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Determination of selenium in biological tissue samples rich in phosphorus using electrothermal atomization with Zeeman-effect background correction and (NH4)3RhCl6+citric acid as a mixed chemical modifier1
Spectrochimica Acta Part B 53 (1998) 1381–1389
Determination of selenium in biological tissue samples rich in phosphorus using electrothermal atomization with Zeeman-effect background correction and (NH 4) 3RhCl 6 + citric acid as a mixed chemical modifier1 Li Mei a, Ni Zhe-ming b,*, Rao Zhu a a
b
National Research Center for Geoanalysis, Beijing 100037, China Research Center for Eco-Environmental Science, Chinese Academy of Sciences, P.O. Box 2871, Beijing 100085, China Received 15 October 1997; accepted 30 January 1998
Abstract Spectral interferences from phosphorus on the determination of selenium in biological tissue materials were not observed when a Zeeman-effect background correction was used with rhodium as a chemical modifier. A suppression effect on the selenium signal resulted when the concentration of phosphorus present was greater than 1.0 mg ml −1. Rhodium was found to be more effective than palladium in overcoming the phosphate interference. Analytical procedures for the direct determination of trace selenium in standard reference materials by graphite furnace atomic absorption spectrometry following sample dissolution in nitric acid and hydrogen peroxide using a microwave oven has been described. The results obtained agreed favourably with the certified values. q 1998 Elsevier Science B.V. All rights reserved Keywords: Se; Biological tissue; Electrothermal atomic absorption spectrometry; Rhodium modifier; Zeeman background correction; Phosphate interference
1. Introduction It has been well recognized that electrothermal atomic absorption spectrometry (ETAAS) is one of the most sensitive and selective analytical methods for the determination of selenium in biological samples [1–5]. However, the technique is subject to spectral interferences especially due to the presence of a relatively large amount of phosphorus in the samples. Since biological samples are usually rich in phosphorus, a number of studies have been carried * Corresponding author 1 Special Issue on China
out on the elimination of phosphorus interferences [6–10]. Baulaugh et al. [8] reported that platinum catalyses the dissociation of phosphorus compounds into atoms so it can be used to minimize the phosphorus interference. Kumpulainen and Saarela [9] employed 0.75% Pt(IV) + 0.25% Mg(NO 3) 2 as the chemical modifier for the determination of selenium in foods and diets. The modifiers were utilized for the reduction of background absorption and spectral interferences due to molecular phosphorus species in the gas phase and enhance the stability of selenium. Radziuk and Thomassen [10] studied in detail the role of palladium and nickel on the thermal stabilization and atomization of selenium and phosphorus.
0584-8547/98/$19.00 q 1998 Elsevier Science B.V. All rights reserved PII S 0 58 4- 8 54 7 (9 8 )0 0 12 0 -7
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L. Mei et al./Spectrochimica Acta Part B 53 (1998) 1381–1389
They found that the most effective way of eliminating spectral interferences was to convert essentially all phosphorus into the stabilized species by the addition of sufficient palladium, e.g. 300–500 mg. However, the relatively large amount of modifier used might accumulate in the graphite tube and cause a memory effect with increasing numbers of determinations. In practical analysis the use of much less modifier is preferable in case the interference of phosphorus can be eliminated. The aim of the present work is to propose a chemical modifier which can be used for selenium determination in biological samples containing phosphorus.
2. Experimental 2.1. Equipment A Hitachi Z-8200 Zeeman atomic absorption spectrometer with a GA-3 graphite furnace was used for all AA measurements. Data collection and processing were carried out by means of an IBM compatible 80386 DX computer with Z-8200 windows-based application program; data and absorbance profiles were output through a Canon NJC-120s printer. A high intensity selenium hollow cathode lamp from the General Research Institute for Non-Ferrous Metals, Beijing, China, operated at 6 mA, was used. The wavelength and slit width were set at 193.7 and 1.3 nm, respectively. 2.2. Reagents A selenium stock solution 1.0 mg ml −1 Se was prepared by dissolving adequate amounts of Na 2SeO 3 (Beijing Chemical Ltd., China) in distilled water. Working solutions were prepared by serial dilution of the stock solution immediately before use. A rhodium solution containing 2 mg ml −1 Rh was prepared by dissolving triammonium rhodium extrapure, hexachloride ((NH 4) 3RhCl 6·1.5H 2O, Tianjin, China) in 6% citric acid solution. A palladium solution containing 2 mg ml −1 Pd was prepared by dissolving diammonium palladium hexachloride ((NH 4) 2PdCl 6, General Research Institute for Non-ferrous Metals, Beijing, China) in 6% citric acid solution.
