Analyst, July 1998, Vol. 123 ( ) Jean Yves Cabon *a and Wolfgang Erler b

Transcription

1 Analyst, July 1998, Vol. 123 ( ) 1565 Determination of selenium species in seawater by flow injection hydride generation in situ trapping followed by electrothermal atomic absorption spectrometry Jean Yves Cabon *a and Wolfgang Erler b a Laboratoire de Chimie Analytique, UMR CNRS 6521-UBO 6, Avenue Le Gorgeu, BP 809, Brest-Cedex, France b Bodenseewerk Perkin-Elmer GmbH, D Überlingen, Germany In this study, Se IV was determined in seawater by flow injection hydride generation, trapping within the graphite furnace, followed by electrothermal atomic absorption spectrometry. The optimized protocol permitted a preconcentration factor of 1000 lowering the detection limit of Se IV in seawater to about 1.5 ng l 21 for a 25 ml sample volume. Because this technique is specific to Se IV, various physico-chemical treatments leading to the conversion of other chemical forms into Se IV were investigated. Thermal and UV irradiation treatments of seawater samples in acidic or basic medium permitted the conversion of Se 2II -selenomethionine and Se VI to Se IV. Based on the different responses of selenium species towards the photochemical treatments, an operational speciation model of selenium species in seawater is proposed. According to our analytical protocol, 21 ng l 21 Se VI, 10 ng l 21 Se IV and 13 ng l 21 organic Se 2II species were determined in a coastal surface seawater collected near Brest. Keywords: Selenium; speciation; seawater; hydride generation; electrothermal atomic absorption spectrometry The toxicity and essential nature of Se in natural waters depends on its concentration but also on its chemical form. 1 5 Se is present in seawater at concentration levels generally below 100 ng l 21 and under various chemical forms: Se VI, Se IV and Se 2II organically bound Due to its low concentration, Se cannot be determined by direct electrothermal atomic absorption spectrometry and a preconcentration factor about 1000 is generally necessary. A widely used preconcentration step is based on batch or flow injection hydride generation followed by atomic absorption measurements in a heated quartz cell or after preconcentration by collection of Se in the atomizer; 15 this technique is specific to Se IV. Consequently, Se VI, organic Se 2II and, obviously total selenium, can be determined only after their conversion to Se IV. For natural waters, various procedures to transform Se VI and organic Se 2II to Se IV have been proposed such as thermal or photochemical treatments but these treatments were very dependent on medium and experimental conditions and sometimes contradictory In the first part of this study, we optimized the experimental conditions for the determination of Se IV by flow injection hydride generation, in situ trapping onto iridium followed by ETAAS; in the second part we investigated different physicochemical treatments to transform Se VI and Se 2II -selenomethionine taken as organic model to Se IV in water or seawater samples. From our results, an experimental protocol permitting an operational differentiation of selenium species in seawater was proposed. Experimental Sodium borohydride solutions were prepared daily from Merck (Darmstadt, Germany) or Acros products (Geel, Belgium). Suprapur hydrochloric acid and sodium hydroxide from Merck were used. The solutions of Se IV, Se VI and Se II were respectively obtained from sodium selenite (Merck), sodium selenate (Sigma, Poole, Dorset, UK) and dl-selenomethionine (Fluka, Buchs, Switzerland). The Perkin-Elmer (Überlingen, Germany) FIAS-400 flow injection system was used according to the schematic diagram presented in Fig. 1. Samples acidified with hydrochloric acid were continuously pumped with blue yellow tubes and the tetrahydroborate solution with red red tubes at 100 rpm. Measurements were performed with a 4100ZL Perkin-Elmer spectrometer equipped with an AS-70 autosampler. A Perkin- Elmer quartz capillary was mounted in place of the PTFE capillary tubing of the sample dispenser arm. This modification permitted injection into the hot furnace of the gaseous hydride. A Perkin-Elmer quartz gas liquid separator was used. The light source used was a Se Perkin-Elmer electrodeless discharge lamp working at 350 ma. The analytical line was set at 196 nm with a 2 nm slit width. Perkin-Elmer end-capped tubes with integrated platforms were used. A portion (30 ml) of a 0.01 m H 2 IrCl m HNO 3 solution (ABCR product, Roth-Sochiel, Lauterbourg, France) were deposited onto the atomizer and pretreated at 1200 C during 30 s. The operation was repeated three times. The sheath gas was argon. Operating conditions were as follows: the selenium hydride carried with an argon flow was trapped into the atomizer at the selected temperature. Then, the atomization took place at 2400 C in the maximum power mode with the gas flow interrupted. A clean step at 2500 C during 3 s with 250 ml min 21 argon was then used. Thermal treatment A beaker containing 150 ml of 6 m hydrochloric acid was placed in a water bath heated at 85 C. After equilibration of the temperature, an aliquot (100 ml) of the Se IV, Se VI or Se 2II - Presented at The Third International Symposium on Speciation of Elements in Toxicology and in Environmental and Biological Sciences, Port Douglas, Australia, September 15 19, Fig. 1 Schematic diagram of the FI HG ETAAS system.

