JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1986, VOL. 1 23 Mechanisms of Transition Metal Interferences in Hydride Generation Atomic Absorption Spectrometry Part 4.* Influence of Acid and Tetrahydroborate Concentrations on Interferences in Arsenic and Selenium Determinations Bernhard Welz and Marianne Schubert-Jacobs Department of Applied Research, Bodenseewerk Perkin-Elmer & Co GmbH, 0-7770 Uberlingen, FRG An increase in the hydrochloric acid concentration from 0.5 to 5 mol 1-1 improves the range of interference-free determination for arsenic and selenium in the presence of cobalt, copper or nickel by factors of 5100. Decreasing the sodium tetrahydroborate(ll1) concentration from 3 to 0.5’/0 mlVincreases the range of interference-free determination for both elements further by factors of 4-50.The only exception is the interference of copper on selenium, which is slightly more pronounced with the less concentrated tetrahydroborate(ll1) solution. The proposed mechanism is that both in higher acid and with lower tetrahydroborate(ll1) concentrations the interferent is reduced to a lesser extent to the metal (the interfering species) owing to ( i ) better solubility of the metal in the more concentrated acid, (ii) the formation of chloro complexes, thus reducing the concentration of free ions, and (iii) a larger percentage of the tetrahydrobor- ate(lll) being consumed by the acid. Copper, however, interferes with selenium in the ionic form. Keywords: Hydride generation atomic absorption spectrometry; arsenic determination; selenium determination; interference mechanisms; sodium tetrah ydroborate(ll1) concentration It is well documented that a number of transition metals, mainly those of Groups VIII and IB, can cause severe signal depressions in hydride generation atomic absorption spec- trometry.Smith1 made the first systematic study on the effect of 48 elements on the determination of hydride-forming elements and found that many of the interfering elements formed precipitates after the addition of sodium tetrahydro- borate(II1). He proposed that preferential reduction of the metal ion interferent in solution to a different oxidation state or to the free metal can cause precipitation of that species, which can then either coprecipitate the analyte element or adsorb the volatile hydride formed and catalytically decom- pose it.Kirkbright and Taddia2 also noticed that, in the presence of elements such as nickel, palladium or platinum, after the addition of the reductant a finely dispersed black precipitate was formed. For the determination of arsenic virtually complete suppression of the signal was observed on addition of nickel powder. The authors pointed out that nickel and other Group VIII elements are hydrogenation catalysts and can adsorb hydrogen in large amounts. Hence capture and decomposition of the hydride by the finely dispersed metal can occur. Meyer et al. 3 reported numerous interferences from transi- tion metals on the determination of selenium but did not mention an interferent precipitation. They proposed that the selenium hydride, after its generation, forms insoluble sele- nides or stable complexes with the free ions of the interfering elements in a secondary reaction when it is transported through the sample solution by the carrier gas and the hydrogen.In Part 1 of this series,4 we investigated transition metal interferences on the determination of selenium in a system in which the selenium hydride is generated in pure acid solution and comes into contact with the metal ions only in a second flask. It could be shown that capture and decomposition of the selenium hydride by the finely dispersed metal is the most * For Part 3 of this series, see reference 5. likely mechanism of interference whenever precipitation of the metallic species occurs. It could also be shown that several orders of magnitude higher concentrations of the interfering metal can be tolerated when its reduction and precipitation are avoided or delayed, e .g . , by the addition of a buffer ion which is more easily reduced.5 One of the simplest and most frequently applied methods of reducing transition metal interferences is to increase the acidity of the reaction mediurn.3.4>6-* The explanation most frequently used for this phenomenon is the increased solubil- ity of the reduced metal and/or of the compound formed between the analyte element and the interferent in the strong a ~ i d . 