GC-ICP-MS determination of dimethylselenide in human breath after ingestion of (77)Se-enriched selenite: monitoring of in-vivo methylation of selenium

Scientific opinion on the tolerable upper intake level for selenium

Following a request from the European Commission, the EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA) was asked to deliver a scientific opinion on the tolerable upper intake level (UL) for selenium. Systematic reviews of the literature were conducted to identify evidence regarding excess selenium intake and clinical effects and potential biomarkers of effect, risk of chronic diseases and impaired neuropsychological development in humans. Alopecia, as an early observable feature and a well-established adverse effect of excess selenium exposure, is selected as the critical endpoint on which to base a UL for selenium. A lowest-observed-adverse-effect-level (LOAEL) of 330 μg/day is identified from a large randomised controlled trial in humans (the Selenium and Vitamin E Cancer Prevention Trial (SELECT)), to which an uncertainty factor of 1.3 is applied. A UL of 255 μg/day is established for adult men and women (including pregnant and lactating women). ULs for children are derived from the UL for adults using allometric scaling (body weight0.75). Based on available intake data, adult consumers are unlikely to exceed the UL, except for regular users of food supplements containing high daily doses of selenium or regular consumers of Brazil nuts. No risk has been reported with the current levels of selenium intake in European countries from food (excluding food supplements) in toddlers and children, and selenium intake arising from the natural content of foods does not raise reasons for concern. Selenium-containing supplements in toddlers and children should be used with caution, based on individual needs.

Absorption, distribution, metabolism and excretion (ADME) of oral selenium from organic and inorganic sources: A review

BACKGROUND

Selenium is a trace element traditionally ingested either in its organic form via food or in its inorganic form through nutritional supplements, while selenium formulated as nanoparticles is a putative long-acting alternative. To understand the physiology and toxicology of the different selenium formulations, it is important to determine how their selenium content is absorbed, distributed, metabolised and excreted; therefore, we reviewed their biokinetics following oral exposure.

METHODS

We retrieved and reviewed the literature on the absorption, distribution, metabolism, and excretion of oral exposure to different forms of selenium.

RESULTS

Selenium in both the organic form (containing carbon to selenium chemical bonds) and the inorganic form is absorbed into the blood in humans. The mean normal blood level of many studies was 139 μg/L. There are indications that selenium from organic sources is more bioavailable than selenium from inorganic sources. Selenium is distributed throughout the body, including in breast milk. The elimination of selenium mainly involves the faecal and urinary pathways, whereas breath, saliva and hair are minor contributors. Urinary metabolites include trimethylselenium ions, selenosugars and Se-methylselenoneine.

CONCLUSION

Selenium is absorbed to a high extent, and selenium from organic sources is more bioavailable than from inorganic sources. Selenium, as expected as an essential trace element, is distributed throughout the body. Selenium is extensively metabolised, and various excretion metabolites have been identified in both urine and breath, while some selenium is also excreted via faeces.

Selenium in plant foods: speciation analysis, bioavailability, and factors affecting composition

Interest in selenium has been increasing over the past few decades with growing knowledge of its importance to overall health. The ability of several plants to accumulate and transform inorganic selenium forms into its bioactive organic compounds has important implications for human nutrition and health. In this review, we present the studies carried out during the last decade to characterize selenium species produced by different plant foods. Attention is also paid to the effect of selenium treatment on chemical composition and antioxidant properties of plants.

Determination of eleven small selenium species in human urine by chromatographic-coupled ICP-MS methods

BACKGROUND

The determination of various selenium species in urine enables a specific biomonitoring of the exposure to different selenium compounds.

METHODS

For this task a coupling of three chromatographic techniques with ICP-MS was developed for the separate quantification of eleven species in urine. The first procedure was based on reverse phase chromatography and was designed for the separate determination of methyl-2-acetamido-2-deoxy-1-seleno-b-d-galactopyranoside (SeSug1), methyl-2-acetamido-2-deoxy-1-seleno-b-d-glucopyranoside (SeSug2), selenomethionine (SeMet), methylselenocysteine (MeSeC), seleno-D,L-ethionine (SeEt), methylselenic acid (MeSeA) and methylselenoglutathione (MeSeG); the second procedure was based on anion exchange chromatography and measured selenate (Se (VI)) and selenite (Se (IV)); the third procedure was based on cationic exchange chromatography and determined methyl-2-amino-2-deoxy-1-seleno-b-d-galactopyranoside (SeSug3) and the trimethylselenium ion (TMSe). A fourth method for the more sensitive determination of TMSe was upgraded by an on-line after-column reaction process.