2 1566 Analyst, July 1998, Vol. 123 selenomethionine standard solutions was added to this solution under continuous agitation. At the selected time, an aliquot (100 ml) of this solution was taken and diluted in 0.2 m HCl solution (100 ml) for selenium determination by hydride generation electrothermal atomic absorption spectrometry. UV irradiation treatment The UV irradiation apparatus consisted of a 700 W mercury vapour lamp which was mounted in the centre of a circular rack surrounded by an aluminium cage. Se IV, Se VI and Se 2II - selenomethionine in acidic or basic solutions were irradiated in 100 ml quartz tubes. At the selected time, an aliquot (100 ml) of this solution was taken and diluted in 0.2 m HCl solution (100 ml) for selenium determination by hydride generation atomic absorption spectrometry. Results and discussion The selenium concentration in non-polluted seawater is well below the mg l 21 detection limit of ETAAS. In these conditions, selenium must be preconcentrated by a factor of about 1000 before its determination by ETAAS. The preconcentration factor to be used when using hydride generation in situ trapping followed by ETAAS is dependent on the detection limit of ETAAS but also on the preconcentration stage, efficiency of hydride generation, transport, trapping and eventually interference effects. As the detection limit is inversely proportional to the sample volume, optimization of spectroscopic, electrothermal parameters and flow injection conditions is necessary to minimize sample volume and time analysis. Optimization of the flow injection conditions permits, moreover, to minimize the concentration of reactants in order to reduce the risks of contamination and operating costs. temperatures > 2000 C due to the shortening of the integration window. The Perkin-Elmer recommended conditions is 2000 C. Optimization of flow injection conditions As shown in Fig. 3, generation of selenium hydride is dependent both on sodium borohydride and hydrochloric acid concentrations. Under our experimental conditions, 1 g l 21 NaBH 4 and 0.1 m hydrochoric acid concentrations are sufficient for an efficient generation of SeH 2. The minimum concentration of hydrochloric acid is about one order of magnitude smaller than the concentration recommended by the manufacturer. As also shown in Fig. 3, the trapping temperature of selenium onto the graphite furnace coated by iridium is optimum for temperatures about 300 C; a similar optimum temperature was obtained with the use of Ir stabilized with Zr or W. 36 Using our experimental device, the integrated absorbance was practically constant for a flow rate of the argon carrier gas between 50 and 200 ml min 21 and no important variation with the distance of the tip from the trapping surface was observed. Under optimized conditions, a detection limit of about 1.5 ng l 21 selenium was obtained for a 25 ml sample volume; a blank value of about 3 ng l 21 was also obtained. The time analysis was about 5 min. This analytical protocol was controlled with the measurement of the Selenium IV content of a NASS-4 seawater sample; using the standard addition method, Optimization of spectroscopic and electrothermal conditions The detection limit of selenium in ETAAS is dependent on spectroscopic, electrothermal parameters and mathematical treatments of the atomization signal. In the absence of a background absorption signal, the mass detection limit can be a priori calculated from the L vov relation; 33,34 for our 4100ZL Perkin-Elmer spectrometer, in the absence of a background absorption signal, the detection limit can be expressed as: 35 m L = E/36 3 t int 3 A (1/t int + 1/t boc ) 3 m/q A(1) E is the energy displayed on the spectrometer, t int is the integration time, t boc is the baseline offset compensation time, m is the mass of analyte and Q A the integrated absorbance. Both lamp current and slit width had an influence on the integrated absorbance and E value; in Fig. 2 is presented the variation of the mass detection limit versus the E parameter. As is clearly shown the best detection limit was obtained for the highest E values, i.e., when the lamp current and slit width were increased. When increasing the E value for this element the improvement of the baseline noise is dramatically improved whilst the increase of the integrated absorbance is less important; consequently, m L is greatly improved. The best detection limit was obtained for a 350 ma lamp current and a 2 nm slit width. According to eqn. (1), the atomization temperature, by modifying the integrated absorbance value and the integration time, had also an important influence on the detection limit. The variation of the mass detection limit versus the atomization temperature obtained in the presence of iridium used as a permanent modifier to trap selenium is presented in Fig. 2. As is also clearly shown, despite a decrease of the integrated absorbance when increasing the atomization temperature, the best detection limit was obtained for the highest atomization Fig. 2 Variation of the detection limit versus E and atomization temperature.

