Selenium

C H A P T E R

1 0

S ELENIUM

10-0.

INTRODUCTION

The element selenium is 66th in abundance in the Earth’s crust and has 6 stable isotopes, of which 78Se and 80Se are the most common (see also Appendix A). Se can be found in four natural oxidation states: þ6 (selenate), þ4 (selenite), 0 (elemental selenium) and 2 (selenide). Selenium is mainly obtained from metal sulfide mineralizations, mostly with Cu, Zn, Ag, Hg, and Pb. Some Se-minerals, e.g. tiemannite and naumanite, contain up to 24% Se. Major organic selenium species are selenoproteins: selenomethionine and selenocysteine (Barceloux, 1999; Finley, 1999). Selenium is an important human (and animal) nutrient (closely related to an antioxidant like vitamin E); component of the enzyme glutathione peroxidase protecting against oxidative damage; and an antidote for Hg, Cd, As and other elements (recommended daily intake of Se: 50–200 mg). Se deficiency has been associated with cardiomyopathy (= Keshan disease, China) among children and increased risk of cardiovascular disease (Finland); high doses are toxic but also depend on the chemical form of selenium. Acute ingestion of selenious acid is almost invariably fatal. Garlic odor of the breath, caused among others by dimethyl selenide expiration, may be an indication of excessive selenium exposure. Se supplementation has shown to inhibit tumorigenesis in several organ system models of animals and is considered to be a cancer-prevention agent in humans. Se is further involved in resistance against viral infections, aging processes and male reproduction. Se is used as a decolorizer (low concentration) or to color (higher concentration) glass or ceramics, in semi-conductor diodes, in photoelectric cells, shampoos, to improve the properties of metal (steel) alloys, as a catalyst in the chemical industry, as an additive to dyes, plastics and lubricants, in the rubber industry (vulcanizing) and in clinical treatment (Rea et al., 1979; Knab & Gladney, 1980; Thomson & Robinson, 1980; Reamer & Veillon, 1981; Young, 1981; Raptis et al., 1983; Mosher & Duce, 1987; Grosser & Heumann, 1988; Heumann & Ra¨dlein, 1989; Patterson et al., 1989; Swanson et al., 1991 Aggarwal et al., 1992b; Barceloux, 1999; Finley, 1999; Johnson et al., 1999; Coplen et al., 2002; Fox et al., 2005). Selenium is emitted into the atmosphere, mostly as volatile compounds (majorly as dimethyl selenide species) by natural (e.g. marine flux) and anthropogenic processes (Mosher & Duce, 1987; Amouroux et al., 2001; Wen & Carignan, 2007), and is also present in the aerosol phase (Mosher & Duce, 1983, 1987). Thermodynamic data and geochemistry of isotopic compounds of Se were reported by Krouse & Thode (1962) and Se´by et al. (2001). The chemical and biological redox reactions of Se(IV) and Se(VI) are the major isotope fractionating processes identified to date (Johnson et al., 1999). Other studies on the isotopic effects of reduction on Se compounds 723

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are found in Rees & Thode (1966), Rashid et al. (1978) and Rashid & Krouse (1985). Bacterial or microbial Se isotopic fractionation was reported by Rashid et al. (1978), Herbel et al. (2000, 2002), and Ellis et al. (2003). Biological reduction of Se oxyanions in slurries or estuary sediments with 80Se/76Se isotopic fractionation was reported by Ellis et al. (2003). Significant Se isotopic fractionation (up to 12‰) by reduction of Se oxyanions by Fe(II) þ Fe(III) hydrous oxides was reported by Johnson & Bullen (2000, 2003). Selenium isotopic values of the Earth mantle and meteorites seem to be limited to a narrow range of less than 0.5‰. (Rouxel et al., 2002). The largest natural 82/76Se variation was found, up to date, by Wen et al. (2007), to be ranging from 12.77‰ to þ4.93‰. A review on stable isotope fractionations in natural systems was presented by Johnson et al. (1999), Johnson (2004), and Johnson & Bullen (2004). Selenium isotope variability in natural (hydrothermal) systems was reported by Rouxel et al. (2004). Figure 10-0.1 shows thermodynamic stability fields for selenium in a pH–Eh(V) diagram. Figure 10-0.2 shows a diagram of selenium isotopic fractionations by different processes between different redox states and/or materials containing Se. Delta values, isotopic scale and enrichment factor – Delta values are defined in the same way as the general definition (see the definition in the Introduction Chapter in this book). Because there are six Se isotopes and the use of ‘-values’ varies between different authors in publications, we need a uniform way to express which Se isotopes are concerned. Therefore, it is required to include two isotopes, instead of the single, heaviest one, in the delta value notation. This, for example, can be done in a form such as 80/76Se or 82/76Se.

1.2

HSeO4–

10–6 M

H2SeO3 0.7

SeO42–

Eh(V)

HSeO3–

0.2

SeO32–

Se0

–0.3

H2Se HSe–

–0.8 0

2

4

6

8

10

12

14

pH

Figure 10-0.1 Eh(V)^ pH diagram showing thermodynamic stable fields for different forms of Se (after Elrashidi et al., 1987).

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Selenium

Se(VI)

Abiotic 7–12‰

Abiotic 0‰

Bacterial 3–5‰

Se(IV)

~0‰

Higher plants

~1‰?

