Anal. Chem. 1996, 68, 4064-4071
Direct Speciation of Selenite and Selenate with Thermospray Sample Introduction Methods Jinfu Yang, Timothy S. Conver, and John A. Koropchak*
Department of Chemistry and Biochemistry, Southern Illinois University at Carbondale, Carbondale, Illinois 62901-4409
Thermospray is a high-temperature and high-pressure process compared with pneumatic nebulization. In this study, these characteristics of thermospray were exploited for selenium speciation with inductively coupled plasma atomic emission spectrometric (ICP-AES) detection. In aqueous solution, the sensitivity for Se(IV) was found to be lower than that for Se(VI). Addition of methanol into the solution further depressed the sensitivity for Se(IV) to a negligible level compared to that for Se(VI). The low sensitivity for Se(IV) arises from its reduction to Se(0) within the thermospray process, after which selenium metal was trapped on the vaporizer. This loss of Se(IV) can be counteracted by addition of an oxidant (K2S2O8 or K2Cr2O7) into the selenium solution, which prevents Se(IV) from being reduced within the vaporizer. Based on these results, a method for speciation of Se(IV) and Se(VI) was developed. The concentration of Se(VI) is selectively determined in a sample aliquot with 20% (v/ v) methanol added. In another sample aliquot with an addition of 5 mmol/L K2S2O8 or 2 mmol/L K2Cr2O7, the total selenium concentration is determined. The selenite content is calculated by difference. The detection limits for Se(VI) at 196.0 nm were measured to be 2 ng/mL for aqueous solution and 9 ng/mL for 20% (v/v) methanol solution. A concomitant with oxidation or reduction ability, which can obstruct the selenite reduction within the thermospray vaporizer or counteract the function of an oxidant added, will be a potential interferent in the determination of Se(VI) or total selenium, although the tolerance limits of such interferents as Cr(VI) and Sn(II) were determined to be well above their concentrations in many environmental samples such as natural water. The effects of methanol concentration, oxidant concentration, and nitric acid concentration in solution were studied in detail. Good accuracy and precision of the method were demonstrated for the analysis of spiked tap water samples. Selenium is an important element from the perspective of human and environmental health. It is an essential micronutrient, yet it promotes toxicity and adverse biological effects at relatively low concentrations. For example, selenium compounds have been reported to have anticarcinogenic activity and prevent heavy metal toxic effects.1,2 On the other hand, excessive intake of selenium can lead to toxic responses including “alkali disease” and “blind staggers”.3 The nutritionally required level is very close to the toxic concentration.4 The toxicity, availability, and environmental (1) Das, N. P.; Ma, C. W.; Salman, Y. M. Biol. Trace Elem. Res. 1986, 10, 215. (2) Magos, L.; Webb, M. Crit. Rev. Toxicol. 1980, 8, 1.
4064 Analytical Chemistry, Vol. 68, No. 22, November 15, 1996