RESULTS

The validation of the methods yielded sensitive detection limits of the species between 0.03 and 0.10 μg Se/L. For TMSe a detection limit of 0.02 μg Se/L resulted by the fourth method. An intra-day precision of 2.7-10.6% and a relative recovery between 87 % and 108 % confirm the robustness of the methods.

CONCLUSION

The developed procedures enable a separate and sensitive determination of eleven selenium species in urine and thus permit the exploring of metabolic factors in the general population and particularly exposed individuals.

Delivery of selenium to selenophosphate synthetase for selenoprotein biosynthesis

BACKGROUND

Selenophosphate, the key selenium donor for the synthesis of selenoprotein and selenium-modified tRNA, is produced by selenophosphate synthetase (SPS) from ATP, selenide, and H₂O. Although free selenide can be used as the in vitro selenium substrate for selenophosphate synthesis, the precise physiological system that donates in vivo selenium substrate to SPS has not yet been characterized completely.

SCOPE OF REVIEW

In this review, we discuss selenium metabolism with respect to the delivery of selenium to SPS in selenoprotein biosynthesis.

MAJOR CONCLUSIONS

Glutathione, selenocysteine lyase, cysteine desulfurase, and selenium-binding proteins are the candidates of selenium delivery system to SPS. The thioredoxin system is also implicated in the selenium delivery to SPS in Escherichia coli.

GENERAL SIGNIFICANCE

Selenium delivered via a protein-bound selenopersulfide intermediate emerges as a central element not only in achieving specific selenoprotein biosynthesis but also in preventing the occurrence of toxic free selenide in the cell. This article is part of a Special Issue entitled "Selenium research in biochemistry and biophysics - 200 year anniversary".

Biology and toxicology of tellurium explored by speciation analysis

Tellurium (Te) is widely used in industry because it has unique physicochemical properties. Although Te is a non-essential element in animals and plants, it is expected to be metabolized to organometallic compounds having a carbon-Te bond in living organisms exposed to inorganic Te compounds. Thus, the speciation and identification of tellurometabolites are expected to contribute to the depiction of the metabolic chart of Te. Speciation by elemental mass spectrometry and identification by molecular mass spectrometry coupled with separation techniques have significantly contributed to the discovery of tellurometabolites in animals and plants. The aim of this mini review is to present recent advances in the biology and toxicology of tellurium as revealed by speciation and identification by molecular mass spectrometry.

Biological Chemistry of Hydrogen Selenide

There are no two main-group elements that exhibit more similar physical and chemical properties than sulfur and selenium. Nonetheless, Nature has deemed both essential for life and has found a way to exploit the subtle unique properties of selenium to include it in biochemistry despite its congener sulfur being 10,000 times more abundant. Selenium is more easily oxidized and it is kinetically more labile, so all selenium compounds could be considered to be "Reactive Selenium Compounds" relative to their sulfur analogues. What is furthermore remarkable is that one of the most reactive forms of selenium, hydrogen selenide (HSe- at physiologic pH), is proposed to be the starting point for the biosynthesis of selenium-containing molecules. This review contrasts the chemical properties of sulfur and selenium and critically assesses the role of hydrogen selenide in biological chemistry.

Human metabolism and renal excretion of selenium compounds after oral ingestion of sodium selenite and selenized yeast dependent on the trimethylselenium ion (TMSe) status