Abiotic 6–12‰

0‰

Bacterial 6–9‰

Se(0)

Algae

Small?

Dissolved organics

0‰

Se(-II) Small?

Figure 10-0.2 Diagram showing reactions between Se species and related processes and Se isotopic fractionations (after Johnson, 1999).

A uniform Se isotopic scale is not yet defined. The most general scale used for Se isotopic calibration is related to the SRM-3149 standard, which is also close to the CDT (Canyon Diable Troilite) standard in Se isotope compositions. Carignan & Wen (2007) determined the 82/76Se composition of SRM-3149 relative to a MERCK-Se solution, which was already used as reference by Rouxel et al. (2002). Obviously, there is need for consensus and for the definition of a unique scale (or better scales, for the various ratios that are applied for analytical purpose). An enrichment factor " is given in literature to report relative differences between -values of samples (in this example for -values based on 80Se/76Se ratios) in reduction, e.g. by Rayleigh-type processes. Values of " were extracted from data by plotting measured 80Se/76Se values against consumed species ln( f ), where f is the fraction of the Se(VI) or Se(IV) remaining (Herbel et al., 2000, Ellis et al., 2003): "¼

dð80 =76 SeÞ dðln f Þ

[10-0.1]

If " is constant over time, the data can be fit to a linear function: ð80 Se=76 SeÞ ¼ "  lnðf Þ þ ð80 Se=76 SeÞo where (80Se/76Se)o is the initial 80Se/76Se of the consumed species.

[10-0.2]

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10-1.

PREPARATION M ETHODS

In general, with the preparation of samples containing Se, losses of selenium during oven-drying were found to be a problem (Crews et al., 1994). Rock and sediment samples – Krouse & Thode (1962) extracted Se (elemental form) from natural samples by HBr–Br2 reaction technique (after methods reported in the 1920’s and 1930’s; see references in Krouse & Thode, 1962). Knab & Gladney (1980) dissolved rock samples by acid digestion in several steps, with HNO3 and HF, followed by HNO3 þ HF þ HClO4 and HNO3 þ HCl þ K2Cr2O7 þ Ta carrier þ H3BO3. In the last step, the solution was evaporated until HClO4 fumed and heating was continued until Cr3þ was oxidized to Cr2O72 (color shift from green to bright orange). The residue was dissolved in HCl and boiled for 20 min to reduce Se(VI) into Se(IV). After cooling, H3PO4 was added. Se was separated in an Al2O3 ion exchange column. Non-siliceous samples (including coal or fly ash samples) followed a slightly different dissolution scheme. If dissolution was difficult, fusion with NaOH þ Na2O2 in a Ni crucible (not for organic samples) was applied before acid digestion. Wachsmann & Heumann (1989) used acid digestion (concentrated HNO3 þ concentrated HF) of sediments, sandy soil and sewer sludge samples (for IDMS a spike solution was added before the decomposition was carried out). After decomposition of the samples, Se(VI) was reduced to Se(IV) by HCl (addition of 20 mL of 5 mol/L HCl). Johnson et al. (1999) found that the chemical reduction process caused isotopic fractionation (they observed >4‰ in 80Se/76Se ratio; this problem can be overcome using the double-spike method; see Section 10-4.2,3 and Volume I, Part 1, Chapter 29). SeH2 was formed by hydride generation [3 w% solution of NaBH4 (Figure 10-1.1)], and H2Se was stripped out of N2 Peristaltic pump

Droplet separator

Teflon tubing NaBH4 solution HNO3 Stripping vessel Absorption tube

Sample + spike Sintered glass plugs Stirrer

Figure 10-1.1 Diagram showing the system for hydride generation of SeH2 and subsequent absorption in HNO3 (afterTanzer & Heumann, 1991; see also Heumann & Ra«dlein, 1989).

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the solution by a N2 carrier gas in a liquid–gas separator and was passed to a fluoropolymer tube containing concentrated HNO3 where it was absorbed into solution. HNO3 was taken to dryness with Se recovery of 85% (Rouxel et al., 2002; Ellis et al., 2003) or an average of 48% (Johnson et al., 1999), with major losses occurring through incomplete reaction and flow of unreacted Se out of the liquid–gas separator. Johnson et al. (1999) removed ferric iron via cation exchange resin to avoid interference with the Se reduction process. Ellis et al. (2003) described a continuous flow hydride generation system (Figure 10-1.2; see also Figures 10-5.2 and 10-6.2 and Rouxel et al., 2002) for purification of large-volume samples of Se for TIMS analytical purpose. All Se was converted to Se(IV). The sample dissolved in a 3–6 M HCl matrix was pumped at 8 mL/min and mixed with 1% NaBH4 solution pumped at 1 mL/min. Organic compounds were removed by oxidation with several additions of 100 mL concentrated HNO3 and 50 mL, 30% H2O2 to the dried samples, each followed by evaporation to dryness (Rouxel et al., 2002; Ellis et al., 2003). Heating to 120C (samples in closed PTFE containers) followed by overnight evaporation to dryness at 70C was applied by Rouxel et al. (2002). Temperature must stay below 80C to prevent Se loss during evaporation. Separate extraction of Se(IV) and Se(VI) by anion exchange methods was applied by Ellis et al. (2003) on sediment samples. Selenium redox change – Isotopic fractionation induced by reduction of Se(IV) to Se(0) was variable (between 19 and 10‰) and depended on the reaction mechanism and rate. Krouse & Thode (1962) and Rashid & Krouse (1985) reduced Se(IV) (5 g Na-selenite dissolved in 50 mL water) with dilute hydroxylamine hydrochloride (excess quantity) to form elemental Se (slow precipitation) following the reaction