3 Analyst, July 1998, Vol we found a 16 ± 2 ng l 21 value (10 measurements) for a 18 ng l 21 information value. Conversion of Se IV and Se 2II -selenomethionine to Se IV Thermal treatment In the literature, the thermal treatment in 4 6 m HCl medium to transform Se VI to Se IV has been widely used; the rate of conversion depends on the temperature, the medium composition and the use of closed or open vessel; this analytical protocol was used to determine inorganic selenium Se IV and Se VI in natural waters 27,30 or seawater. 8,12,17 In this work, we examined the transformation of Se VI as SeO 4 but also Se 2II - selenomethionine to Se IV in water and seawater in 5 m HCl medium at 85 C in an open vessel. The percentage of conversion of Se VI and Se 2II -selenomethionine in seawater versus time is presented in Fig. 4; similar curves were obtained in water. As observed, at 85 C in an open vessel the conversion of Se VI to Se IV is quite rapid and complete. The conversion of Se 2II -selenomethionine to Se IV is much slower but after a long heating time Se 2II -selenomethionine could be also practically entirely converted to Se IV in water or seawater. Therefore, under our experimental conditions, not only inorganic selenium but also an important part of Se 2II -organic species can probably be transformed to Se IV in natural waters by using such a thermal treatment. As previously shown, 27,28 we could observe also an important decrease of Se IV concentration, spiked or resulting from the transformation of Se VI or Se 2II, when long heating times in a closed vessel were used; probably due to the back oxidation of Se IV consecutively to the generation of chlorine in 5 m HCl medium. 37 UV irradiation treatment As for the thermal treatment, the conversion of Se VI to Se IV by UV irradiation is highly dependent on ph and medium conditions and eventually degassing or not. Basic medium. It has been previously shown that degassing water samples dramatically modified the rate of conversion of Se VI to Se IV in basic medium at ph about Therefore, we practiced the conversion of Se VI SeO 4 and Se 2II -selenomethionine to Se IV in water and seawater at ph about 10 in borate medium, with or without degassing with nitrogen. As observed in Fig. 5, Se 2II -selenomethionine was rapidly transformed to Se IV in seawater (or water) and degassing with nitrogen did not modify the rate of conversion of Se 2II -selenomethionine to Se IV. On the contrary, the rate of conversion of Se VI to Se IV was rapid under nitrogen but severely decreased when the sample was not degassed. Under our experimental conditions, UV irradiation at ph about 10 with degassing permitted the determination of total selenium in water or seawater after a relatively short UV irradiation time. Acidic medium. The reduction of Se VI to Se IV by UV irradiation in acidic medium is also very dependent on experimental conditions and eventually the presence of organic substances. 24 In this work, we followed the behaviour of Se IV, Fig. 4 Conversion of Se VI and Se 2II -selenomethionine to Se IV at 85 C in seawater (5 m in HCl). Fig. 3 Influence of the trapping temperature on the integrated absorbance of Se. Influence of HCl concentration on the integrated absorbance of Se for various concentrations of NaBH 4 (g l 21 ). Fig. 5 Conversion of Se VI and Se 2II -selenomethionine to Se IV in borate medium, ph 10.6 under UV irradiation in seawater with and without degassing.

