Anal. Chem. 2001, 73, 4711-4716
Microwave Digestion of Environmental and Natural Waters for Selenium Speciation Zijian Wang,*,† Yu-Xi Gao,† and Nelson Belzile‡
State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, Beijing 100085, China, and Department of Chemistry and Biochemistry, Laurentian University, Sudbury, Ontario, Canada P3E 2C6
A microwave preparation procedure is proposed for selenium speciation in natural and drinking waters. Different chemical reagents were tested, and the conditions for Se speciation were optimized. The effect of the different reagents on various oxidation states of selenium under microwave digestion conditions was investigated. Most of the Se(-II) was converted to selenite when digested with HNO3 and 0.997). Linear relationships between dosage of Se(IV) and fluorescence intensity can be observed in a range of 0-1.5 ng Se. For a sensitive analysis method, the blank has to be considered. In the proposed procedure for selenium determination, interference may be introduced from fluorescent impurity in DAN and traces of selenium in chemical reagents. Samples can also be contaminated during the pretreatment of samples and the subsequent selenium determination. The interference coming from the fluorescent impurity can be eliminated by DAN purifica(11) Wiedmeyer, R. H.; May, T. W. Arch. Environ. Contam. Toxicol. 1993, 25, 67.
4714 Analytical Chemistry, Vol. 73, No. 19, October 1, 2001
tion and an HPLC separation.8 To assess the interference from reagents other than DAN and the whole procedure, 10 duplicate determinations were performed on 2.0 mL of quartz-distilled water. The results were 0.0205 ( 0.0022, and 0.0521 ( 0.0032 ng of selenium per sample respectively, which corresponds to a detection limit (3Φ) of 0.0066 and 0.0096 ng of selenium for HNO3 and H2O2/H2SO4 as digestion reagents, respectively. The blank is comparatively high for H2O2/H2SO4 because of the trace amounts of Se present in concentrated H2SO4. The detection limit is 0.06 ng for the hot plate digestion technique.8 The microwave digestion technique can supply a lower detection limit. This is a benefit to the improvement of accuracy and precision of determination. A biological standard reference material, rice flour from Japan Environmental Agency (NIES 10), was employed to validate the total selenium analytical process. The results are shown in Table 3. The recoveries were 99.4 ( 2% for HNO3 as digesting reagent and 97.8 ( 2.5% when the H2O2/H2SO4 mixture was used (N ) 3). In addition to reducing the treatment time, the microwave digestion technique performed in closed vessels can avoid contamination of samples and eliminate the uncontrolled loss of volatile molecular species, thus reducing errors of measurement. A microwave digestion with HNO3 combines the advantage of a simple reagent, a short sample treatment, and the lowest detection limit. Speciation of Natural Waters. When appropriate reagents are used, the transformation of different selenium species into Se(IV) can be achieved, and Se(IV) can be easily determined by HPLC-FLD after DAN derivation. This makes possible the speciation of selenium in natural and environmental samples. The proposed methodology was used for the speciation of Se in four water samples. A 20-mL sample was required for Se(IV)
Table 4. Selenium Speciation in Different Types of Water Samples from Beijing (µg L-1)
total Seb Se(IV+VI)c Se(IV)d Se(VI)e Se(-II,
0)e
tap water
spring water
pond water
rainwater
0.20 ( 0.01 0.19 ( 0.01 0.02 ( 0.00 (10.0)a 0.17 ( 0.01 (85.0) 0.01 ( 0.01 (5.0)
0.90 ( 0.03 0.89 ( 0.05 0.02 ( 0.00 (2.2) 0.87 ( 0.05 (96.7) 0.01 ( 0.06 (1.1)
0.55 ( 0.02 0.23 ( 0.01 0.01 ( 0.00 (1.8) 0.22 ( 0.01 (40.0) 0.32 ( 0.02 (58.2)
0.54 ( 0.03 0.42 ( 0.02 0.20 ( 0.01 (37.0) 0.22 ( 0.02 (40.7) 0.12 ( 0.04 (22.0)
a
Data in parentheses is % of Se species with respect to the total Se (N ) 3). b Total Se was determined by HNO3 digestion followed by a reduction process. c Inorganic Se was determined after a reduction process. d 20 mL of water was mixed with 1 mL of concentrated H3PO4. After evaporation of the mixture to ∼2 mL under nitrogen protection, the amount of Se(IV) in the sample was determined. e Calculated data by subtraction.