H2Se + N2 to concentrated nitric acid trap

H2Se + N2 + solution

Frit

NaBH4 (1% w/v) (1 mL /min)

Drain Sample (1–4 M HCl) (8 mL/min) 3–6 M HCl matrix

N2 Peristaltic pump

Figure 10-1.2 Schematic diagram of the continuous flow hydride generation system developed for purification of Se before TIMS analysis (after Ellis et al., 2003).The frit is a 4 - to 5.5-mm fine-glass frit.

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Handbook of Stable Isotope Analytical Techniques

2Hþ þ 2NH2 OH þ SeO2 3 ! Se # þN2 O " þ4H2 O

[10-1.1]

Rashid & Krouse (1985) determined the isotopic fractionation in this system for 80Se/ 76Se and 82Se/76Se. Selenate (Se[VI]) was reduced to selenite (Se[IV]) by HCl (1 M) following the reaction (Rees & Thode, 1966) SeO42 þ 8Cl þ 8Hþ ! SeCl62 þ Cl2 þ 4H2 O

[10-1.2]

Rees & Thode (1966: 82Se/76Se) studied isotopic fractionations in the system of equation [10-1.2]. Rees & Thode (1966: 82Se/76Se) reduced Na-selenite (in solution) with ascorbic acid (C6H8O6) following the reactions SeO32 þ 2Hþ ¼ H2 SeO3

[10-1.3]

H2 SO3 þ 2C6 H8 O6 ¼ Seþ2C6 H6 O6 þ 3H2 O

[10-1.4]

and studied the Se isotopic fractionations in this system. Rashid & Krouse (1985) reduced SeO32 to H2Se with a mixture of HI–H3PO2–HCl and studied the isotopic fractionations, and compared these with fractionations in other reduction methods given above. They concluded that Se(IV) reduction was a multi-step process and the relative rates of the steps determined the amount of isotopic fractionation involved. Rees and Thode (1966) found a fractionation of around 18‰ for Se(VI) to Se(IV) reduction by strong HCl at 25C. Johnson et al. (1999) measured a much smaller fractionation (8.3 ‰) at 70C. Brimmer et al. (1987) described a quantitative method for reduction of selenate into selenite. Johnson et al. (1999) used 6 N HCl with heating at 100C for 20 min. See also Janghorbani & Ting (1989) and Ting et al. (1989). Water samples – Water samples were filtered (0.45-mm cellulose nitrate filter; polycarbonate filter) and any material passing the filter was considered to be dissolved. A procedure for Se extraction from water samples for isotopic measurement (by N-TIMS), mostly based on reductive reactions is given in Figure 10-1.3. Heumann & Grosser (1989) separated selenite and selenate from water solutions by a weakly basic DEAE (diethylaminoethyl; 2 g) cellulose exchange column. Formic acid (1 mol/L) was added first and then the sample solution. Selenite is eluted with 1 mol/L formic acid and selenate with 0.1 mol/L HNO3. Tanzer & Heumann (1991) separated compounds from water samples by GC and used HNO3/HClO4 reaction for extraction of Se from the thus separated organic compounds. For details of the acid reaction, see the next section and Figure 10-1.4 [Tanzer & Heumann (1991) used different ratios of acid mixture and heating procedures]. Organic or biological samples – For enriched Se isotope tracer studies, Se can be derivatized in the form of 5-nitropiazselenol (NPD) from organic or biological samples by digestion with HNO3, H3PO4 and H2O2 (rapid reaction: 10–20 min to get clear solution: Reamer & Veillon, 1983) followed by reaction of the acid solution with (4-nitro-)o-phenylenediamine (purification/preparation procedure described by Reamer & Veillon, 1981, 1983 and Aggarwal et al., 1992b). The remaining HNO3, which will react with the chelating agent, was removed by the addition of formic acid (Reamer & Veillon, 1983; Swanson

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Weighing sample (~200 mL) Addition 82SeO32– tracer (~1 g)

Bring at pH 11–12 with 2 M KOH solution Evaporate to near dryness

Reduction of SeO42– to SeO32– with 10 mL 5 M HCl; cover heat ~30 min at 90–100°C

Reduction of SeO32– to Se0 with ascorbic acid (5 g; 200 mg/g solution)

Elementary Se (colored red) filtered off (filter with 1–2 mL methanol and drops of water Dissolve in concentrated HNO3/HCl Evaporate to dryness Dissolve in 20 μL water

Figure 10-1.3 Diagram for preparation of Se from water samples for N-TIMS measurement (after Grosser & Heumann, 1988).

