Anal. Cham. 1991, 63, 1984-1989
1984
Determination of Dissolved Selenium Species in Environmental Water Samples Using Isotope Dilution Mass Spectrometry Dieter Tanzer and Klaus G. Heumann* Institut für Anorganische Chemie, Universitat Regensburg, Universitátsstrasse
distinguishing the different chemical forms are scarce. One of the difficulties in exploring the versatile species of selenium in the environment is its usually low concentration. This demands a very sensitive analytical determination method (7). Recommended methods for selenium analyses include hydride generation atomic absorption spectrometry (8), neutron activation analysis (9), fluorometry (10), and gas chromatography (11). A major disadvantage of these methods is the requirement of an external calibration for quantification. We describe here analytical procedures for the selective determination of different selenium species in natural waters applying isotope dilution mass spectrometry (IDMS), which yields relatively precise and accurate results even at low concentration levels. For IDMS, internal calibration by an enriched spike isotope is used. The species investigated include selenite and selenate as well as organic selenium compounds, e.g., the trimethylselenonium ion (TMSe+). Hints to the existence of further selenium species in the samples may be obtained from the comparison of the sum of the single species to the total selenium content determined separately. A prerequisite for this IDMS method was the development of a technique to form Se' ions by negative thermal ionization mass spectrometry (NTI-MS) (12). This NTI technique was already successfully applied to the determination of selenium traces in food, sediments, and groundwater samples (13-15).
In order to clarify the species composition of selenium In environmental water samples, analytical methods have been developed for the selective determination of Afferent chemical forms of this element (selenite, selenate, and organic species Including trimethylselenonium) using Isotope dilution mass spectrometry (IDMS). The species analysts was made pósetele by means of chromatographic separation procedures and an “Se-enrlched selenate, selenite, and trimethylselenonium spike for the Isotope dilution process. The total selenium concentration was determined after decomposition of organic compounds with a HN0S/HCI04 mixture. Selenium was measured In the mass spectrometer by producing negative Se' thermal Ions for detection. Precise determination at the parts-per-trllHon level was achieved. This new methodology was applied to different types of natural water samples (groundwater, pond water, river water, moorland lake water) with total selenium concentrations In the range of 200 pg/g to 15 ng/g. Selenite and selenate have been the only detected species In most of the Investigated samples, with selenate dominating all except one. In samples with high contents of dissolved organic carbon, however, different organoselenlum compounds Including trimethylselenonium ions were additionally quantified In the range of 10-95 pg/g. In these cases, the sum of all selenium species agreed wel with the Independently determined total element concentration.
EXPERIMENTAL SECTION NTI-MS Measurement. The mass spectrometric measure-
ments were carried out with a single focusing magnetic sector field instrument, Type MAT 261 (Finnigan MAT). The isotope ratio ®°Se/82Se was determined by forming negative thermal ions on the surface of a hot rhenium filament in a double-filament ion source. 82Se was selected as the spike isotope and the most abundant natural ^Se as the reference isotope because no interferences are known in NTI-MS for both mass numbers. In addition, the small mass difference between these isotopes reduces possible isotopic fractionations in the mass spectrometer to values negligible for IDMS. The relative standard deviation for the spike “Se/^Se measurement was determined to be 0.3%. A detailed
INTRODUCTION In recent years, there has been increasing interest in the trace determination of selenium because of its dual role as an essential nutrient at low concentration levels and as a toxic substance at higher concentration levels. Selenium deficiency, for example, can be related to necrotic degeneration of the liver, pancreas, heart, and kidney. High selenium concen-
trations can establish toxicity phenomena like inflammation of the feet, softening and loss of hoofs and horns in animals, or loss of hairs and nails and irritation of skin and eyes in humans (1-3). The narrow concentration range between the two contrary effects requires precise knowledge of the selenium content in the environment (4). The accurate determination of selenium, however, is still a major challenge for analysts. Detailed information about the availability and mobility of an element in the environment and its behavior in biological and geochemical systems, however, requires the additional knowledge of the different chemical forms in which the element exists. Species determination with respect to selenium, which can exist in different oxidation states (-2, selenide; 0, elemental selenium; +4, selenite; +6, selenate) and multiple chemical forms within the oxidation state -2 (e.g., organic and inorganic selenide), is especially important in aquatic systems, because the occurrence of selenium is mainly influenced by its chemical form (5,6). Therefore, analytical methods that can provide such information are receiving more attention. While a number of methods for the determination of the total selenium content are available, methods that are capable of 0003-2700/91 /0363-1984502.50/0
31, D-8400 Regensburg, Germany
description of the preparation technique together with the principles and advantages of this ionization technique has been given in previous papers (12,16). Reagents and Standards. All reagents were analytical grade, and bidistilled water was used throughout the study. HN03 p.a. and HC1 p.a. (p.a. = pro analyst) were further purified by subboiling distillation in a quartz apparatus. Radiotracer experiments were carried out with a commercial tracer 76Se in form of selenious acid with a nominal specific activity of 37 MBq/mL (Amersham Buchler, Braunschweig, FRG). A selenate tracer was obtained by oxidation of the selenite form with H202 (30%). A selenite standard solution was prepared by diluting a commercial standard solution (Titrisol; Merck, Darmstadt, FRG) to the appropriate volume with bidistilled water. Laboratory standards of selenate and trimethylselenonium were prepared by dissolution of Na2Se04 (Fluka, Ulm, FRG) and TMSeCl in 0.1 mol/L HNG3 and 0.1 mol/L HC1, respectively. TMSeCl was synthesized from dimethyl selenide (DMSe) according to the modified method of Nahapetian et al. (17).