Ammonium dihydrogenphosphate solution (Beijing Chemical Works), 10 mg ml −1 P, was prepared by dissolving the salt in water. Calcium nitrate solution, 10 mg Ca 2+ ml −1, was obtained by dissolving calcium carbonate (standard material from National Research Center for Certified Reference Materials) in dilute nitric acid. The phosphate and calcium solutions were mixed in a mole ratio of P:Ca 2+ = 1:1 before use. All chemicals were of analytical reagent grade in which no selenium was detected. 2.3. Biological reference materials The Mussel Standard Reference Material (GBW 08571) certified by the Research Center for EcoEnvironmental Sciences, Chinese Academy of Sciences and the Second Institute of Oceanology, National Bureau of Oceanology, contains 1.35% phosphorus. The certified value of selenium is 3.65 6 0.09 mg mg −1. The Shrimp Standard Reference Material (GBW 08572) certified by the Research Institute of Food Monitoring, Ministry of Commerce, China had a phosphorus content of 0.845% and the certified value for selenium is 1.52 6 0.20 mg g −1. The Oyster Standard Sample ESA-2 (GZBL 1900295) certified by China Monitoring Station, Beijing, contains 0.68% phosphorus and 4.37 6 0.58 mg g −1 selenium. The Liver Standard Reference Material (GBW 08551) certified by the Shanghai Research Institute of Atomic Nuclear Energy, Chinese Academy of Sciences, contains 1.3% phosphorus and 0.94 6 0.03 mg g −1 selenium. The selenium values of the above standard reference materials were certified by using the analytical results obtained by different analytical techniques, such as instrumental nuclear activation analysis, atomic absorption, polarography and spectrofluorometry. 2.4. Sample decomposition Approximately 0.1–0.2 g of dry mass of reference material was digested with 2 ml of nitric acid (65%) and 0.5 ml of H 2O 2 (30%) in 7 ml PTFE vials using a microwave oven (Milstone 1200). The module was
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L. Mei et al./Spectrochimica Acta Part B 53 (1998) 1381–1389 Table 1 Temperature program Stage no.
Stage
1 2 3 4
Dry Dry Pyrolysis Pyrolysis
5 6 7
Atom. Clean Cool
a b
Start temp. (8C) 50 100 140 400 2200 2600
End temp. (8C)
Ramp (s)
Hold (s)
Gas flow (ml min −1)
100 140 400 1000 a 800 b 2200 2600
20 10 10 20
10 10 10 10
200 200 200 200
10 4 10
200 200
Pyrolysis temperature for Se with Rh. Pyrolysis temperature for Se with Pd.
rated at 1200 W and microprocessor controlled from 1 to 100%. The heating program was set at 120 W, 2 min; 300 W, 2 min; 400 W, 2 min; 600 W, 5 min. The digested material was then cooled and evaporated to near dryness on a hot plate below 1008C. A few drops of citric acid (6%) were added to dissolve the residue and the solution was diluted to 10 ml with distilled water. 2.5. Furnace program A volume of 10 ml of the sample solution was injected into a pyrolytically coated graphite tube on a pyrolytic platform followed by adding 10 ml of rhodium or palladium solution. A 1 mg ml −1 solution of the modifier was used for the matrix free selenium and 2 mg ml −1 was used for selenium solution containing phosphorus or reference material. The furnace program is listed in Table 1. Integrated absorbance was measured throughout for quantification.