et al., 1991; Aggarwal et al., 1992b; Moser-Veillon et al., 1992), and the undigested lipids were, after addition of 10 mL deionized H2O (pH of solution is 0.5–1.5), removed with 5 mL of chloroform (Reamer & Veillon, 1983). Se–NPD was extracted into chloroform (2 mL 15 min in mechanical shaker: Reamer & Veillon, 1983) for injection in a GC-MS device (Reamer & Veillon, 1981; Patterson et al., 1989; Veillon et al., 1990; Aggarwal et al., 1992b). The above described methods were not meant for high-precision Se isotope determinations, but to detect enrichment of Se isotopes in tracer experiments or tests. Ting et al. (1989) used a mixture of HNO3–H2O2 (plasma, red blood cells, faeces, food) or a mixture of HNO3–HClO4 (urine) as an oxidant for wet-ashing samples. This was followed by boiling for 10 min with HCl to reduce Se into selenite form for analysis by hydride generation – ICP-MS. Buckley et al. (1992) added Mg(NO3)2 (20 mL = 23 g Mg/L concentrated HNO3) as an ashing aid to orchardgrass hay (1.0 g) or bovine serum (1.0 mL). Ashing was applied in covered beakers at 105C for 30 min, followed by 185C for 2 h; then, uncovered, the solutions were heated at 275C until the beakers were dry at the bottom. The cover was replaced and the beakers were heated at 500C in a muffle furnace for 4 h. Ash was dissolved in HCl (3.8 M in 20 mL). Use of 4-trifluoromethyl-o-phenylenediamine (TFMPD) (Veillon et al., 1990; Aggarwal et al., 1992b) or 3,5-dibromo-o-phenylenediamine (DBPD) (Aggarwal et al., 1992b) as

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Handbook of Stable Isotope Analytical Techniques

Weighed sample (0.3–2 g) Addition 82Se-enriched spike Exact weight; pH ≈ 1

Decomposition of sample + 5 mL bidest. H2O + 6 mL of concentrated HNO3 /HClO4 (10:1) Heating at 100°C for 30 min + 140°C for 20 min (= step 1)

Adding 6 mL HNO3/HClO4 (2:1); Heating at 180°C for 45 min + 200°C for 5 min (= step 2)

Concentration of solution (200°C) ± complete evaporation (Risk: HClO4 may form explosive compounds)

Reduction of Se(VI) to Se(IV) with 15 mL 5 mol/L HCl Heating at 130°C for 30 min

Formation of SeH2 with NaBH4 (3% solution) Selective distillation

Absorption of SeH2 in concentrated HNO3 evaporated to dryness

Figure 10-1.4 Diagram for preparation of Se from food samples for N-TIMS measurement (after Heumann & Ra«dlein, 1989).

a chelating agent, instead of 4-nitro-o-phenylenediamine (NPD), was reported and tested for GC behavior (memory; precision and accuracy of isotopic ratio measurement). – Blood samples (serum – red blood cells) were collected into 12- or 25-mL disposable, trace-metalfree syringes containing acid citrate dextrose and were centrifuged (2000 = g for 15 min), and plasma was drawn off with a disposable pipette and aliquots were placed in cryotubes and frozen quickly within 45 min after collection and stored at 20C (Janghorbani et al., 1982a: without citrate dextrose in syringe; Patterson et al., 1989). Samples were further treated by acid digestion followed by derivatization as is reported above.

Selenium

731

– Urine samples (100 mL) were placed in plastic bottles and stored at 20C until analysis (Janghorbani et al., 1982a; Patterson et al., 1989). Samples were further treated by acid digestion followed by derivatization as is reported above. – Fecal samples were collected in polystyrene containers and, in the laboratory, homogenized in a colloid mill using deionized water. Weighed samples were lyophilized for 6 days and stored frozen at 20C (Janghorbani et al., 1982a; Patterson et al., 1989). Samples were further treated by acid digestion followed by derivatization as is reported above. – Food contains, commonly, Se in the form of selenomethionine (Veillon et al., 1990; Swanson et al., 1991). Food samples were decomposed with a HNO3/HClO4 mixture and eventually a spike solution was added (Heumann & Ra¨dlein, 1989). Se was recovered by hydride formation (SeH2) and the hydride was trapped in a HNO3 solution and evaporated to dryness (see procedure in Figure 10-1.2). – Oil products from refineries, waste streams were processed either by ferric iron coprecipitation methods or by hydride generation (see above; Johnson et al., 2000). – Plant material (0.2 Mg) was dried, ground and digested at 40C with concentrated HNO3 (10 mL) and periodic additions of 200 mL of 30% H2O2 was done over a period of 5 days. The solution was taken to dryness and dissolved with 6 M HCl (Herbel et al., 2002). Marin et al. (2003) derived concentrated selenium from lichens and plants using matrix separation and pre-concentration of samples with thiol cotton (see also below). Thereafter, samples were digested with HNO3–H2O2–HF and analyzed for Se content by graphite furnace atomic absorption spectrometry (GFAAS). Column separation techniques Al2O3 column separation of Se(IV)–Se(VI): Acidic (1–8 M HCl) Al2O3 has been found to be an effective and selective ion exchanger for Se(IV) and Se(VI) (Knab & Gladney, 1980). Phosphate added to the preconditioning solution of the column precludes adsorption of Se(VI) without affecting Se(IV) adsorption. For complete Se collection on the column, Se(VI) must be reduced in HCl (which is temperature and pH dependent: 6 M HCl and 20 min boiling are required for complete reduction; see Knab & Gladney, 1980 for details). Herbel et al. (2002) applied a ferric hydroxide co-precipitation technique to separate Se(VI), which did not adsorb strongly when sulfate was present, from other dissolved Se species which did adsorb strongly. Anion-exchange chromatography: Johnson et al. (2000) collected Se on an anion-exchange resin [Se(IV) and Se(VI)] (but only a small part of organic Se) or, after conversion of Se(VI) and organic Se into Se(IV), by co-precipitation with hydrous ferric oxide. Herbel et al. (2000) used anion chromatography for separation of Se(IV) and Se(VI) if they coexisted in the samples after a method as was given in Ornemark & Olin (1994). Ellis et al. (2003) applied a similar anion chromatography Se separation technique. Se extraction by TCF (thiol cotton fiber): TCF, cotton to which thiol groups had been attached by reaction with thioglycollic acid, quantitatively absorbed elements such as Se, Te, As and Sb, depending on their various oxidation states, and were used for separation of Se from the sample solution (Yu et al., 1983; Xiao-Quan & Kai-Jing, 1985; Rouxel et al., 2002, 2004; Marin et al., 2003). This method enabled rapid sample preparation, as Se was extracted from solutions with a single step and was then released from the TCF via oxidation by nitric acid. Separation of Se from Ge (an isobaric interferent) was complete, but separation from As was not.