mobility of selenium are very much dependent on its chemical form.5-7 Therefore, precise knowledge of the concentrations of the selenium compounds present in a system, in addition to the total selenium concentration, is required for an accurate assessment of the environmental and biological impact of selenium, which has resulted in an increasing need of analytical methods for selenium speciation at trace to ultratrace levels. Selenium can exist in a variety of oxidation states (-II, 0, IV, and VI) in inorganic and organic forms. In many environmental matrices such as natural water, soils, and fly ash,7-12 the predominant valence states of selenium are Se(IV) and Se(VI), corresponding to the most environmentally mobile and biogeochemically important forms of selenium: selenite (SeO32-) and selenate (SeO42-), respectively. Most of the studies published so far on selenium speciation deal with these two species.13,14 Hydride generation techniques have been explored for the determination of selenite and selenate.14-21 Se(IV) is selectively reduced to volatile selenium hydride usually by sodium tetrahydroborate in hydrochloric acid medium, which is introduced into a detector for obtaining signal; in another sample aliquot, the total inorganic selenium concentration is determined after the prereduction of Se(VI) to Se(IV); then the Se(VI) content is calculated by the difference. This method, however, is very prone to interferences by concomitants which reduce the hydride generation efficiency.14,22 Kolbl et al.13 reviewed the identification and quantification of different selenium compounds with highperformance liquid chromatography (HPLC), which probably is (3) National Research Council. In Mineral Tolerance of Domestic Animals: Selenium; National Academy of Sciences: Washington, DC, 1980; pp 393401. (4) Dubois, F.; Belleville, F. Pathol. Biol. 1988, 36, 1017. (5) Shamberger, R. J. Biochemistry of Selenium; Plenum: New York, 1983; p 185. (6) Wheeler, A. E.; Zingaro, R. A.; Irgolic, K. J.; Bottino, N. R. J. Exp. Mar. Biol. Ecol. 1982, 57, 151. (7) Deverel, S. J.; Millard, S. P. Environ. Sci. Technol. 1988, 22, 697. (8) Tanzer, D.; Heumann, K. G. Anal. Chem. 1991, 63, 1984. (9) Ferri, T.; Sangiorgio, P. Anal. Chim. Acta 1996, 321, 185. (10) Neal, R. H.; Sposito, G.; Holtzclaw, K. M.; Traina, S. J. Soil Sci. Soc. Am. J. 1987, 51, 1161. (11) Fujii, R.; Derevel, S. J.; Hatfield, D. B. Soil Sci. Soc. Am. J. 1988, 52, 1274. (12) Niss, N. D.; Schabrob, J. F.; Brown, T. H. Environ. Sci. Technol. 1993, 27, 827. (13) Kolbl, G.; Kalcher, K.; Irgolic, K. J.; Magee, R. J. Appl. Organomet. Chem. 1993, 7, 443. (14) Dedina, J.; Tsalev, D. L. Hydride Generation Atomic Absorption Spectrometry; John Wiley & Sons: Chichester, England, 1995; Chapter 13. (15) Gutter, G. A. Anal. Chim. Acta 1978, 98, 59. (16) Pitts, L.; Worsfold, P. J.; Hill, S. J. Analyst 1994, 119, 2785. (17) Roden, J. R.; Tallman, D. E. Anal. Chem. 1982, 54, 307. (18) Gutter, G. A. Anal. Chem. 1985, 57, 2951. (19) Apte, S. C.; Howard, A. G. J. Anal. At. Spectrom. 1986, 1, 379. (20) Sinemus, H. W.; Melcher, M.; Welz, B. At. Spectrosc. 1981, 2, 81. (21) Yu, M.; Liu, G.; Jin, Q. Talanta 1983, 30, 265. (22) Welz, B.; Stauss, P. Spectrochim. Acta, Part B 1993, 48, 951. S0003-2700(96)00610-5 CCC: $12.00
© 1996 American Chemical Society