4 1568 Analyst, July 1998, Vol. 123 Se 2II -selenomethionine and Se VI under UV irradiation, in water and seawater, 0.01 m and 0.1 m in HCl. In 0.01 m HCl solution, it appeared that Se IV was not stable under UV irradiation and its concentration decreased dramatically with time (Fig. 6), probably also due to its back-oxidation to Se VI. Under these conditions, Se VI could practically not be converted to Se IV. Se 2II - selenomethionine was rapidly converted to Se IV as in basic medium; but as Se IV was not stable under UV irradiation treatment in this medium, its concentration decreased also dramatically for long UV irradiation times. Similar behaviour was observed in seawater medium but Se IV was more stable, probably due to a slower back-oxidation to Se VI (Fig. 6). Consequently, under these experimental conditions after a 30 min UV irradiation time, Se 2II -selenomethionine was converted to Se IV whilst only about 10% of Se VI SeO 4 was converted to Se IV. In 0.1 m HCl solution, Se IV was more stable under UV irradiation treatment in water (Fig. 6). Consequently, Se 2II - selenomethionine was converted to Se IV which was stable in this medium under UV irradiation treatment (Fig. 6). The rate of conversion of Se VI to Se IV in this medium was very slow and degassing did not improve the yield of conversion. The same behaviour was observed in seawater but the rate of conversion of Se 2II -selenomethionine and Se VI to Se IV was slightly lower. Speciation model By considering the different responses of Se VI, Se IV and Se 2II - selenomethionine to a thermal treatment in 5 m HCl or UV irradiation treatments in acidic or basic medium, we could propose an experimental protocol to differentiate selenium species in seawater. Only Se IV was determined in seawater without treatment. Total selenium could be determined in seawater after a thermal treatment in 5 m HCl at about 100 C or UV irradiation in basic medium with degassing under nitrogen. We used the second procedure because the first procedure led to an important dilution effect (longer analysis time) and to more important costs of analysis and risks of contaminations. In seawater 0.01 m in HCl, after 30 min UV irradiation time Se 2II -selenomethionine was converted to Se IV whilst only about 10% of Se VI was converted to Se IV. Consequently, from these three different experimental selenium determinations, Se IV, Se VI and Se 2II -selenomethionine could be determined in seawater with a good recovery. Application Using this experimental protocol, we determined selenium species in a surface coastal seawater sample collected near Brest. In seawater, various organic Se 2II species may be Fig. 7 Variation of Se IV concentration in a surface coastal seawater versus UV irradiation time in borate medium at ph 10 under nitrogen and in 0.01 m HCl medium. Fig. 6 Conversion of Se IV versus irradiation time in water and seawater at ph 2 and ph 1 and conversion of Se VI and Se 2II selenomethionine to Se IV.

5 Analyst, July 1998, Vol naturally present and may have a different kinetic response to the proposed UV irradiation treatments. In order to check our experimental speciation model, we followed the conversion of selenium species to Se IV in seawater under UV irradiation at ph 10 under nitrogen and in 0.01 m HCl medium. As observed in Fig. 7, as expected from Fig. 5 with UV irradiation at ph 10 under nitrogen, total selenium could be determined in seawater with the use of an UV irradiation time about 30 min. In 0.01 m HCl medium, the shape of the curve obtained showed that Se 2II organic species present in seawater had a similar behaviour to Se 2II -selenomethionine. Therefore, organic Se 2II species could also be probably converted to Se IV in 0.01 m HCl seawater after 30 min UV irradiation time under