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Handbook of Stable Isotope Analytical Techniques

10-2.

F LUORINATION M ETHOD

Elemental Se was fluorinated into SeF6 by Krouse & Thode (1962), basically following fluorination methods such as those described for sulfur isotope analysis (Chapters 8-1.7 and 8-1.8.2 and Volume I, Part 1, Chapter 20.4.5). Krouse & Thode (1962) used Monel steel for their sample-handling system and avoided any contact with grease or Hg in their system. Se isotopes (82Se/76Se ratio) were measured on the most abundant ion species: SeFþ 5 (masses 171 and 177). Rees & Thode (1966) and Rashid & Krouse (1985) described methods to prepare elemental Se from selenite and selenate. Doctoral theses by Krouse (1960) and Rashid et al. (1978) included studies of fluorination of Se. A repeatability for 82/76Se analysis in natural samples between –0.1 and 1‰ was reported by Krouse & Thode (1962).

10-3.

GC-MS M ETHOD

A GC-MS (quadrupole) equipment was used, with stable isotope dilution [added: Se (= stable) and 75Se (= radiogenic); see Volume I, Part 1, Chapter 37 for explanation of isotope dilution techniques], for quantitative measurement of Se (Reamer & Veillon, 1981; Patterson et al., 1989; Veillon et al., 1990). Because isotopes are involved in the method, it is included in this review. From isotope ratio measurements in the Se-NPDþ parent ion cluster, the amount of ‘Se-tracer’ [74Se, 75Se (= radiogenic), 76Se, and 82Se were reported], the un-enriched natural Se and total Se can be quantitated. 82 Se/74Se, 82Se/76Se and 82Se/80Se ratios were determined by Veillon et al. (1990). Solutions were injected (1 mL; at 175C) into a column (1.2 m  2 mm i.d. silanized glass column packed with 1% SP2401 on 100/120 mesh Chromosorb 750; at 160C). A He carrier gas flow (20 mL/min) was applied (Reamer & Veillon, 1981). Aggarwal et al. (1992b) brought forward that the key problem in GC-MS methods is the lack of chelating agents without appreciable memory problem at nanogram levels. Other studies using GC-MS can be found in Mangels et al. (1990, following the method by Reamer & Veillon (1983); tracers: 74Se in selenomethionine and 76Se in sodium selenite], Swanson et al. (1991), Moser-Veillon et al. (1992; tracers: 74Se in selenomethionine and 76Se in sodium selenite) and Van Dael et al., (2002: 76Se-selenate and 74Se-selenite in a milk-based infant formula). Boza et al. (1995) analyzed Se isotopes in a comparative intrinsic and extrinsic 78Se and 82 Se and radiogenic (75Se) tracer study. Rat feces and yeast (Saccharomydes cerevisiae) were dried, ground and well mixed and isotopically spiked. These preparates were digested with equal volumes of HNO3 (14 mol/L) and HClO4 (70%) at 180C for 36 h. Selenate was reduced to selenite by the addition of HCl (6 mol/L) and neutralized with NH4OH (7 mol/L). Se was derivatized with 4-nitro-o-phenylene-diamine, extracted with chloroform and dried under an atmosphere of N2 and measured for its isotopic composition by GC-MS. Wolf & Zainal (2002) analyzed the five Se isotopes in selenomethionine and methylselenocystine in food materials by GC-MS with 74Se-enriched selenomethionone as the tracer. 82

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Selenium

An overall repeatability of about 1% in a day and of about 3% day-to-day was reported. Note that many of these studies concern enriched tracer methods, where repeatability is less of a concern.

10-4. 10-4.1.