measurement and a 2.0-mL sample was sufficient for total inorganic Se(IV, VI) or total Se determination. The results obtained are shown in Table 4 (N ) 3). In most natural waters, total Se levels range from 0.04 to 5 µg L-1.12 Selenium in water can exit in different oxidation states (-II, 0, IV, VI); Se(IV) and Se(VI) are the most common forms.13 For the water samples investigated in this study, total Se levels ranged from 0.2 to 0.90 µg L-1. Among its four valences, Se(VI) was the most common state, and Se(0) was not discriminated from Se(-II) in most Se speciation procedure. The total Se concentration in tap water was the lowest of our four samples at 0.20 µg L-1. About 85% of Se was present as Se(VI), 5% as Se(-II, 0), and 10% as Se(IV), which is slightly different from the values (total Se 0.15 µg L-1; Se(VI) 68%, Se(IV) 32%) in its source water (Huairou Reservoir, Miyun Reservoir, Beijing) which was reported previously.14 In pond water, the total Se concentration was at 0.55 µg/L. There was more selenide than selenate, with 58.2% of the total Se being in the form of Se(-II, 0) and 40% of total Se as Se(VI). It has been reported that reduced species can be produced from biotic process in surface water, and Se(-II, 0) is correlated with indicators of biological activity, such as chlorophyll, bioluminescence, and primary productivity.15,16 Phytoplankton is abundant in this pond, which could explain the dominance of Se(-II, 0). In spring water, the total Se content was 0.90 µg L-1. Almost all of total Se was in Se(VI) form, which may partly be explained by the Se form present in bedrock. Similar results were reported in our previous work.12 Se content in rainwater was 0.53 µg L-1, and not one of the four selenium species was obviously predominant. The total Se level in rainwater in the Beijing area has been reported to range from 0.03 to 0.43 µg L-1, and more generally in the 0.25-0.35 µg L-1 range,17 which is lower than the value measured in this present work. Many factors can affect the Se content and species in (12) Peng, A.; Wang, Z.; Whanger, P. D.; Combs, G. F.; Yeh, J. Y. In Environmental Bioinorganic Chemistry of Selenium (in Chinese); China Environmental Science Press: Beijing, 1995; p 7. (13) Conde, J. E.; Sanz Alaejos, M. Chem. Rev. 1997, 97, 1979. (14) Wang, Z.; Peng, A. J. Environm. Sci. 1989, 1, 116. (15) Cutter, G. A. Mar. Chem. 1992, 40, 65. (16) Cutter, G. A.; Bruland, K. W. Limnol. Oceanogr. 1984, 29, 1179. (17) Zhao, F. Q.; Tan, J. N. Acta Scietiae Circumstantiae (in Chinese) 1988, 8, 428.
Table 5. Recovery of Spiked Se in Different Forms from the Rainwater Sample Se forms addeda
Se in sample ng
Se added ng
Se found ng
recovery %
Se(IV) Se(IV, VI) Se(-II, IV, VI)b
0.20 ( 0.01 0.42 ( 0.02 0.54 ( 0.03 (0.55 ( 0.02) 0.22 ( 0.02 0.12 ( 0.04 (0.13 ( 0.03)
0.20 0.40 0.55
0.38 ( 0.02 0.84 ( 0.03 1.10 ( 0.03 (1.13 ( 0.04) 0.46 ( 0.04 0.26 ( 0.04 (0.28 ( 0.05)
95.0 102.4 99.1 (103.7) 109.5 96.3 (100)
Se(VI)c Se(-II, 0)c
0.20 0.15
a To 1.0 mL of the rainwater sample (Se speciation see Table 4), an equal amount of Se in different forms was spiked. b Data show results obtained by using HNO3; data in parentheses show results obtained by using H2SO4/H2O2. c Calculated data by addition or subtraction.
rainwater, such as weather conditions, rainfall, atmospheric pollution, and microorganism activity in the soil.17,18 A higher Se content in rainwater may result from more serious air pollution. To validate the proposed speciation approach, a recovery test was performed. In the absence of a proper standard reference material for selenium speciation in natural waters, known amounts of selenomethionine, selenite, and selenate were spiked to a water sample. For the test, 1.0 mL of the rainwater containing 0.12 ng of Se(-II, 0), 0.20 ng of Se(IV), and 0.22 ng of Se(VI) was spiked with equal amounts of different selenium compounds (selenomethionine, selenite, and selenate). The percentage of recovery of each species was calculated by eq 1 using the Se amount determined and the combined mass of sample Se and spiked Se. The results showed that the recoveries of the different selenium species were 95.0-109.5%, as shown in Table 5.
Recovery (%) ) Se measured/(Se in sample + Se added) × 100 (1)
CONCLUSIONS A microwave preparation procedure for selenium speciation in natural waters was established. The optimized procedure for 2 mL water consists of using 1.0 mL of HNO3 or of a mixture of H2O2/H2SO4 (5:1 v:v) as the reagent, heating the solution 20 min with HNO3 or 30 min with H2O2/H2SO4 with the microwave power at 80% and a pressure of 0.55 Mpa. The digestion with H2O2/H2SO4 can transform all Se species to selenite; digestion with HNO3, to selenite and selenate. Selenite, Se(IV), can be determined using a normal phase HPLC-FLD after a precolumn derivatization with DAN, and selenium speciation in a water sample can be carried out. The proposed procedure was applied to four different water samples from the Beijing area. Total Se ranged from 0.20 to 0. 90 µg L-1 with Se(VI) being the dominant species in samples, with the exception of pond water, in which Se(-II, 0) was predominant. The proposed methodology combines the advantages of a shorter digestion time, a lower risk of sample contamination, and (18) Dall’aglio, M.; Ghiara, E.; Proietti, W. Rend. Soc. Ital. Mineral Petrol. 1978, 34, 591.
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a minimized amount of sample requirement. Detection limits of 0.0066 and 0.0096 ng Se were obtained for HNO3 and H2O2/ H2-SO4, respectively. The optimized microwave preparation procedure was also applied to rice flour, and recoveries reached 99.4 ( 2% and 97.8 ( 2.5% for HNO3 and H2O2/H2SO4, respectively, as digestion reagents, confirming that the procedure can also be used to measure total Se in biological samples.
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Analytical Chemistry, Vol. 73, No. 19, October 1, 2001
ACKNOWLEDGMENT This work was supported by the Chinese Academy of Sciences (KZCX2-410; RCEES-KIP-9901) and project NKBRSF-G1999045710. Received for review March 19, 2001. Accepted July 17, 2001. AC010330H