our experimental conditions. Using our experimental protocol, the concentration of Se VI, Se IV and organic Se 2II species determined by standard additions in our seawater sample were respectively 21 ± 4, 10 ± 2 and 13 ± 4 ng l 21 for a total selenium concentration of 44 ± 3 ng l 21 (10 measurements). Conclusion Flow injection hydride generation, trapping within the graphite furnace followed by electrothermal atomic absorption spectrometry permits the determination of Se IV in seawater at the ng l 21 concentration level. Optimization of the experimental conditions: spectroscopic and electrothermal parameters, trapping temperature and reagent concentrations leads to a detection limit of 1.5 ng l 21 for a 25 ml seawater sample. A relatively rapid and non polluting UV irradiation treatment in acidic or basic medium before hydride generation followed by electrothermal atomic absorption determination permits an experimental differentiation of Se VI, Se IV and organic Se 2II species in seawater. References 1 Robberecht, H., and Van Grieken, R. V., Talanta, 1982, 29, Cooke, T. D., and Bruland, K. W., Environ. Sci. Technol., 1987, 21, Mayland, H. F., James, L. F., Panter, K. E., and Sonderegger, J. L., Soil Sci. Soc. Am. Am. Soc. Agron., SSSA Special Publication, 1989, 23, Olivas, R. M. O., Donard, O. F. X., Camara, C., and Quevauviller, P., Anal. Chim. Acta, 1994, 286, Kolbl, G., Mar. Chem., 1995, 48, Measures, C. I., and Burton, J. D., Earth Planet. Sci. Lett., 1980, 46, Measures, C. I., and Burton, J. D., Anal. Chim. Acta, 1980, 120, Guan, D. M., and Martin, J. M., Marine Chem., 1991, 36, Cutter, G. A., Science, 1982, 217, Cutter, G. A., and San Diego-McGlone, M. L. C., Sci. Total Environ., 1990, 97, Tanzer, D., and Heumann, K. G., Intern. J. Environ. Anal. Chem., 1992, 48, Cutter, G. A., and Cutter, L. S., Mar. Chem., 1995, 49, Abdullah, M. I., Shiyu, Z., and Mosgfen, K., Mar. Pollut. Bull., 1995, 31, Apte, S. C., Howard, A. G., Morris, R. J., and McCartney, M. J., Mar. Chem., 1986, 20, Matusiewicz, H., and Sturgeon, R. E., Spectrochim. Acta, Part B, 1996, 51, Ferri, T., and Sangiorgio, P., Anal. Chim. Acta, 1996, 321, Cutter, A., Anal. Chim. Acta, 1978, 98, Batley, G. E., Anal. Chim. Acta, 1986, 187, Campanella, L., Ferri, T., Morabito, R., and Paoletti, A. M., Chim. Ind. (Milan), 1987, 10, Aoño, T. A., Nakaguchi, Y., and Hiraki, K., Geochem. J., 1991, 25, Petterson, J., and Olin, A., Talanta, 1991, 38, Tanzer, D., and Heuman, K. G., Anal. Chem., 1991, 63, Ornemark, U., Petterson, J., and Olin, A., Talanta, 1992, 39, Mattson, G., Nyholm, L., Olin, A., and Ornemark, U., Talanta, 1995, 42, Bryce, D. W., Izquierdo, A., and Luque De Castro, M. D., J. Anal. At. Spectrom., 1995, 10, Bryce, D. W., Izquierdo, A., and Luque De Castro, Analyst, 1995, 120, Seby, F., Potin-Gautier, M., Castetbon, A., and Astruc, M., Analusis, 1995, 23, Hill, S. J., Pitts, L., and Worsfold, P., J. Anal. At. Spectrom., 1995, 10, Elleouet, C., Quentel, F., and Madec, C., Water Res., 1996, 30, Diaz, J. P., Navarro, M., Lopez, H., and Lopez, M. C., Sci. Total Environ., 1996, 186, Ferri, T., and Sangiorgio, P., Anal. Chim. Acta, 1996, 321, LaFuente, J. M. G., Sanchez, M. L. F., Marchante-Gayon, J. M., Sanchez Uria, J. E., and Sanz-Medel, A., Spectrochim. Acta, Part B, 1996, 51, L vov, B. V., Polzik, L. K., Borodin, A. V., Fedorov, P. N., and Novichikhin, A. V., Spectrochim. Acta, Part B, 1994, 49, L vov, L. V., Polzik, L. K., Borodin, A. V., Dyakov, A. O., and Novichikin, A. V., J. Anal. At. Spectrom., 1995, 10, Cabon, J. Y., and Le Bihan, A., Analyst, 1997, 122, Tsalev, D. L., D Ulivo, A., Lampuganni, L., Di Marco, M., and Zamboni, R., J. Anal. At. Spectrom., 1996, 11, Krivan, V., J. Anal. At. Spectrom., 1992, 7, 155. Paper 8/02541J Received April 2, 1998 Accepted May 28, 1998