TIMS M ETHOD

Negative ion TIMS

Dual filament approach – Heumann et al. (1985), Grosser & Heumann (1988), Wachsmann & Heumann (1989, 1992) and Heumann & Wachsmann (1989) described a negative TIMS (N-TIMS) method for measuring Se isotopes. They used a double-Re-filament ion source with distance between the filaments of 2 mm for optimal performance. Se (0.5–15 mg) was loaded as H2SeO3 or BaSeO3 solution and evaporated to dryness, and 10 mL silica gel suspension was added dropwise and also evaporated to dryness (causes enhancement of the Se current by a factor of 40; ion current >1011 A). Wachsmann & Heumann (1989) and Heumann & Wachsmann (1989) first mixed the silica gel and dried selenium sample to load on the filament. After reaching a vacuum of 106 Torr, the ionization filament is heated to 1.4 A at a rate of 0.15 A/min, after which the rate was decreased to about 0.04 A/min. At a temperature between 850C and 900C, the first Se ions were observed. Now, the heating rate is 0.01 A/min until the signal stops growing and becomes almost stable (at 930–960C). In some cases, the evaporation filament needs to be heated to 0.5–1 A (e.g. if Se is loaded as BaSeO3 or Na2SeO4 or if Se was separated from the matrix by ion exchange chromatography, introducing an additional retarding effect of organic material originating from the resin). In earlier stages, techniques for Se isotope measurement with Ba(OH)2 loaded on the ionization filament without additions to the Se salt loaded on the evaporation filament were applied (e.g. Grosser & Heumann, 1988), but with the disadvantage that relatively large sample amounts are necessary to generate sufficiently high ion currents (>1012 A). In this method, SeO and SeO 2 were also produced as minor abundance ions. Further, the dependence of the Se ions on filament temperature is similar to the above described method. Ba(OH)2 was continued to be loaded on the ionization filament with the Se sample prepared with silica gel on the evaporation filament (e.g. Heumann & Wachsmann, 1989). Grosser & Heumann (1988) reported a factor 4 higher Se beam when selenious acid instead of barium selenite or sodium selenate is loaded on the filament. A maximum ion intensity was reached at temperatures between 970C and 1000C (1.8 A) of the ionization filament. Se isotope ratios can be determined with this negative TIMS method with a relative standard deviation in the order of 0.3–0.6% (Grosser & Heumann, 1988), improved to 0.1–0.3% by application of the silica gel technique (Wachsmann & Heumann, 1989). Wachsmann & Heumann (1989) also applied loading of samples in a resin bead, where better results were obtained then with the ‘Ba(OH)2-only-technique’ used before. However, the silica gel method gave results better (more thermal Se ions

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Handbook of Stable Isotope Analytical Techniques

produced) than those of the resin-bead method and this method was not followed up for that reason. It was noted that a smaller sample amount is necessary with negative TIMS compared with positive TIMS (0.5 mg versus 10 mg). IUPAC (1991) considered results by the negative TIMS method by Wachsmann & Heumann (1992) as ‘best results’ for Se isotopes. Single-filament approach – Johnson et al. (1999) developed a single-filament N-TIMS method for Se isotopic measurement. Ba(OH)2 (1 mL saturated solution = 24 mg Ba) was added to the Re filament to enhance ionization (see above) and the filament was dried. Volatility of the Se was reduced by adding graphite to the Se sample (100–500 ng Se as selenious acid þ 20.2 mg colloidal graphite). This mixture was loaded on top of the Ba(OH)2 and gently evaporated to dryness. The filament was heated for 5 min at 900C to remove interfering contaminations. Ion beams of 1011 A 80Se are yielded with this technique. Maximum signal intensity for Se occurs between 950C and 1000C, although data were collected at a lower temperature to minimize drift in instrumental discrimination. Ratios 82Se/80Se, 78Se/80Se, 77Se/80Se, 76Se/80Se and 74Se/80Se were determined, some in steps (caused by detector configuration limitations). A similar technique was applied by Ellis et al. (2003).

10-4.2.

Isotope dilution TIMS

The N-TIMS technique, in combination with IDMS, has been applied successfully (e.g. Heumann et al., 1985) in determination of Se and Se species in aquatic samples with very low concentrations (range of 10–100 pg/g) (Grosser & Heumann, 1988). Johnson et al. (1999) used ‘double spike (74Se, 80Se) TIMS’ (IDMS) methods for measuring Se (quantitatively) on shale samples. Johnson et al. (2000) used the same technique for water, oil refinery waste water and sediment samples, Herbel et al. (2000) for samples related to bacterial respiratory reduction processes and Herbel et al. (2002) for sediment cores and pore waters, plants, algae and shallow ground waters in a study on an agricultural drainage management system.

10-4.3.

Double-spike TIMS

Currently, high-precision Se isotope ratio data (better than 1‰ precision) can only be obtained by TIMS if a double isotope spike approach is employed. Double-spike TIMS techniques are discussed in Volume I, Part 1, Chapter 29. Briefly, two stable spike isotopes (e.g., 74Se and 82Se) are mixed to form a double spike solution, which is then added to the sample prior to sample preparation. The two spike isotopes are analyzed along with the target natural ratio isotopes (e.g. 76Se and 80Se), and their ratio is used to determine the mass bias of the instrument and correct it. Johnson et al. (1999) used double-spike TIMS methods for measuring 80Se/76Se to a precision of 0.2‰. Johnson et al. (2000b) used the same technique for water, oil refinery waste water and sediment samples, Herbel et al. (2000) for samples related to bacterial respiratory reduction processes and Herbel et al. (2002) for sediment cores and pore waters, plants, algae and shallow ground waters in a study on an agricultural drainage management system.