A human in vivo metabolism study was carried out to investigate the impact of the trimethylselenium ion (TMSe) status on metabolism and toxicokinetics of sodium selenite and selenized yeast. Nine healthy human volunteers were orally exposed to 200 µg selenium as sodium selenite and seven with selenized yeast (100 µg selenium). In each intervention group, three subjects belong to TMSe eliminators. Blood samples were withdrawn before and up to 6 h after administration. Urine samples were collected before and within 24 h after administration. Total selenium (Se) was quantified in blood plasma and urine and low molecular Se species in urine. Selenium concentration in plasma increased from 84.5 ± 13.2 µg Se/L before to 97.4 ± 13.2 µg Se/L 2-3 h after selenite supplementation and 89.5 ± 12.9 µg Se/L to 92.1 ± 13.9 µg Se/L after selenized yeast intake. The oral ingestion caused an additional Se elimination via urine of 16.9 ± 10.6 µg/24 h (TMSe elim.: 10.8 ± 6.9 µg/24 h; non-TMSe elim.: 20.0 ± 11.3 µg Se/24 h) after selenite exposure and 11.8 ± 4.1 µg/24 h (TMSe elim.: 10.8 ± 4.6 µg/24 h; non-TMSe elim.: 12.6 ± 4.2 µg Se/24 h) after selenized yeast exposure. Methyl-2-acetamido-2-deoxy-1-seleno-β-D-galactopyranoside (SeSug1) was the main metabolite in all urine samples, whereas TMSe was another main metabolite in TMSe eliminators' urine. After selenite exposure, a small amount of the dose (0.5 ± 0.2 %) was oxidized to selenate and rapidly excreted via urine. With the exception of selenite exposure in TMSe eliminators, the comparison of total Se and the sum of quantified Se species revealed a high renal portion of unidentified species. The study indicated a different metabolism of inorganic and organic Se compounds in human, but also crucial differences of Se metabolism in TMSe eliminators and non-TMSe eliminators.

Selenium cycling across soil-plant-atmosphere interfaces: a critical review

Selenium (Se) is an essential element for humans and animals, which occurs ubiquitously in the environment. It is present in trace amounts in both organic and inorganic forms in marine and freshwater systems, soils, biomass and in the atmosphere. Low Se levels in certain terrestrial environments have resulted in Se deficiency in humans, while elevated Se levels in waters and soils can be toxic and result in the death of aquatic wildlife and other animals. Human dietary Se intake is largely governed by Se concentrations in plants, which are controlled by root uptake of Se as a function of soil Se concentrations, speciation and bioavailability. In addition, plants and microorganisms can biomethylate Se, which can result in a loss of Se to the atmosphere. The mobilization of Se across soil-plant-atmosphere interfaces is thus of crucial importance for human Se status. This review gives an overview of current knowledge on Se cycling with a specific focus on soil-plant-atmosphere interfaces. Sources, speciation and mobility of Se in soils and plants will be discussed as well as Se hyperaccumulation by plants, biofortification and biomethylation. Future research on Se cycling in the environment is essential to minimize the adverse health effects associated with unsafe environmental Se levels.

Biomarkers of selenium status

The essential trace element, selenium (Se), has multiple biological activities, which depend on the level of Se intake. Relatively low Se intakes determine the expression of selenoenzymes in which it serves as an essential constituent. Higher intakes have been shown to have anti-tumorigenic potential; and very high Se intakes can produce adverse effects. This hierarchy of biological activities calls for biomarkers informative at different levels of Se exposure. Some Se-biomarkers, such as the selenoproteins and particularly GPX3 and SEPP1, provide information about function directly and are of value in identifying nutritional Se deficiency and tracking responses of deficient individuals to Se-treatment. They are useful under conditions of Se intake within the range of regulated selenoprotein expression, e.g., for humans <55 μg/day and for animals <20 μg/kg diet. Other Se-biomarkers provide information indirectly through inferences based on Se levels of foods, tissues, urine or feces. They can indicate the likelihood of deficiency or adverse effects, but they do not provide direct evidence of either condition. Their value is in providing information about Se status over a wide range of Se intake, particularly from food forms. There is need for additional Se biomarkers particularly for assessing Se status in non-deficient individuals for whom the prospects of cancer risk reduction and adverse effects risk are the primary health considerations. This would include determining whether supranutritional intakes of Se may be required for maximal selenoprotein expression in immune surveillance cells. It would also include developing methods to determine low molecular weight Se-metabolites, i.e., selenoamino acids and methylated Se-metabolites, which to date have not been detectable in biological specimens. Recent analytical advances using tandem liquid chromatography-mass spectrometry suggest prospects for detecting these metabolites.