3 ? ~ Another possible explanation would be that more of the tetrahydroborate(II1) is used up by the higher acid concentration and converted into hydrogen so that less is available for the reduction of the interfering element to the metal.If this is true, a reduction of the tetrahydroborate(II1) concentration should have the same effect. However, there is no indication in the literature that would support this assumption. Sodium tetrahydroborate(II1) concentrations reported in the literature vary between 0.3 and 10% mlV, but most workers have used concentrations between 1 and 3% mlV. The only criterion applied to the selection of the optimum tetrahydroborate concentrations is typically the best sensitiv- ity that can be obtained for reference solutions.9-13 Thompson and Thomersonl4 even found that the tetrahydroborate concentration should be increased in order to ensure adequate reduction of the analyte element when analysing samples with high concentrations of transition metal ions.Evans et al. 15 also felt that it is desirable to maximise the amount of tetrahydro- borate(II1) used to overcome consumption of this reagent by other elemental species present. Dittrich et al. ,I6 on the other hand, found that only a very small percentage (<0.1%) of the tetrahydroborate(lI1) is used to generate the hydride, and that the consumption of reductant by competitive reactions cannot be the only reason for the observed interferences. In this work, we have systematically investigated the influence of different sodium tetrahydroborate(II1) concen- trations (0.5, 1.0 and 3.0% mlV) on some typical transition metal interferences in the determination of arsenic and24 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1986, VOL.1 ~ Table 1. Operating parameters for the MHS-20 hydride system Cell tempera- Element Purge I/s Reaction/s Purge II/s ture/OC 10 40 900 8 35 900 As . . . . . . 35 Se . . . . . . 35 0.5 mol 1-1 HCI 0'4 1 3% I I 1 %o 5.0 mot I-' HCI 0.3 0.5% 5: 0.2 - 0.1 - I I 0 5 10 Ti me/m i n 15 Fig. 1. Influence of hydrochloric acid concentration (0.5 and 5 moll-1) and sodium tetrahydroborate(II1) concentration (0.5, 1 and 3% m/V) on the signal of 50 ng of As(V) in 10 ml of solution selenium in low (0.5 moll-1) and high (5 moll-1) hydrochloric acid concentrations. The purpose of the investigation was to shed more light on the sometimes contradictory reports about the influence of the tetrahydroborate(II1) concentration on interferences, and to obtain a greater insight into the interference mechanisms in hydride generation AAS.Experimental Apparatus A Perkin-Elmer Model 4000 atomic absorption spectrometer equipped with electrodeless discharge lamps, operated at 8 W for arsenic and at 6 W for selenium from an external power supply, was used for all determinations. A spectral band pass of 0.7 nm was selected to isolate the 193.7-nm arsenic line, and 2 nm was used for the determination of selenium at the 196.0-nm line. The signals were recorded on a Perkin-Elmer Model 56 recorder set at the 10-mV range. All measurements are expressed as peak height unless stated otherwise. The Perkin-Elmer Model MHS-20 hydride system used has been described in detail elsewhere17; the instrumental settings used for the determination of arsenic and selenium are summarised in Table 1.Reagents All reagents used, except for the sodium tetrahydrobor- ate( 111) powder, were of analytical-reagent grade or higher purity. Hydrochloric acid was further purified by sub-boiling distillation. Sodium tetrahydroborate(III) solution, 0.5, 1 and 3% mlV. Prepared by dissolving sodium tetrahydroborate( 111) powder (Riedel-de-Haen) in de-ionised, distilled water and stabilising with 1% mlV sodium hydroxide solution. The solutions were prepared freshly every day and filtered before use. Arsenic( V) and selenium(IV) stock standard solutions, 1000 mg 1-1. Prepared by diluting Titrisol solutions (Merck) containing 1000 mg of the element to 1 1 with de-ionised, distilled water. Aliquots were diluted with 0.5 moll-1 HCl to obtain appropriate working reference solutions.0.5 0.4 0) 0 0 0.3 5: a 11 0.2 0.1 r 1 O/O 0.5% 5.0 mol 1-1 HCI 1 . . I 0 5 10 15 Ti me/m i n Fig. 2. Influence of hydrochloric acid concentration (0.5 and 5 moll-l) and sodium tetrahydroborate I11 concentration (0.5, 1 