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Selenium

10-5.

ICP-MS

All Se isotopes suffer polyatomic interferences (e.g. 37Cl2 þ, 40Ar36Arþ, Ar37Clþ, Ar Arþ, 40Ar2 þ; see Table 10-4.1) (Lyon et al., 1988; using ICP-QMS). Results on 82Se are anomalously high, indicating that there must be an unidentified interference at this mass number (although a possible interfering molecule was reported by Crews et al., 1994; see Table 10-5.1). To decrease interferences of other ions with Se ions, collision cells or dynamic reaction cells can be applied (see Chapter 12-0.2.5.1). The use of a hexapole collision cell with hydrogen as the collision gas for measuring Se isotopes was reported by Palacz et al., (2001). Layton-Matthews et al. (2006) presented a continuous-hydride-generation

40

38

Table 10-4.1 List of possible common mass interferences encountered in an ICP-MS analysis of Se isotopes Isotope

Potential interference

74

Se

40

Ar S, Ni O, Cl2, Ar Ar, Ge, 39K35Cl, 42Ca16O2, 152Sm2þ, 154 Gd2þ

Lyon et al. (1988), Crews et al. (1994), Rouxel et al. (2002), Elwaer & Hintelmann (2007)

Se

36

Ar40Ar, 38Ar38Ar, 40Ar36S, 60Ni16O, AsH, 76Ge, 58Fe18O, 39K37Cl, 40 Ca18O2, 154Sm2þ, 154Gd2þ

Lyon et al. (1988), Buckley et al. (1992), Crews et al. (1994), Boulyga & Becker (2001), Palacz et al. (2001), Rouxel et al. (2002), Elwaer & Hintelmann (2007)

Se

40

Ar37Cl, 36Ar41K, 76SeH, 61Ni16O, K37Cl, 40Ar36ArH, 38ArH, 59Co18O, 155 Gd2þ

Lyon et al. (1988), Buckley et al. (1989), Crews et al. (1994), Palacz et al. (2001), Rouxel et al. (2002), Chatterjee et al. (2003), Elwaer & Hintelmann (2007)

Se

38

Ar40Ar, 77SeH, 62Ni16O, 41K37Cl, Ar36ArH2, 38Ar40Ca, 156Dy2þ, 156 Gd2þ

Lyon et al. (1988), Buckley et al. (1989), Crews et al. (1994), Palacz et al. (2001), Rouxel et al. (2002), Elwaer & Hintelmann (2007)

Se

40

Lyon et al. (1988), Buckley et al. (1989), Crews et al. (1994), Boulyga & Becker (2001), Palacz et al. (2001), Patterson & Veillon (2001), Rouxel et al. (2002), Elwaer & Hintelmann (2007)

Se

40

76

77

78

80

82

34

58

16

37

Reference 36

38

74

75

40

40

Ar2, 40Ar40Ca, 40Ar36ArH2, Ar40K, 64Ni16O, 32S16O3, 48 Ca16O2, 158Dy2þ, 158Gd2þ 40

80

Kr,

Ar42Ca, 34S16O3, 40Ar40ArH2, 82Kr BrH, 36Ar46Ti, 66Zn16O, 164Er2þ, 164 Gd2þ 81

Crews et al. (1994), Palacz et al. (2001), Chatterjee et al. (2003), Elwaer & Hintelmann (2007)

[It must be noted that this list is not exhaustive and many optional other interferences are possible.]

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Handbook of Stable Isotope Analytical Techniques

dynamic-reaction-cell ICP-MS technique, where mass interference was strongly reduced and a precision for 82/76Se of – 0.84‰ was obtained. Janghorbani & Ting (1989) used two ways of sample introduction: (1) a pneumatic nebulizer (PN), and (2) a hydride generator (HG). For the PN system the sample material was introduced into an Ar plasma via a peristaltic pump (sample solution flow rate 0.96 mL/min). For the HG system, a peristaltic pump was used to introduce the reagent (solution of 10 g NaBH4 and 2.5 g NaOH diluted with H2O to 1 L) and sample solution into the mixing chamber of the HG (Figure 10-5.1). Ting et al., (1989) also used ‘on-line’ hydride generation by reacting selenite sample solution (formed by boiling and reducing the oxidized selenium compound with HCl) with NaBH4. A two-channel peristaltic pump pumping both analyte and reagent (NaBH4) into the mixing chamber of the hydride generator was used. The output of the mixing chamber was send through a liquid–gas separator before introduction (H2Se) into the Ar plasma. Isotope dilution with 82Se was applied and 82Se/77Se and 74Se/77Se ratios were measured. For details of machine settings, see Ting et al. (1989). Minor influences by matrix interferences and over-all memory effects (pneumatic nebulizer) were discussed by Ting et al. (1989). Similar methods were used by Buckley et al. (1992: biological materials, measuring on enriched 76Se, 77Se, 82Se, and using 78Se as reference isotope). Addition of 0.2 M NaI to the NaBH4 solution during hydride generation suppressed interference of Cu2þ. Finley (1999) applied Se tracer methods (74Se, 82Se) for the study of retention and distribution of Se by humans consuming Se-labeled selenite, selenate or broccoli. Se extracted from urine, blood plasma, selenite, selenate and plant material was measured by HG-ICP-MS.