the most widely used method for selenium speciation. Nonelement-specific detectors, mostly conductivity detectors, which have a disadvantage of poor selectivity, have been conventionally used for signal detection. Selectivity can be greatly improved, and the chromatographic task can, therefore, be simplified by using selenium-specific detectors. In this respect, inductively coupled plasma (ICP) spectrometry, among others such as graphite furnace atomic absorption spectrometry,23,24 atomic fluorescence,25 and isotope dilution mass spectrometry,8 is very promising.26-31 One aspect of ICP spectrometry that has long been considered to be a hindrance to detection is the sample introduction process.32 Conventionally, the eluent exiting from an HPLC column is nebulized using a pneumatic nebulizer, and the generated aerosol is introduced into the ICP. This type of sample introduction, however, is highly inefficient. Typically, 98-99% of the analyte goes to waste and never reaches the ICP. In attempts to improve sample introduction efficiency for ICP spectrometry, thermospray techniques have been developed.33 With thermospray, aerosols are generated by pumping liquids through an electrothermally heated capillary, where partial vaporization occurs at appropriate temperatures, resulting in a jet of vapor and aerosol. It has been shown that thermospray aerosols have smaller particle sizes on average than pneumatic ones,34 leading to higher analyte transport efficiencies, on the order of 20-50%34-36 and to improved detection limits typically 20-50 times lower than those for pneumatic nebulization.34-42 The applicability of a thermospray system as an interface between HPLC and ICP spectrometry has also been demonstrated with significant advantages over pneumatic nebulization.43,44 One inherent characteristic with HPLC separations is the discrete sample injection, which unavoidably causes a loss of sensitivity due to dilution and dispersion of the injected sample during the chromatographic, nebulization, and transport processes compared to continuous sample introduction. (23) Chakraborti, D.; Hillman, D. C. J.; Irgolic, K. J.; Zingaro, R. A. J. Chromatogr. 1982, 249, 81. (24) Laborda, F.; Chakraborti, D.; Mir, J. M.; Castillo, J. R. J. Anal. At. Spectrom. 1993, 8, 643. (25) Pitts, L.; Fisher, A.; Worsfold, P.; Hill, S. J. J. Anal. At. Spectrom. 1995, 10, 519. (26) Hill, S. J.; Bloxham, M. J.; Worsfold, P. J. J. Anal. At. Spectrom. 1993, 8, 499. (27) LaFreniere, K. E.; Fassel, V. A.; Eckels, D. E. Anal. Chem. 1987, 59, 879. (28) Thompson, J. J.; Houk, R. S. Anal. Chem. 1986, 58, 2541. (29) McCarthy, J. P.; Caruso, J. A.; Fricke, F. L. J. Chromatogr. Sci. 1983, 21, 389. (30) Reohl, R.; Alforque, M. M. At. Spectrosc. 1990, 11, 210. (31) Irgolic, K. J.; Stockton, R. A.; Chakraborti, D.; Beyer, W. Spectrochim. Acta, Part B 1983, 38, 437. (32) Browner, R. F.; Boorn, A. W. Anal. Chem. 1984, 56, 786A. (33) Koropchak, J. A.; Veber, M. Crit. Rev. Anal. Chem. 1992, 23, 113. (34) Koropchak J. A.; Winn, D. H. Appl. Spectrosc. 1987, 41, 1311. (35) Yang, J.; Conver, T. S.; Koropchak, J. A. Spectrochim. Acta, Part B, in press. (36) Koropchak, J. A.; Aryamanya-Mugisha, H.; Winn, D. H. J. Anal. At. Spectrom. 1988, 3, 799. (37) Koropchak, J. A.; Veber, M.; Herries, J. Spectrochim. Acta, Part B 1992, 47, 825. (38) Conver, T. S.; Koropchak, J. A. Spectrochim. Acta, Part B 1995, 50, 341. (39) Koropchak, J. A.; Aryamanya-Mugisha, H. Anal. Chem. 1988, 60, 1838. (40) Elgersma, J. W.; Maessen, F. J. M. J.; Niessen, W. M. A. Spectrochim. Acta, Part B 1986, 41, 1217. (41) Vermeiren, K. A.; Taylor, P. D. P.; Dams, R. J. Anal. At. Spectrom. 1987, 2, 383. (42) de Loos-Vollebregt, M. T. C.; Tiggelman, J. J.; Bank, P. C.; Degraeuwe, C. J. Anal. At. Spectrom. 1989, 4, 213. (43) Roychowdhury, S. B.; Koropchak, J. A. Anal. Chem. 1990, 62, 484. (44) Laborda, F.; de Loos-Vollebregt, M. T. C.; de Galan, L. Spectrochim. Acta, Part B 1991, 46, 1089.