3. Results and discussion 3.1. Comparison of rhodium and palladium as a matrix modifier The purpose of using a chemical modifier is to stabilize a relatively volatile element in the furnace so that as much as the matrix can be driven out of the furnace before the atomization of the analyte takes place. The higher the maximum tolerable pyrolysis temperature an analyte can withstand, the less will
be the interference. Since rhodium has a higher melting point (19708C) than palladium (15508C), it is expected that the former can retain selenium to a higher pyrolysis temperature. Fig. 1(A,B) shows the results of pyrolysis and atomization of selenium. In the presence of rhodium (Fig. 1A(a)), the loss-free pyrolysis temperature found for selenium in aqueous solution is 12008C, while with palladium (Fig. 1A(b)) selenium can only be stabilized to a temperature of 10008C, lower than that with rhodium. The optimal atomization temperatures for selenium were in the range 1900–22008C when the pyrolysis temperatures were set at 12008C for rhodium and 10008C for palladium. The peak profiles of selenium at atomization temperatures below 22008C became broadened and less symmetrical so that a temperature of 22008C was selected at the atomization cycle. The pyrolysis curves for selenium in mussel digest is similar to that in matrix-free solution in the presence of rhodium (Fig. 1B(a)), but with palladium (Fig. 1B(b)) as a modifier selenium can only tolerate a temperature of 8008C in the mussel digest. Because rhodium was found to be more efficient in retaining selenium both in aqueous solution and sample digest, this modifier was used in the analysis of biological tissue samples. 3.2. Phosphate interference on selenium In the selenium determination the occurrence of spectral interferences especially caused by phosphate has been well documented [1,6,9]. The decomposition products of phosphate, PO and HPO, can be detected
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L. Mei et al./Spectrochimica Acta Part B 53 (1998) 1381–1389
L. Mei et al./Spectrochimica Acta Part B 53 (1998) 1381–1389
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Fig. 2. (A) Peak profile for (a) 1 ng of Se and (b) background absorption in aqueous solution in the presence of 20 mg of Rh. (B) As (A) with addition of 500 mg ml −1 of P.
by molecular non-thermal excitation spectrometry and it has been reported that these molecules are stable enough to exist at temperatures between 1000 and 20008C [11]. Welz and Schlemmer [1] stated that spectral interferences in the vicinity of the selenium resonance line caused by phosphate disappeared with a Zeeman-effect background correction. However, other workers found that the presence of 670 mg ml −1 of phosphorus caused a negative and then a positive Zeeman effect background corrected
signal at the 196.0 nm selenium line [9]. These discrepancies obtained experimentally may probably be due to different experimental conditions. According to our experimental results no spectral interferences due to 500 mg ml −1 of phosphorus were observed when 20 mg of rhodium or palladium was present, as shown in Figs 2 and 3. However, with palladium a suppression effect on the selenium signal occurred with an amount of phosphorus greater than 5 mg, while in the presence of rhodium selenium is less
Fig. 1. (A) Pyrolysis and atomization curves for 1 ng of Se in aqueous solution: (a) in presence of 10 mg of Rh; (b) in presence of 10 mg of Pd. (B) Pyrolysis and atomization curves for 1 ng of Se in mussel digest: (a) in presence of 20 mg of Rh; (b) in presence of 20 mg of Pd. Atomization temperature 22008C, pyrolysis temperature 10008C with Rh and 8008C with Pd.
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L. Mei et al./Spectrochimica Acta Part B 53 (1998) 1381–1389
Fig. 3. (A) Peak profile for (a) 1 ng of Se and (b) background absorption in aqueous solution in the presence of 20 mg of Pd. (B) As (A) with addition of 500 mg ml −1 of P.
effected by phosphorus (Fig. 4), because chemical interferences are generally minimized if a high pyrolysis temperature can be achieved via improved thermal stability. The mechanism of removing the interference from phosphate on selenium by rhodium or palladium is not clear. It was proposed that lantha-
num modifier formed a thermally stable adduct of phosphorus preventing the volatilization of PO which causes the spectral interferences in the vicinty of the selenium resonance line [11]. Stabilization of phosphorus as a palladium adduct [12] or through formation of several Pd xPy phases has also been sug-
Table 2 Results of sample analysis Standard reference material
Mussel (GBW 08571) Shrimp (GBW 08572) Oyster ESA-2 (GZBL 19002-95) Liver (GBW 08551) a
Mean 6 SD (n = 5).