and 3% mlV) on the signal of 100 ng of Se[IV] in 10 ml of solution Results and Discussion The influence of three transition metals, cobalt, nickel and copper, on the absorbance signal of arsenic and selenium was investigated in 0.5 and 5.0 moll-1 hydrochloric acid solution using 3, 1 and 0.5% mlV sodium tetrahydroborate(II1) solution as the reducing agent. Both the hydrochloric acid and the tetrahydroborate(II1) concentrations have an influence on the peak-height sensitivity and signal shape, as shown in Fig.1 for arsenic. A higher acid concentration reduces the signal height and width. A reduction in tetrahydroborate(II1) concentration decreases the signal height but increases its width. The apparently small peak recorded in 5 moll-1 HC1 with 3% mlVNaBH4 is an artifact because the recorder cannot follow the very rapid signal accurately. The behaviour of selenium and the signal shapes recorded are similar to those of arsenic, and are shown in Fig. 2. These differences in absorbance and in peak shape are not taken into consideration when transition metal interferences are investigated because the acid and tetrahydroborate(II1) concentrations are typically kept constant during a set of experiments. The interferences are therefore expressed as relative sensitivity (Yo), and the absorbance (peak height) of the pure analyte solution in the same acid and with the same tetrahydroborate(II1) concentration is set to 100%.A typical graph of relative sensitivity against interferent concentration is shown in Fig. 3 for arsenic in the presence of nickel. It is obvious that an increase in the acid concentration and a decrease in the tetrahydroborate(II1) concentration both result in a substantial increase in the range of interference-free determination of arsenic, which means that higher nickel concentrations can be tolerated without influencing the arsenic absorbance. In most instances the interference starts at a well defined transition metal concentration and becomes very pronounced within an order of magnitude increase in the interferent concentration.The only exception from this otherwise uniform behaviour was found for 5 mol 1-1 HC1 and 0.5% mlV NaBH4. The moderate signal depression caused by nickel concentrations of 200 mg 1-1 or more becomes progressively more pronounced with every determination and irreparably deteriorates the sensitivity of the whole system. Regular cleaning of the quartz tube is essential under these conditions. We believe that aJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1986, VOL. 1 25 0 0.1 1 .o 10 100 1000 Ni concentration/mg I-’ Fig. 3. Influence of sodium tetrahydroborate(II1) concentration (0.5, 1 and 3% mlv) on the interference of nickel on the determina- tion of arsenic in 10 ml of 0.5 and 5 moll-1 HCl Table 2. Range of interference-free determination of 50 ng of As(V) in 10 ml of 0.5 and 5.0 moll-’ hydrochloric acid using 0.5,l and 3% m/V NaBH, solution.Values given are highest concentrations of interfer- .ent in mg 1-1 that can be tolerated without affecting the peak-height sensitivity for arsenic 0.5 moll-’ HCl 5 moll-’ HC1 3% 1% 0.5% 3% 1% 0.5% Interferent NaBH, NaBH4 NaBH4 NaBH, NaBH4 NaBH, Co(I1) . . 1 1 5 50 100 200 Cu(I1) . . 10 100 100 50 500 500 Ni(I1) . . 0.1 0.2 3 3 5 100 ~ ~~~~ ~ ~ Table 3. Range of interference-free determination of 100 ng of Se(1V) in 10 ml of 0.5 and 5.0 mol 1-1 hydrochloric acid using 0.5, 1 and 3% m/V NaBH, solution. Values given are highest concentrations of interferent in mg 1-1 that can be tolerated without affecting the peak-height sensitivity for selenium 0.5 mol 1-I HCl 5 moll-’ HCl 3% 1% 0.5% 3% 1% 0.5% Interferent NaBH, NaBH4 NaBH, NaBH, NaBH, NaBH4 Co(I1) .. 1 5 10 100 200 400 Ni(I1) . . 0.2 3 10 10 50 200 Cu(I1) . . 