Compared with pneumatic nebulization, thermospray is a hightemperature and high-pressure process. Under such conditions, a compound may have different properties, such as chemical reactivity and stability, from those under normal conditions. It was observed, for example, that the sensitivity for arsenic with thermospray is species dependent.43 In this work, this characteristic of thermospray was exploited as a means for selenium speciation. With thermospray nebulization and ICP atomic emission spectrometry (ICP-AES) detection, the response for selenite was found to be lower than that for selenate in aqueous solutions at the same concentration. Addition of methanol to the solution further depresses the sensitivity for selenite to a level that is negligible compared to that for selenate. Thus, the concentration of selenate can be selectively determined. The low sensitivity for selenite results from its reduction during the thermospray process to metallic selenium, which deposits on the vaporizer and, therefore, is not transported to the ICP. Such a reduction can be prevented by adding an oxidant to the aqueous solution, and the total selenium concentration can be determined. The effects of the concentrations of methanol, nitric acid, and oxidant and potential interferences from concomitants will be described. The feasibility of the method for speciation of selenite and selenate will be demonstrated with analyses of spiked water samples. EXPERIMENTAL SECTION Instruments and Operating Conditions. All spectroscopic measurements were performed using a Varian Liberty 220 ICP spectrometer (Victoria, Australia), consisting of a plasma source powered by a crystal-controlled high-frequency generator operating at 40.68 MHz and a vacuum path monochromator with a Czerny-Turner mount. The spectrometer has a resolution of 18 pm for the first order. Se(I) 190.6 nm was used as the analysis line at first order. All data were collected at an integration time of 5 s with five replicates for standards and 10 replicates for blanks. Detection limits were calculated as the concentration giving a signal equal to 3 times the standard deviation of the blank, based on 10 replicate measurements. The sample introduction system is shown in Figure 1. A fused silica aperture thermospray nebulizer, details of which have been previously described,37 was employed for aerosol generation in this work. Briefly, the thermospray vaporizer consists of a resistively heated stainless steel tube with 1.6 mm outer diameter and 0.5 mm inner diameter, into which a fused silica capillary, having an outer diameter of 0.36 mm, an inner diameter of 150 µm, and a 1 mm tip with an inner diameter of 50 µm, was inserted and placed such that the capillary tip is just beyond the end of the stainless steel tube. Liquid samples were pumped through the fused silica capillary. Solvent vaporization and aerosol generation result from the energy transfer from the heated stainless steel tube through the fused silica wall and the surrounding annular space to the flowing liquid stream. Based on previous work,37 a solution flow rate of 1.5 mL/min was used. The particular thermospray system used herein was a prototype constructed by Leeman Labs, Inc. This system employs a fused silica aperture thermospray nebulizer and glassware arrangement as described by Koropchak et al.,37 but uses digital controllers to monitor and control the operating temperatures as opposed to the analog triac controllers used in the past publications. Specifically, two CN 9000A digital temperature controllers from Omega Engineering, Inc. (Stamford, CT) were used to monitor and Analytical Chemistry, Vol. 68, No. 22, November 15, 1996
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Figure 1. Diagram of the sample introduction system used in this work. Table 1. Experimental Conditions set 1 solutions methanol concentration nitric acid concentration (mmol/L) uptake rate (mL/min) ICP conditions forward power (kW) outer Ar flow (L/min) intermediate Ar flow (L/min) carrier Ar flow (L/min) viewing height above load coil (mm) control temperature for thermospray (°C) spray chamber temperature (°C) condenser temperature (°C)
0 80 1.5 1.25 15.0 1.5 0.55 7 200 160 -7
set 2 20% (v/v) 80 1.5 1.50 16.5 2.25 0.45 6 190 140 -10