Torch Spray chamber 2-channel pump Analyte (10% HCl) Mixer NaBH4

Flow meter

Waste

Carrier gas Separator

Hydride generator

Figure 10-5.1

Schematic diagram of the hydride generator (after Janghorbani & Ting, 1989).

737

Selenium

Fox et al. (2005) studied absorption of Se by humans from wheat, garlic and cod, which were intrinsically labeled with 77Se and 82Se isotopes. All Se was converted into Se(IV), as this is the valence state of Se that can efficiently generate a hydride for HG-ICP-MS stable isotope analysis. Boulyga & Becker (2001; discussing analysis of Se, Ca and Fe) brought forward the fact that the use of a double focusing ICP-MS in high-resolution mode reduces drastically ion interferences, but involves a significant loss of ion intensity and therefore increased detection limits. Boulyga & Becker (2001) used a hexapole collision cell (see Chapter 12-0.2.5) in an ICP-QMS to depress molecular ion interferences. All of the above studies’ measured isotope ratios have precisions of about 1‰ or worse because single collector mass spectrometers were applied. High-precision isotope ratio measurements were obtained by Rouxel et al. (2002, 2004) by use of a continuous-flow hydride generator (Figure 10-5.2) coupled to an MC-ICP-MS device. The reducing agent used for the hydride generation was NaBH4 (1%) in NaOH (0.05%) solution, which was pumped with the sample or standard solution through a mixing coil for reaction. Liquid was separated from the analyte hydride gas, which was transported by Ar (carrier gas) to the torch for isotopic analysis in the MC-ICP-MS. All Se isotopes were measured by the array of nine Faraday cups of the MC-ICP-MS. Calibration was carried out by the ‘matching standard’ procedure as is also used for Fe or Cu isotopes (Belshaw et al., 2000; Zhu et al., 2000a). Minimum amount of Se for determination of 76Se/82Se ratios is 10 ng, while routine analysis was performed on amounts of 50 ng Se or more. Overall external analytical precision for the ratios 82Se/ 76Se, 82Se/78Se and 82Se/77Se was estimated to be 0.25‰ (95% confidence level). For details on the analytical procedure and settings of the analytical device, see Rouxel et al. (2002).

Autosampler

Sample/standard in HCl 1.7 N Pump

Ar1

Stripping and carrier Ar Ar2

Mixing coil To MC-ICP-MS torch

NaBH4 (1%) in NaOH (0.05%)

PTFE aerosol filter Gas–liquid separator (at 4°C)

Drain

Figure 10-5.2 Schematic diagram of the continuous-flow hydride generator coupled to a MCICP-MS device (after Rouxel et al., 2002).

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Handbook of Stable Isotope Analytical Techniques

The use of ICP-MS technique for Se isotope analysis is briefly discussed in Volume I, Part 1, Chapter 20.4.5.

10-6.

N EUTRON ACTIVATION M ETHOD

The Se metabolic pathway was studied on feces, plasma, red blood cells and urine for Se absorption, turnover and excretion budgets by neutron activation methods (Janghorbani et al., 1981b,1982a). Subjects took an oral dose of 74Se (74SeO2 3 ) for tracer purpose. It is possible to measure five isotopes of Se by neutron activation: 74

Se Se 78 Se 80 Se 82 Se 76

(n,g: 120 d, 265 keV) 75Se (n,g: 117.6 s, 160 keV) 77mSe (n,g: 3.9 m, 96 keV) 79mSe (n,g: 57.0 m, 103 keV) 81mSe (n,g: 23.0 m, 356 keV) 83Se

Precision of the method is relatively low, in the order of 10%, which is sufficient in most of this type of tracer studies, but can be too large for other purposes. A procedure for measurement of isotopes 74Se, 76Se and 80Se is given in Figure 10-6.1. See also Chapter 12-0.2.6 for general discussion on NAA methods and Table 12-0.4 for some Se activation reactions [given for fecal samples, after Janghorbani & Young (1980)]. Environmental samples were studied for Se contents by NAA techniques using the 74 Se(n,g)75Se reaction by Knab & Gladney (1980). Irradiated samples (7 h; thermal neutron flux: 1  1013 n/cm3.s) were dissolved in acid (5 M HCl–1 M H3PO4 solution) and 75Se was adsorbed on Al2O3 (chromatographic grade, 60 mesh) in an ion exchange column (0.7 cm i.d.  19 cm tubes with tapered tip; 100-mL reservoir on top). The adsorbed 75Se was measured by 4 large (60–80 cm3) Ge(Li) g detectors at 265 and 280 keV peaks or the more sensitive 136 keV peak for low-level samples. Measurements first were carried out at least 3 weeks after irradiation (to reduce personal exposure to radiation and to lower interferences by other radio nuclides with shorter half-lifes). Krivan et al. (1981) used the 74Se(n,g)75Se reaction for (quantitative) Se determination in bovine liver, blood plasma and erythrocytes.

Selenium

739

Figure 10-6.1 Schematic of the neutron activation procedure for measurement of 74Se, 76Se and 80 Se in human feces, plasma, red blood cells (RBC) and urine (after Janghorbani et al., 1981b).