Se found (mg g −1) a This work
Certified value
3.60 6 0.27 1.46 6 0.17 4.40 6 0.16 0.96 6 0.17
3.69 6 0.09 1.52 6 0.02 4.37 6 0.58 0.94 6 0.03
L. Mei et al./Spectrochimica Acta Part B 53 (1998) 1381–1389
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Fig. 4. Effect of phosphorus on the recovery of selenium with addition of 20 mg of chemical modifier: (a) Rh and (b) Pd.
gested [10,13]. Similar to the above speculations, a reaction may occur between rhodium and phosphorus forming an adduct or compound in the furnace so that the co-volatilization of selenium with PO at the preatomization cycle can be prevented. In addition to the co-volatilization loss, the suppression effect on selenium caused by the presence of phosphorus may also stem from the intercalated phosphorus formed on the inner surface of the graphite tube after heating the graphite tube at a relatively high temperature so that the active sites of carbon were reduced causing deactivation of the graphite surface, thus lowering the atomization efficiency of selenium. According to our observations when a solution containing phosphorus in the form of phosphate greater than 1.5 mg ml −1 had been injected into the furnace several times, the selenium absorbance continued to decrease with increasing number of replicate measurements. This problem can be remedied by firing the furnace at 26008C to remove the intercalated phosphorus. In real sample analysis it is advisable to make the phosphorus concentration in the solution not greater than 1.0 mg ml −1. The peak profiles for selenium in mussel digest are shown in Fig. 5.
3.3. Sample analysis The biological tissue samples were decomposed by microwave heating using nitric acid and hydrogen peroxide. Nitric acid was used because it absorbs microwave energy better than other mineral acids, implying complete digestion of organoselenium compounds could be achieved [3]. The addition of perchloric acid or hydrofluoric acid was avoided because these acids will cause interferences in the furnace. The samples after decomposition with a mixture of nitric acid and hydrogen peroxide remained clear in solution. In order to test the validity of the recommended method, the analysis of certified reference materials were carried out. The results are summarized in Table 2. 3.4. Analytical figures of merit The detection limit for selenium defined as the mass based on the variability of the blank (i.e. 3j) was 25 pg (n = 11) when rhodium was used as a modifier. The characteristic mass, defined as the mass of analyte which produces an integrated absorbance signal of
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L. Mei et al./Spectrochimica Acta Part B 53 (1998) 1381–1389
Fig. 5. (A) Peak profiles of Se in mussel digest in the presence of (a) 20 mg of Rh; (b) background absorption. (B) As (A) with addition of 1 ng Se.
0.0044s was found to be 20 pg. The regression equation y = 0.2296x − 0.0002 (where y = integrated absorbance and x = analyte mass in ng) with a regression coefficient of 0.9973 was obtained from the calibration graph.
by 10 mg of phosphorus on the determination of 1 ng of selenium. The method has been applied to the determination of trace selenium in biological samples containing phosphorus. References
4. Conclusion A mixture of (NH 4) 3RhCl 6 + citric acid can be used as a chemical modifier for the removal of phosphorus interference in selenium determination. The results obtained are free from spectral interference, the background absorption observed is much lower than that observed previously. Only 20 mg of rhodium is required to minimize the suppression effect caused
[1] B. Welz, G. Schlemmer, Determination of arsenic, selenium and cadmium in marine biological tissue samples using a stabilized temperature platform furnace and comparing deuterium arc with Zeeman-effect background correction atomic absorption spectrometry, J. Anal. At. Spectrom. 1 (1986) 119–124. [2] M.B. Knowles, K.G. Brodia, Determination of selenium in blood by Zeeman graphite furnace atomic absorption spectrometry using a palladium–ascorbic acid chemical modifier, J. Anal. At. Spectrom. 3 (1988) 511–516.
L. Mei et al./Spectrochimica Acta Part B 53 (1998) 1381–1389 [3] I. Harrison, D. Littlejohn, G.S. Fell, Determination of selenium in human hair and nail by electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 10 (1995) 215–219. [4] M. Deaker, W. Maher, Determination of selenium in seleno compounds and marine biological tissues using electrothermal atomization atomic absorption spectrometry, J. Anal. At. Spectrom. 10 (1995) 423–431. [5] H. Docekalova, B. Docekal, J. Komarek, I. Novotny, Determination of selenium by electrothermal atomic absorption spectrometry: 1. Chemical modifiers, J. Anal. At. Spectrom. 6 (1991) 661–668. [6] F.I. Fernandez, R. Giddings, Elimination of spectral interferences using Zeeman effect background correction, At. Spectrosc. 3 (1982) 61–65. [7] K. Saeed, Y. Thomassen, Spectral interferences from phosphate matrices in the determination of arsenic, antimony, selenium and tellurium by electrothermal atomic absorption spectrometry, Anal. Chim. Acta 130 (1981) 281–287. [8] J. Bauslaugh, B. Radziuk, K. Saeed, Y. Thomassen, Reduction of effects of structured non-specific absorption in the determination of arsenic and selenium by electrothermal atomic absorption spectrometry, Anal. Chim. Acta 165 (1984) 149– 157.
