under these conditions were strictly linear and the precision of the results was excellent, in spite of these obvious analyte losses, indicating that the losses must be very reproducible. The addition of modifiers to the aqueous solutions in order to stabilize the mercury could obviously be a solution to prevent these losses. This approach, however, was not further investigated in this feasibility study. 4. Conclusion The proposed method of analyte transfer from a solid sample to the graphite tube wall, coated with

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a permanent modifier, did not necessarily provide a highly accurate means of determining the mercury content in solid coal samples and calibration against aqueous standards was not feasible yet. However, the results for all but one reference material were within the 95% confidence interval of the certificate when calibration against the slope of a correlation curve was used, so that the proposed method could at least be applied advantageously for a fast screening analysis of coal. However, the purpose of the present work was not to find a new, accurate method for the determination of mercury in coal, as this problem has already been solved in an earlier publication w38x. The purpose was to investigate the possibility to transfer the analyte quantitatively to the tube wall, coated with a permanent modifier, in order to avoid spectral andyor non-spectral interferences caused by concomitants. The results of this investigation suggest that this approach is feasible and might be applied to other analytes and matrices, e.g. to eliminate the notorious chloride interference on the determination of thallium w43x. One of the assumptions in this approach is that the analyte is trapped on the modified graphite tube surface independent of its chemical form, i.e. as an atom or as a molecule, which has yet to be demonstrated. In addition, these future experiments could also provide further information about the mechanism of modifier action. Acknowledgments

w2 x

w3 x w4 x

w5 x

w6 x

w7 x

w8 x

w9 x

w10x

w11x

The authors are grateful for financial support ¸ ˜ de Aperfeicoamento ¸ from Coordenacao de Pessoal ´ de Nıvel Superior (CAPES) and from Conselho ´ Nacional de Desenvolvimento Cientıfico e Tecnol´ogico (CNPq). S.M. Maia has a doctorate scholarship from CAPES, and B. Welz has a research scholarship from CNPq. The authors are also grateful to Analytik Jena AG for the loan of an atomic absorption spectrometer.

w14x

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