0.2 0.2 0.1 10 5 5 small amount of nickel salt is carried into the heated quartz cell as an aerosol where it is deposited. We have observed a similar effect for metals or metal salts placed in the quartz cell.l* As in this earlier experiment, the sensitivity could be restored by cleaning the quartz cell with hydrofluoric acid. The influence of acid and tetrahydroborate(II1) concentra- tion on the interference of all three investigated transition metals in the determination of arsenic and selenium is summarised in Tables 2 and 3, respectively. We have chosen the “range of interference-free determination” to describe the effect because the highest transition metal concentration that has no influence on the analyte signal is usually fairly well defined and reproducible. When the data in Tables 2 and 3 are considered, a very consistent pattern appears for all analyte - interferent combi- nations except for the influence of copper on selenium.This pair will therefore be discussed separately. For all the other combinations, however, a substantial increase in the range of interference-free determination is observed at higher acid and lower tetrahydroborate(II1) concentrations. However, there is more than this qualitative trend in the data in Tables 2 and 3. Table 4 lists the improvement in the range of interference-free determination of arsenic and selenium when the hydrochloric acid concentration is increased from 0.5 to 5 mol 1-1.With the exception of the copper - selenium pair, which will be discussed later, there is an obvious decrease in the improvement factor with increas- Table 4. Improvement in the range of interference-free determination of arsenic and selenium with an increase in hydrochloric acid concentration from 0.5 to 5 mol 1-1 Improvement factor NaBH, concentration, % m/V Analyte Interferent 3 1 0.5 Average As . . . . Co 50 100 40 63 Ni 30 25 33 29 c u 5 5 5 5 Se . . . . Co 100 40 40 60 Ni 50 17 20 29 c u 50 25 50 42 Table 5. Improvement in the range of interference-free determination of arsenic and selenium with a decrease in tetrahydroborate(II1) concentration from 3 to 0.5% mlV Improvement factor HC1 concentration/mol 1- 1 Analyte Interferent 0.5 5 Average As .. . . Co 5 4 4.5 Ni 30 33 32 c u 10 10 10 Se . . . . Co 10 4 7 Ni 50 20 35 c u 0.5 0.5 0.5 ingly positive electrochemical potential (Co*+ < Ni2+ < Cu*+). The average improvement factors for cobalt (63 and 60) and nickel (29 and 29) are essentially identical for the two analyte elements,_ arsenic and selenium. A similar correlation can be found for the improvement in the range of interference-free determination with tetrahydro- borate(II1)’ concentration decreasing from 3 to 0.5% mlV (Table 5). The average improvement factors for cobalt (4.5 and 7) and nickel (32 and 35) are again very close for arsenic and selenium. This indicates that the changes in acid and tetrahydroborate(II1) concentrations act predominantly on the interferent and not on the analyte.Compound formation between the analyte element and the interferent as the primary reaction step and the reason for the interference, as proposed for the interference from some Main Group elements,16 is unlikely. The arsenides and selenides of cobalt and nickel are all insoluble in hydrochloric acid. It would therefore be difficult to explain a two to three orders of magnitude better solubility by a ten-fold increase in acid concentration and a six-fold decrease in tetrahydroborate( 111) concentration. A competitive reaction of the type where some of the sodium tetrahydroborate(II1) is used up by matrix elements, and less is therefore available for reduction of the element of interest to the hydride and a lower signal is obtained, has been proposed by several workers as the mechanism of transition metal interferences.1914,15 The signals and sensitivities obtained for arsenic and selenium at different concentrations of hydrochloric acid could be interpreted in the same way by the more highly concentrated acid consuming more of the tetrahydroborate(II1) so that less is available for hydride formation. It has been shown, however, with radioactively labelled selenium that the efficiency of hydride formation is 95 k 3% even under the conditions where the smallest integrated signal (peak area) is obtained, that is, 5 moll-’ HCl and 3% mlV NaBH4.19 The difference in the signals obtained for arsenic and selenium is therefore not due to insufficient hydride generation but to incomplete atomisation of the hydrides formed. A competition for the sodium tetrahydro-26 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1986, VOL.1 borate(II1) as an explanation for the interferences becomes totally unlikely when the observations in Table 5 are con- sidered. A lower tetrahydroborate(II1) concentration results in all instances, with the exception of the copper - selenium interference, in a substantial increase in the range of inter- ference-free determination. A very small proportion (<0.1%) of the tetrahydroborate(II1) is