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[9] K. Kumpulainen, K.-E. Saarela, Determination of selenium in staple foods and total diets by electrothermal atomic absorption spectrometry without solvent extraction, J. Anal. At. Spectrom. 7 (1992) 165–170. [10] B. Radziuk, Y. Thomassen, Chemical modification and spectral interferences in selenium determination using Zeeman-effect electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 7 (1992) 397–403. [11] K. Dittrich, H. Fuchs, Analytical applications of furnace atomic non-thermal excitation spectrometry (FANES) and molecular non-thermal excitation spectrometry (MONES): 5. Study of the MONES of PO an HPO for the determination of trace amounts of phosphorus, J. Anal. At. Spectrom. 5 (1990) 39–43. [12] R.E. Sturgeon, S.N. Willie, Excitation and detection of molecular species with furnace atomization plasma emission spectrometry, J. Anal. At. Spectrom. 7 (1992) 339–342. [13] H.J. Heinrich, G. Emrich, E. Schierhorn, Investigation of the stabilization mechanism for arsenic by palladium in graphite furnace AAS, in: D.L. Tsalev (Ed.), 26th CSI Selected Papers, vol. 7, 1989, p. 239.
Determination of selenium in biological tissue samples rich in phosphorus using electrothermal atomization with Zeeman-effect background correction and (NH 4) 3RhCl 6 + citric acid as a mixed chemical modifier1 Li Mei a, Ni Zhe-ming b,*, Rao Zhu a a
b
National Research Center for Geoanalysis, Beijing 100037, China Research Center for Eco-Environmental Science, Chinese Academy of Sciences, P.O. Box 2871, Beijing 100085, China Received 15 October 1997; accepted 30 January 1998
Abstract Spectral interferences from phosphorus on the determination of selenium in biological tissue materials were not observed when a Zeeman-effect background correction was used with rhodium as a chemical modifier. A suppression effect on the selenium signal resulted when the concentration of phosphorus present was greater than 1.0 mg ml −1. Rhodium was found to be more effective than palladium in overcoming the phosphate interference. Analytical procedures for the direct determination of trace selenium in standard reference materials by graphite furnace atomic absorption spectrometry following sample dissolution in nitric acid and hydrogen peroxide using a microwave oven has been described. The results obtained agreed favourably with the certified values. q 1998 Elsevier Science B.V. All rights reserved Keywords: Se; Biological tissue; Electrothermal atomic absorption spectrometry; Rhodium modifier; Zeeman background correction; Phosphate interference
1. Introduction It has been well recognized that electrothermal atomic absorption spectrometry (ETAAS) is one of the most sensitive and selective analytical methods for the determination of selenium in biological samples [1–5]. However, the technique is subject to spectral interferences especially due to the presence of a relatively large amount of phosphorus in the samples. Since biological samples are usually rich in phosphorus, a number of studies have been carried * Corresponding author 1 Special Issue on China
out on the elimination of phosphorus interferences [6–10]. Baulaugh et al. [8] reported that platinum catalyses the dissociation of phosphorus compounds into atoms so it can be used to minimize the phosphorus interference. Kumpulainen and Saarela [9] employed 0.75% Pt(IV) + 0.25% Mg(NO 3) 2 as the chemical modifier for the determination of selenium in foods and diets. The modifiers were utilized for the reduction of background absorption and spectral interferences due to molecular phosphorus species in the gas phase and enhance the stability of selenium. Radziuk and Thomassen [10] studied in detail the role of palladium and nickel on the thermal stabilization and atomization of selenium and phosphorus.