Isotope Dilution Technique and Spike Solutions. The
change in the isotopic composition of an element caused by adding an enriched isotope (spike) to the sample is the basis for the ©
1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 18, SEPTEMBER 15, 1991
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1965
Table I. Concentration and Isotopic Composition of Spike and Standard Solutions Used for IDMS Analyses isotopic compn, %
soln
concn, Se atoms/g of soln
”Se
MSe
selenite spike selenate spike TMSe+ spike selenite std selenate std TMSe* std
(2.336 ± 0.008) X 1016 (2.28 ± 0.01) X 1016 (1.71 ± 0.01) X 1016 (9.945 ± 0.008) X 1018 (9.077 ± 0.01) X 1018 (1.859 0.01) X 1018
5.35 5.35 5.36 49.61 49.78 49.64
91.16 91.16 91.15 8.73 8.82 8.78
Table II. Selenite and Selenate Portions of Spike Solutions
spike selenite selenate TMSe+
concn, Se atoms/g of soln selenite selenate
of
analyses
0.5) X 1014
7 7
(1.9 ± 0.7) X 1018
3
(1.5 (1.4 ± 0.3) X 1014 (1.9 ± 0.2) X 1018
no.
parallel
determination of its concentration by IDMS. Important preconditions for the application of IDMS in element speciation are the synthesis of a labeled spike in the chemical form of the species to be determined (or its conversion into the chemical form of the spike after quantitative separation) and the necessity that no isotopic exchange between different species takes place until complete chemical separation from one another. More detailed descriptions are given elsewhere (16,18). The selenite, selenate, and trimethylselenonium spike solutions enriched in 82Se were synthesized from metallic gray selenium (Hempel, Düsseldorf, FRG). The preparation of the selenite and selenate spike was described elsewhere (15). These HN03 acidic solutions (pH = 1) were stored in polyethylene (PE) bottles. As has been already shown (15), this storage method using HN03 acidic solutions does not influence the concentration or identity of selenite and selenate. This was checked over a period of 2 years. However, possible alterations of one species into another one were investigated from time to time as described later. The synthesis of the TMSe+ spike was carried out as follows: 13.5 mg of elemental selenium and 72 mg of methyl iodide were heated for 10 h at 220 °C in a sealed tube to give a viscous oil that was dissolved in a mixture of 200 mL of bibistilled water and 200 µ , of ethyl acetate. The solution was treated with hydrogen sulfide until colorless, filtered, and poured into 2 mL of acetone to precipitate the crude salt. For further purification, the salt was dissolved in bidistilled H20, and the solution was passed through a glass column (8 mm X 25 cm) packed with a cationexchanger resin (Dowex 50W-X8; H+ form). After rinsing the column with water and hydrochloric acid (20 mL of 0.1,0.5, and 1 mol/L each) the pure product was eluted with 5 mol/L hydrochloric acid following the method that was used for the determination of TMSe+ ions in water samples. The solution was diluted with bidistilled water to give a pH value of approximately 1. The exact selenium content of the spike solutions was determined by a reversed IDMS technique using selenium standard solutions of natural isotopic composition (16). The concentrations of the spikes and standards as well as the measured abundances of “Se and “Se are listed in Table I. For species analyses, it is necessary to know the possible contributions of the spike solution to other species. For example, the selenite spike may contain selenate and vice versa, and the TMSe+ spike may contain selenite and selenate as well. In order to determine these possible contributions, the selenite spike was mixed with a selenate standard solution and the selenate spike with a standard solution of selenite. The TMSe+ spike was mixed with a standard solution of both selenite and selenate. Then, selenite and selenate were separated chromatographically with the anion-exchanger resin AG1-X8 by using the procedure described later. The corresponding fractions were measured in the mass spectrometer. The results of these investigations (Table ) showed that the spike solutions contain only very little portions of the other species. They must only be taken into account for
Figure 1. Sample treatment for the determination of total selenium In water samples with IDMS.
analyses with aspired accuracies of less than 1%.