regulate the thermospray control and spray chamber temperatures. An Autochrom (Berlin, Germany) Model M500 HPLC pump continuously delivered a carrier flow to the nebulizer, and sample solutions were introduced in flow injection manner with a Rheodyne (Cotati, CA) Model 7125 metal-free injector fitted with a 5 mL PEEK injection loop, followed by a 2 µm particle filter. Finally, the Ar carrier gas flow rate was monitored and controlled using a mass flow controller (Model FC-260, Tylan General, Torrance, CA). Thermospray aerosols were input into a heated cylindrical Pyrex glass chamber. Desolvation of the aerosols was accomplished with a Friedrichs condenser cooled to -7 °C for aqueous solutions and -10 °C for methanol solutions. The condenser temperature was regulated using a refrigerated recirculating bath (NESLAB Instruments, Inc., Newington, NH). The dry aerosol from the condenser was introduced into the ICP. For each methanol level in solution, the ICP operating conditions (viewing height, forward power, and carrier gas flow rate), the control temperature for thermospray, and the spray chamber temperature were optimized to obtain maximum signalto-noise ratio for Se(VI) at 190.6 nm. Table 1 lists two sets of optimal conditions: set 1 for aqueous solutions and set 2 for 20% (v/v) methanol solutions. These conditions were used throughout this work unless stated otherwise. 4066 Analytical Chemistry, Vol. 68, No. 22, November 15, 1996
Chemicals and Reagents. Analytical grade chemicals and deionized water were used for the preparation of all the solutions used in this study. Selenium stock solutions were prepared from sodium selenite and selenate from Sigma (St. Louis, MO) dissolved in water, and working solutions were prepared daily by diluting the stock solutions. Methanol was distilled before use. Analyte Transport Measurements. The procedure used for analyte transport measurements was similar to that described previously.34,35,45 Solutions of 100 µg/mL of Se(VI) or Se(IV) in 20% (v/v) methanol containing 80 mmol/L nitric acid were nebulized by the thermospray nebulizer. Aerosols exiting from a torch injection tube connected to the outlet of the Friedrichs condenser were collected on glass microfiber filters (Whatman EPM 2000, Whatman International Ltd., Maidstone, England), fixed by a Teflon filter holder. An excess of air was drawn through the filters by an external vacuum source. The aerosol injection tube was placed inside a larger diameter sampling tube connected to the filter holder to minimize the effects of the vacuum on aerosol transport. The collected aerosol was then leached from the filters with 160 mmol/L nitric acid solution and an ultrasonic bath and transferred into volumetric flasks. The solutions were analyzed with the same Varian ICP-AES spectrometer but using a type K pneumatic nebulizer (Varian, Victoria, Australia). The results provided a direct measure of analyte transport. By collecting the drain wastes, separately rinsing the spray chamber and the condenser, the selenium losses were also measured. 1% (v/v) nitric acid was used as the rising solution. But in evaluating the selenite loss in spray chamber, the spray chamber was also rinsed by concentric sulfuric acid before using 1% (v/v) nitric acid. HPLC Separation of Selenite and Selenate. The HPLC system consisted of an Autochrom (Berlin, Germany) Model M500 pump, a Rheodyne (Cotati, CA) Model 7125 injector fitted with a 20 µL injection loop, and a Kromasil C18 column (Alltech, Associates, Inc, IL) with 4.6 mm i.d., 25 cm length, and 10 µm particle size. The mobile phase used was 5 mmol/L tetrabutylammonium phosphate (from Fisher Scientific) in 50/50 (v/v) water/methanol at a pH of 3.4. The flow rate of the mobile phase was 1 mL/min. The eluent from the column was nebulized by the type K pneumatic nebulizer and introduced into the ICP described above for obtaining signal. RESULTS AND DISCUSSION Thermospray Behavior of Selenite and Selenate. One of the primary operating parameters with a thermospray system is the vaporizer temperature, which controls the degree of solvent vaporization, influences the pressure within the nebulizer, and hence modifies the physical and chemical environment that analytes experience. With the vaporizer used in this study, the control temperature is the vaporizer temperature measured with the aid of a thermocouple located about 30 cm distant from the capillary tip. The dependence of the sensitivity for Se(IV) and Se(VI) on control temperature is shown in Figure 2 with log scales for the Y axes. With aqueous media (Figure 2a), a typical trend was observed for both Se(IV) and Se(VI), i.e., sensitivity first increases with control temperature, reaches a maximum, and subsequently declines. However, the sensitivity for Se(IV) is lower than that for Se(VI) by a factor of about 4. Moreover, the maximum sensitivity for Se(IV) occurs at a control temperature 20 °C lower than that where the sensitivity for Se(VI) has its (45) Smith, D. D.; Browner, R. F. Anal. Chem. 1982, 54, 533.
Figure 3. Effect of methanol concentration in solution on sensitivity ratio of Se(VI) to Se(IV). All the data were collected at the optimal control temperature and spray chamber temperature at each methanol concentration for maximum Se(VI) sensitivity. Table 2. Selenium Transport Measurements for Se(IV) and Se(VI) Solutions Containing 20% (v/v) Methanol selenium loss (%)
Se(IV) Se(VI)
transport efficiency (%)a
waste draina
condenser
spray chamber