0584-8547/98/$19.00 q 1998 Elsevier Science B.V. All rights reserved PII S 0 58 4- 8 54 7 (9 8 )0 0 12 0 -7
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L. Mei et al./Spectrochimica Acta Part B 53 (1998) 1381–1389
They found that the most effective way of eliminating spectral interferences was to convert essentially all phosphorus into the stabilized species by the addition of sufficient palladium, e.g. 300–500 mg. However, the relatively large amount of modifier used might accumulate in the graphite tube and cause a memory effect with increasing numbers of determinations. In practical analysis the use of much less modifier is preferable in case the interference of phosphorus can be eliminated. The aim of the present work is to propose a chemical modifier which can be used for selenium determination in biological samples containing phosphorus.
2. Experimental 2.1. Equipment A Hitachi Z-8200 Zeeman atomic absorption spectrometer with a GA-3 graphite furnace was used for all AA measurements. Data collection and processing were carried out by means of an IBM compatible 80386 DX computer with Z-8200 windows-based application program; data and absorbance profiles were output through a Canon NJC-120s printer. A high intensity selenium hollow cathode lamp from the General Research Institute for Non-Ferrous Metals, Beijing, China, operated at 6 mA, was used. The wavelength and slit width were set at 193.7 and 1.3 nm, respectively. 2.2. Reagents A selenium stock solution 1.0 mg ml −1 Se was prepared by dissolving adequate amounts of Na 2SeO 3 (Beijing Chemical Ltd., China) in distilled water. Working solutions were prepared by serial dilution of the stock solution immediately before use. A rhodium solution containing 2 mg ml −1 Rh was prepared by dissolving triammonium rhodium extrapure, hexachloride ((NH 4) 3RhCl 6·1.5H 2O, Tianjin, China) in 6% citric acid solution. A palladium solution containing 2 mg ml −1 Pd was prepared by dissolving diammonium palladium hexachloride ((NH 4) 2PdCl 6, General Research Institute for Non-ferrous Metals, Beijing, China) in 6% citric acid solution.
Ammonium dihydrogenphosphate solution (Beijing Chemical Works), 10 mg ml −1 P, was prepared by dissolving the salt in water. Calcium nitrate solution, 10 mg Ca 2+ ml −1, was obtained by dissolving calcium carbonate (standard material from National Research Center for Certified Reference Materials) in dilute nitric acid. The phosphate and calcium solutions were mixed in a mole ratio of P:Ca 2+ = 1:1 before use. All chemicals were of analytical reagent grade in which no selenium was detected. 2.3. Biological reference materials The Mussel Standard Reference Material (GBW 08571) certified by the Research Center for EcoEnvironmental Sciences, Chinese Academy of Sciences and the Second Institute of Oceanology, National Bureau of Oceanology, contains 1.35% phosphorus. The certified value of selenium is 3.65 6 0.09 mg mg −1. The Shrimp Standard Reference Material (GBW 08572) certified by the Research Institute of Food Monitoring, Ministry of Commerce, China had a phosphorus content of 0.845% and the certified value for selenium is 1.52 6 0.20 mg g −1. The Oyster Standard Sample ESA-2 (GZBL 1900295) certified by China Monitoring Station, Beijing, contains 0.68% phosphorus and 4.37 6 0.58 mg g −1 selenium. The Liver Standard Reference Material (GBW 08551) certified by the Shanghai Research Institute of Atomic Nuclear Energy, Chinese Academy of Sciences, contains 1.3% phosphorus and 0.94 6 0.03 mg g −1 selenium. The selenium values of the above standard reference materials were certified by using the analytical results obtained by different analytical techniques, such as instrumental nuclear activation analysis, atomic absorption, polarography and spectrofluorometry. 2.4. Sample decomposition Approximately 0.1–0.2 g of dry mass of reference material was digested with 2 ml of nitric acid (65%) and 0.5 ml of H 2O 2 (30%) in 7 ml PTFE vials using a microwave oven (Milstone 1200). The module was
1383
L. Mei et al./Spectrochimica Acta Part B 53 (1998) 1381–1389 Table 1 Temperature program Stage no.
Stage
1 2 3 4
Dry Dry Pyrolysis Pyrolysis
5 6 7
Atom. Clean Cool
a b
Start temp. (8C) 50 100 140 400 2200 2600
End temp. (8C)
Ramp (s)
Hold (s)
Gas flow (ml min −1)
100 140 400 1000 a 800 b 2200 2600
20 10 10 20
10 10 10 10
200 200 200 200
10 4 10
200 200
Pyrolysis temperature for Se with Rh. Pyrolysis temperature for Se with Pd.