required for hydride generation anyway,l6 so that a depletion is unlikely a priori. By the method of exclusion we therefore end up with a mechanism that was already proposed earlier for transition metal interferences, namely preferential reduction of the interferent, and capture and decomposition of the hydride formed at the finely dispersed metal precipitate .43 This mechanism allows all the observed phenomena to be explained.Increasing hydrochloric acid concentration can result in a smaller concentration of bivalent ions of the interfering metal to be present owing to the formation of chloro complexes. Most transition metals show a higher solubility in more highly concentrated acids. Both effects will result in a shift of the metal precipitation to higher concentra- tions. Although a competition for the sodium tetrahydrobor- ate(II1) between the analyte and the interferent was excluded because of the very small amount of reductant required for hydride formation, there may well be a competition between the acid and the interfering metal ions.Both do consume relatively large amounts of tetrahydroborate(II1) so that high acid concentrations (e.g., 5 mol 1-1) can lead to a noticeable depletion, particularly if lower concentrations of the reductant (e.g., 0.5% mlv) are used. This again is in agreement with the experimental observations. The strongest argument for the preferential reduction mechanism, however, is the finding that the factors for improvement in the range of interference-free determination (Tables 4 and 5 ) , although very different for different interfering elements, are essentially identical for arsenic and selenium determinations in the presence of the same interfer- ent. This can only be explained by a direct interaction of the acid and/or tetrahydroborate(II1) concentration with the formation of the interfering metal species, e.g., its precipita- tion. Copper was excluded from the discussion of the results up to now because its behaviour is clearly different in some instances. The influence of copper on the determination of arsenic still fits into the general pattern discussed for the other transition metals. The increase, by a factor of 5, in the range of interference-free determination of arsenic caused by an increase of hydrochloric acid concentration from 0.5 to 5 mol 1-1 is not as impressive as for the other elements. This, however, is within expectations because with increasingly positive electrochemical potential the interfering element will be reduced more easily and the acid concentration cannot influence this process very much. Perhaps the hydrochloric 100 >= 80 'G 60 8 c .- > .- 4- (u $ 40 .- c - g 20 I I-' HCI \?\ \, 5.0 mol I- \$\'\ HCI rnl V NaBH4 0 0.1 1 .o 10 100 1000 Cu concentrationlmg I-' Fig.4. Influence of sodium tetrahydroborate(II1) concentration (0.5, 1 and 3% mlV) on the interference of copper on the determination of selenium in 10 ml of 0.5 and 5 moll-' HC1 acid acts through the formation of chloro complexes, reducing the number of copper ions available for reduction. The influence of copper on the selenium determination is significantly different from all other analyte - interferent combinations discussed here. In comparison with the arsenic determination, an increase in the hydrochloric acid concentra- tion has a substantially more pronounced influence on the range of interference-free determination of selenium in the presence of copper (Fig. 4).A decrease in the tetrahydrobor- ate(II1) concentration, however, has very little influence or even an adverse effect, which means that greater freedom from interferences is observed with higher tetrahydrobor- ate(II1) concentrations. This behaviour cannot be explained by a preferential reduction of the interfering metal ion and an interaction of the reduced species with the hydride formed. In earlier work we found evidence that the copper interference on selenium, at least in 5 mol 1-1 HCl, is probably caused by the copper(I1) ion.4 The most likely mechanism is then a reduction of the selenium to the hydride, which, after its generation, reacts with the free copper ions to form relatively insoluble copper selenide, as proposed by Meyer et al.3 Copper selenide is slightly soluble in hydrochloric acid, so that the strong influence of the higher acid concentration can at least in part be explained by a better