SAMPLE TREATMENT Total Selenium. Immediately after collection, each sample was filtered through a 0.45-µ cellulose nitrate filler (Sartorius, Góttingen, FRG). Only material that passed through this filter
to be in the dissolved form, which is internationally accepted for 0.45-µ pore size filters. In order to determine the concentration of total dissolved selenium in a water sample with IDMS, all species must be converted into the same chemical form for equilibration with the spike. The sample treatment for the determination of the total selenium content is shown in Figure 1. The water sample (about 250 g, depending on the selenium concentration) was mixed in a glass beaker with the 82Se032~ spike was considered
solution, treated with 2 mol/L KOH until the pH was approximately 11 and covered with a special Teflon cap. The reason for the addition of KOH is the conversion of the free selenious acid into the selenite form, thus avoiding possible selenium losses during the subsequent evaporation step. After cooling, the sample reduced in volume (about 10 mL) was treated with a mixture of 4 mL of concentrated HN03 and 1 mL of concentrated HC104, heated for 30 min at 120 °C and then heated until white fumes of perchloric acid appeared. The solution was concentrated until the solvent was almost completely evaporated. For the reduction of selenate to selenite, 10 mL of 5 mol/L HC1 were added after cooling and the sample was heated for 30 min at 90-100 °C. The reduction step is necessary for the precipitation of elemental selenium with ascorbic add as well as for the formation of selenium hydride (SeH^ with sodium tetrahydridoborate (NaBH4). These two methods were applied alternatively to isolate selenium from the digestion solution for mass spectrometric measurement. Usually the preripitation method was applied, because this method requires simpler instrumentation and offers a higher recovery of selenium. The hydride generation method was applied only in cases where a residue remained after the digestion step. The reduction of selenite to elemental selenium was performed with 6 mL of an ascorbic add solution (200 mg of L-ascorbic add/g of solution). After standing for at least 4 h, the mixture was passed through a 5-µ pore size poly(tetrafluoroethylene) (PTFE) filter and the precipitate was dissolved with 300 µL of a concentrated HNG3/concentrated HC1 mixture (2:1). The resulting solution was evaporated to dryness in a Teflon vessel by using a surface heating apparatus. The residue was then mixed with 10 µL of a silica gel suspension, and this mixture was deposited on the evaporation filament of a double-filament thermal ion source for mass spectrometric measurement as previously described (12). For the generation of SeH2, the solution was adjusted to 4 mol/L HC1 and transferred into a gas stripping vessel made of glass (3 cm X 20 cm). The conversion of Se(IV) into SeH2 was carried out with a 4% (by weight) solution of NaBH4, which was introduced continuously with a peristaltic pump (1.5 mL/min).
1986
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 18, SEPTEMBER 15, 1991
peristaltic pump
~
_i
j
0
*8«
wSeOfwide8eOf spike addition
^
spike addition
Separation by anion exchange
-Teflon
tubing
V
Button with
Elution with 3 mol/l HO
L
droplet separator
Reduction
1mot/l HCOOH
|
Conversion ¡rrto DMSe
Cation exchange
—absorption tube
vessel-
ra—
-»
/ sample and spike
Adjustment of
Adsorption at
Adsorption at
Evaporation and abeorp. of DMSe
Button with 5 mol/l HO
Evaporation to dfyneaa
·
sintered glass plugs
XAM
chremMography
Button with
[
Bullón with
eSeOf apikaaddttion 1
...
j
resin
,ß . (pH-10) |
[
toSe{0}
stripping
Adjustment of
Ic
Swagelok c
1
HN08/HCI04 digestion and reduction toSe(0)
__L__1_1_ Mass spectrometric
^Sa^Se ratio measurement with
Trimethyl seienonlum
Neutr. and basic
Addc
organoaelenhuri
organoeelenlum
Figure 3. Sample treatment for the determination of selenium species In water samples with IDMS.
Figure 2. Hydride generation and absorption system for the formation of SeH2 and Its subsequent absorption In HN03.