rated at 1200 W and microprocessor controlled from 1 to 100%. The heating program was set at 120 W, 2 min; 300 W, 2 min; 400 W, 2 min; 600 W, 5 min. The digested material was then cooled and evaporated to near dryness on a hot plate below 1008C. A few drops of citric acid (6%) were added to dissolve the residue and the solution was diluted to 10 ml with distilled water. 2.5. Furnace program A volume of 10 ml of the sample solution was injected into a pyrolytically coated graphite tube on a pyrolytic platform followed by adding 10 ml of rhodium or palladium solution. A 1 mg ml −1 solution of the modifier was used for the matrix free selenium and 2 mg ml −1 was used for selenium solution containing phosphorus or reference material. The furnace program is listed in Table 1. Integrated absorbance was measured throughout for quantification.
3. Results and discussion 3.1. Comparison of rhodium and palladium as a matrix modifier The purpose of using a chemical modifier is to stabilize a relatively volatile element in the furnace so that as much as the matrix can be driven out of the furnace before the atomization of the analyte takes place. The higher the maximum tolerable pyrolysis temperature an analyte can withstand, the less will
be the interference. Since rhodium has a higher melting point (19708C) than palladium (15508C), it is expected that the former can retain selenium to a higher pyrolysis temperature. Fig. 1(A,B) shows the results of pyrolysis and atomization of selenium. In the presence of rhodium (Fig. 1A(a)), the loss-free pyrolysis temperature found for selenium in aqueous solution is 12008C, while with palladium (Fig. 1A(b)) selenium can only be stabilized to a temperature of 10008C, lower than that with rhodium. The optimal atomization temperatures for selenium were in the range 1900–22008C when the pyrolysis temperatures were set at 12008C for rhodium and 10008C for palladium. The peak profiles of selenium at atomization temperatures below 22008C became broadened and less symmetrical so that a temperature of 22008C was selected at the atomization cycle. The pyrolysis curves for selenium in mussel digest is similar to that in matrix-free solution in the presence of rhodium (Fig. 1B(a)), but with palladium (Fig. 1B(b)) as a modifier selenium can only tolerate a temperature of 8008C in the mussel digest. Because rhodium was found to be more efficient in retaining selenium both in aqueous solution and sample digest, this modifier was used in the analysis of biological tissue samples. 3.2. Phosphate interference on selenium In the selenium determination the occurrence of spectral interferences especially caused by phosphate has been well documented [1,6,9]. The decomposition products of phosphate, PO and HPO, can be detected
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L. Mei et al./Spectrochimica Acta Part B 53 (1998) 1381–1389
L. Mei et al./Spectrochimica Acta Part B 53 (1998) 1381–1389
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Fig. 2. (A) Peak profile for (a) 1 ng of Se and (b) background absorption in aqueous solution in the presence of 20 mg of Rh. (B) As (A) with addition of 500 mg ml −1 of P.
by molecular non-thermal excitation spectrometry and it has been reported that these molecules are stable enough to exist at temperatures between 1000 and 20008C [11]. Welz and Schlemmer [1] stated that spectral interferences in the vicinity of the selenium resonance line caused by phosphate disappeared with a Zeeman-effect background correction. However, other workers found that the presence of 670 mg ml −1 of phosphorus caused a negative and then a positive Zeeman effect background corrected
signal at the 196.0 nm selenium line [9]. These discrepancies obtained experimentally may probably be due to different experimental conditions. According to our experimental results no spectral interferences due to 500 mg ml −1 of phosphorus were observed when 20 mg of rhodium or palladium was present, as shown in Figs 2 and 3. However, with palladium a suppression effect on the selenium signal occurred with an amount of phosphorus greater than 5 mg, while in the presence of rhodium selenium is less
Fig. 1. (A) Pyrolysis and atomization curves for 1 ng of Se in aqueous solution: (a) in presence of 10 mg of Rh; (b) in presence of 10 mg of Pd. (B) Pyrolysis and atomization curves for 1 ng of Se in mussel digest: (a) in presence of 20 mg of Rh; (b) in presence of 20 mg of Pd. Atomization temperature 22008C, pyrolysis temperature 10008C with Rh and 8008C with Pd.