solubility of the selenide under these conditions. The formation of chloro complexes in the more highly concentrated acid, which has already been discussed for arsenic, could have an additional effect by reducing the number of free copper ions available for reaction. Such a gas - liquid reaction would depend mainly on the speed of diffusion of the selenium hydride to the gas - liquid interface, the selenium hydride concentration in the gas bubble and the residence time of the gas bubble in the solution.With higher concentrations of acid and tetrahydro- borate(II1) the reaction becomes more violent and the amount of hydrogen formed increases substantially. Under these conditions both the concentration of selenium hydride in the gas bubble and the residence time of the gas bubble in the solution may be reduced, which is in agreement with the observations. Finally, a change in the colour of the solution to reddish is observed on addition of tetrahydroborate(II1). This is pro- bably due to a reduction of Cu(I1) to Cu(I), which means a depletion in the interfering ions. This depletion should be more pronounced and the related interferences less pronoun- ced with more highly concentrated tetrahydroborate(II1) solutions, which was also confirmed by the experiment.Conclusion The interferences of cobalt, copper and nickel on the determination of arsenic, and the interference of cobalt and nickel on selenium, are caused by a preferential reduction of the metal ion to a lower oxidation state or to the metal. The finely dispersed precipitate then captures and decomposes the hydride formed in a secondary reaction. The interference of copper on selenium is caused by a reaction of selenium hydride with free copper(I1) ions to form fairly insoluble copper selenide. One way to extend the range of interference- free determination of arsenic and selenium in the presence of transition metals is to increase the acid concentration, and 5 moll-1 HC1 has been found to be favourable.An additional improvement is obtained when the concentration of the sodium tetrahydroborate(II1) solution is reduced to 0.5% mlV when the interference is caused by a reduced species, e.g., the precipitated metal. Interferent concentrations of 100 mg 1-1 or more can be tolerated for all analyte - interferent combinations investi- gated, except for copper in the determination of selenium, when the proposed acid and tetrahydroborate(II1) concentra-JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1986, VOL. 1 27 tions are used. The only disadvantage appears to be that the sensitivity for arsenic and selenium is lower under these conditions than at lower acid and higher tetrahydroborate(II1) concentrations. This lower sensitivity, however, is not due to less efficient hydride formation but to less efficient atomisa- tion of the hydrides. A higher atomisation efficiency would further increase the usefulness of the proposed measures. References 1. 2. 3. Smith, A. E., Analyst, 1975, 100, 300. Kirkbright, G. F., and Taddia, M., Anal. Chim. Acta, 1978, 100, 145. Meyer, A., Hofer, Ch., Tolg, G., Raptis, S., and Knapp, G., Fresenius Z . Anal. Chem., 1979, 296, 337. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. McDaniel, M., Shendrikar, A. D., Reiszner, K. D., and West, P. W., Anal. Chem., 1976, 48, 2240. Vijan, P. N., and Wood, G. R., Talanta, 1976, 23, 89. Verlinden, M., Baart, J . , and Deelstra, H., Talanta, 1980, 27, 633. Sturman, B. T., Appl. Spectrosc., 1985, 39, 48. Branch, C. H., and Hutchison, D., Analyst, 1985, 110, 163. Thompson, K. C., and Thomerson, D. R., Analyst, 1974, 99, 595. Evans, W. H., Jackson, F. J., and Dellar, D., Analyst, 1979, 104, 16. Dittrich, K . , Vorberg, B., and Wolthers, H., Talanta, 1979,26, 747. Welz, B., and Melcher, M., Anal. Chim. Acta, 1981, 131, 17. Welz, B., and Melcher, M., Analyst, 1983, 108, 213. Krivan, V., Petrick, K., Welz, B., and Melcher, M., Anal. Chem., 1985, 57, 1703. 4. Welz, B., and Melcher, M., Analyst, 1984, 109, 569. NOTE-References 4,5 and 18 are to Parts 2, 3 and 1 of this series, 5. Welz, B., and Melcher, M., Analyst, 1984, 109, 577. 6. Welz, B., and Melcher, M., Spectrochim. Acta, Part B, 1981, 36, 439. Paper 35/21 7. Welz, B., and Melcher, M., Vom Wasser, 1982, 59, 407. Received August 7th, 1985 8. Welz, B., and Melcher, M., Vom Wasser, 1984, 62, 137. Accepted August 23rd, 1985 respectively.