The produced SeH2 was absorbed in 10 mL of concentrated HNOa by using a glass absorption tube (1 cm X 20 cm). The hydride generation/absorption system is schematically represented in Figure 2. The sintered glass plugs in the stripping vessel and absorption tube distribute the gas flow by very small gas bubbles, thus increasing the formation and the absorption efficiency of SeH2. After absorption, the HN03 solution was transferred into a Teflon vessel where the sample was evaporated to dryness as described above. To prevent a passage of HN03 through the glass plug, the nitrogen carrier gas was continuously passed through the absorption tube. Adsorptive loss of SeH2 within the apparatus was reduced by deactivating all glass surfaces with a solution of dimethyldichlorosilane in toluene prior to the analytical procedure and by connecting all components of the system with Teflon tubings using Swagelok connectors made of Nylon. The selenium recovery for the two different isolation methods was determined with a radioactive 76Se tracer in the form of selenite to be 90% and 80%, respectively, for the precipitation and the hydride generation/absorption method. These recoveries are sufficient for an analysis with IDMS and have no influence on the result, because these losses of substance appear after equilibration between sample and spike. Selenite and Selenate. Because Se" thermal ions measured in the mass spectrometer are formed by different selenium species, it is not possible to determine a mixture of species with NTI-MS (16). Therefore, a complete chemical separation of the species from each other must precede the mass spectrometric measurement. The sample treatment applied for the determination of the different selenium species is schematically represented in Figure 3. For the selective determination of selenate and selenite, the samples (up to 250 g) were weighed into PE bottles, which had been cleaned by repeated shaking with diluted HNOg and bidistilled water. Then, an exactly weighed amount (approximately 1 g) of the selenate and the selenite spike solution was added. The sample was passed through a column (plastic tube with a diameter of 0.6 cm) that was packed to a height of 9 cm with the anion-exchanger resin Dowex AG1-X8 in the chloride form (200-400 mesh). After rinsing the column with bidistilled water, selenite and selenate were successively eluted with 25 mL of 1 mol/L HCOOH and 25 mL of 3 mol/L HC1, respectively. Before eluting selenate, the column was washed with 10 mL of bidistilled water. Radiotracer experiments were carried out to investigate the separation behavior of selenite and selenate with this anion-exchanger resin. The result is shown in Figure 4. The selenite fraction can be directly evaporated to dryness for the mass spectrometric measurement. The selenate fraction, however, is reduced to elemental selenium according to the total selenium determination method, because the simultaneously separated sulfate would cause selenium losses during an evaporation step. Trimethylselenonium. Two different separation procedures were used for the determination of TMSe+ in water samples
Figure 4. Chromatographic separation of selenite and selenate In a column filled with the strongly basic anion-exchanger resin using an aqueous solution containing selenite and selenate labeled with "Se. A NaI(TI) scintillation detector coupled with a single-channel analyzer was used for -countlng.
(Figure 3). The chromatographic procedure was based on the method of Oyamada and Ishizaki (19). TMSe+ was isolated at the cation-exchanger resin Dowex 50W-X8 in the H+ form by using a Pyrex glass column 25 cm high and 0.8 cm in diameter. After an exactly known amount of the TMSe+ spike (approximately 3 g) was weighed to the sample (about 500 g), 10 mL of a 0.1 mol/L sodium thiosulfate solution was added. Many cations react with thiosulfate to form negatively charged complexes, which will not be absorbed by the resin. Then, the column was subsequently rinsed with bidistilled water and 20 mL of 0.1,0.5, and 1 mol/L HCL TMSe+ was eluted with 5 mol/L HCL Afterwards, the eluate was concentrated until the solvent was almost completely evaporated. The selenium content was determined after decomposing TMSe+ with a mixture of 4 mL of concentrated HN03 and 1 mL of concentrated HCIO* following the method which was applied for the determination of the total selenium content. The alternatively applied method for TMSe+ analysis is based on the thermal decomposition of TMSe+ in highly alkaline solutions forming the volatile compound dimethyl selenide (DMSe). For that, the spike solution was first added to the water sample. This solution was then evaporated to a volume of about 30 mL by using a rotary evaporater. While this sample solution was warmed up to 90 °C in a stripping vessel (Figure 2), a 65% (by weight) KOH solution was continuously introduced. The generated DMSe was absorbed in 10 mL of concentrated HN03. The preparation of the sample for the mass spectrometric measurement followed the procedure described for the hydride generation method. Investigations with a TMSe+ standard solution showed that the recovery of selenium for the DMSe formation and absorption step averaged about 80%. Other Organoselenium Compounds. The separation of organoselenium compounds other than TMSe+ from water samples was carried out by adsorbing these compounds on XAD-2 resin at pH = 8 and 3, respectively. As it was shown by Suzuki
ANALYTICAL CHEMISTRY, VOL. 63, NO. 18, SEPTEMBER 15, 1991
Table III. Description of the Analyzed Water Samples and Important Water Parameters
sample
characterization
amt of IC,“
amt of
pH
mg/L
mg/L
G-2 G-3
R-l R-2
L-l L-2 L-3 L-4
karst aquifer karst aquifer' gravel aquifer
7.0 7.0 6.5
38 39
Samples selenium concn, ng/g selenite selenate
DOC,6
River Water Danube River near 6.2 Regensburg, uncontaminated region 6.2 like R-l, other place Lake Water from a coal mining area artificial lake on the campus of the University of Regensburg moorland lake Id moorland lake II'
4
6 5
sample total selenium