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L. Mei et al./Spectrochimica Acta Part B 53 (1998) 1381–1389
Fig. 3. (A) Peak profile for (a) 1 ng of Se and (b) background absorption in aqueous solution in the presence of 20 mg of Pd. (B) As (A) with addition of 500 mg ml −1 of P.
effected by phosphorus (Fig. 4), because chemical interferences are generally minimized if a high pyrolysis temperature can be achieved via improved thermal stability. The mechanism of removing the interference from phosphate on selenium by rhodium or palladium is not clear. It was proposed that lantha-
num modifier formed a thermally stable adduct of phosphorus preventing the volatilization of PO which causes the spectral interferences in the vicinty of the selenium resonance line [11]. Stabilization of phosphorus as a palladium adduct [12] or through formation of several Pd xPy phases has also been sug-
Table 2 Results of sample analysis Standard reference material
Mussel (GBW 08571) Shrimp (GBW 08572) Oyster ESA-2 (GZBL 19002-95) Liver (GBW 08551) a
Mean 6 SD (n = 5).
Se found (mg g −1) a This work
Certified value
3.60 6 0.27 1.46 6 0.17 4.40 6 0.16 0.96 6 0.17
3.69 6 0.09 1.52 6 0.02 4.37 6 0.58 0.94 6 0.03
L. Mei et al./Spectrochimica Acta Part B 53 (1998) 1381–1389
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Fig. 4. Effect of phosphorus on the recovery of selenium with addition of 20 mg of chemical modifier: (a) Rh and (b) Pd.
gested [10,13]. Similar to the above speculations, a reaction may occur between rhodium and phosphorus forming an adduct or compound in the furnace so that the co-volatilization of selenium with PO at the preatomization cycle can be prevented. In addition to the co-volatilization loss, the suppression effect on selenium caused by the presence of phosphorus may also stem from the intercalated phosphorus formed on the inner surface of the graphite tube after heating the graphite tube at a relatively high temperature so that the active sites of carbon were reduced causing deactivation of the graphite surface, thus lowering the atomization efficiency of selenium. According to our observations when a solution containing phosphorus in the form of phosphate greater than 1.5 mg ml −1 had been injected into the furnace several times, the selenium absorbance continued to decrease with increasing number of replicate measurements. This problem can be remedied by firing the furnace at 26008C to remove the intercalated phosphorus. In real sample analysis it is advisable to make the phosphorus concentration in the solution not greater than 1.0 mg ml −1. The peak profiles for selenium in mussel digest are shown in Fig. 5.
3.3. Sample analysis The biological tissue samples were decomposed by microwave heating using nitric acid and hydrogen peroxide. Nitric acid was used because it absorbs microwave energy better than other mineral acids, implying complete digestion of organoselenium compounds could be achieved [3]. The addition of perchloric acid or hydrofluoric acid was avoided because these acids will cause interferences in the furnace. The samples after decomposition with a mixture of nitric acid and hydrogen peroxide remained clear in solution. In order to test the validity of the recommended method, the analysis of certified reference materials were carried out. The results are summarized in Table 2. 3.4. Analytical figures of merit The detection limit for selenium defined as the mass based on the variability of the blank (i.e. 3j) was 25 pg (n = 11) when rhodium was used as a modifier. The characteristic mass, defined as the mass of analyte which produces an integrated absorbance signal of
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Fig. 5. (A) Peak profiles of Se in mussel digest in the presence of (a) 20 mg of Rh; (b) background absorption. (B) As (A) with addition of 1 ng Se.
0.0044s was found to be 20 pg. The regression equation y = 0.2296x − 0.0002 (where y = integrated absorbance and x = analyte mass in ng) with a regression coefficient of 0.9973 was obtained from the calibration graph.
by 10 mg of phosphorus on the determination of 1 ng of selenium. The method has been applied to the determination of trace selenium in biological samples containing phosphorus. References
4. Conclusion A mixture of (NH 4) 3RhCl 6 + citric acid can be used as a chemical modifier for the removal of phosphorus interference in selenium determination. The results obtained are free from spectral interference, the background absorption observed is much lower than that observed previously. Only 20 mg of rhodium is required to minimize the suppression effect caused
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