Effects of dietary nutrients on volatile breath metabolites

Breath analysis is becoming increasingly established as a means of assessing metabolic, biochemical and physiological function in health and disease. The methods available for these analyses exploit a variety of complex physicochemical principles, but are becoming more easily utilised in the clinical setting. Whilst some of the factors accounting for the biological variation in breath metabolite concentrations have been clarified, there has been relatively little work on the dietary factors that may influence them. In applying breath analysis to the clinical setting, it will be important to consider how these factors may affect the interpretation of endogenous breath composition. Diet may have complex effects on the generation of breath compounds. These effects may either be due to a direct impact on metabolism, or because they alter the gastrointestinal flora. Bacteria are a major source of compounds in breath, and their generation of H₂, hydrogen cyanide, aldehydes and alkanes may be an indicator of the health of their host.

Environmental selenium research: from microscopic processes to global understanding

Selenium is a natural trace element that is of fundamental importance to human health. The extreme geographical variation in selenium concentrations in soils and food crops has resulted in significant health problems related to deficient or excess levels of selenium in the environment. To deal with these kinds of problems in the future it is essential to get a better understanding of the processes that control the global distribution of selenium. The recent development of analytical techniques and methods enables accurate selenium measurements of environmental concentrations, which will lead to a better understanding of biogeochemical processes. This improved understanding may enable us to predict the distribution of selenium in areas where this is currently unknown. These predictions are essential to prevent future Se health hazards in a world that is increasingly affected by human activities.

Se bioavailability and glutathione peroxidase activity in iron deficient rats

Little information is available on the relationship of Se deposit in target organs and GPx activity in iron deficiency anemia. As red blood cells (RBCs) play a crucial role on Se metabolism and during Fe deficiency anemia a lower count of RBCs is featured, we aimed to investigate the influence of this pathology on Se bioavailability and the relationship with antioxidant status. 20 male Wistar rats were randomly divided into two groups, a control group receiving AIN-93G diet with normal Fe content (45mg/kg diet) and the Fe-deficient group receiving AIN-93G diet with low Fe content (5mg/kg diet) for 40 days. Both diets were prepared with an adequate Se content (0.180mg/kg diet). The digestive and metabolic utilization of Se, the distribution in target organ, the GPx activity and TBARS production were measured after receiving the diets. Se retention increased (P<0.001) in the anemic group, fact that contributes to keep the enzymatic antioxidant activity of GPx in normal levels and the tendency observed is that stored Se increased in the organs, especially in kidney (P<0.01), however, a lower Se deposit was found in sternum of anemic rats (P<0.001). The lower count of RBCs featured in this pathology (P<0.001) causes a decrease of Se concentration in sternum meanwhile the increase in kidney deposit is a consequence of the lower urinary losses (P<0.001).

Headspace hollow fiber protected liquid-phase microextraction combined with gas chromatography-mass spectroscopy for speciation and determination of volatile organic compounds of selenium in environmental and biological samples

A simple and novel speciation method for the determination of volatile organic compounds of selenium (dimethylselenide (DMSe) and dimethyldiselenide (DMDSe) has been developed using a headspace hollow fiber protected liquid-phase microextraction (HS-HF-LPME) combined with capillary gas chromatography-mass spectrometry (GC-MS). The organic solvent impregnated in the pores and filled inside the porous hollow fiber membrane was used as an extraction interface in the HS-HF-LPME of the compounds. The effect of different variables on the extraction efficiency was studied simultaneously using an experimental design. The variables of interest in the HS-HF-LPME were sample volume, extraction time, temperature of sample solution, ionic strength, stirring rate and dwelling time. A Plackett-Burman design was performed for screening in order to determine the significant variables affecting the extraction efficiency. Then, the significant factors were optimized by a Box-Behnken design (BBD) and the response surface equations were derived. Under optimum conditions, preconcentration factors up to 1250 and 1170 were achieved for DMSe and DMDSe respectively. The detection limit and relative standard deviation (RSD) (n=5, c=50 μg L(-1)) for DMSe were 65 ng L(-1) and 4.8%, respectively. They were also obtained for DMDSe as 57 ng L(-1) and 3.9%, respectively. The developed technique was found to be applicable to spiked environmental and biological samples.

Surveying selenium speciation from soil to cell--forms and transformations

The aim of this review is to present and evaluate the present knowledge of which selenium species are available to the general population in the form of food and common supplements and how these species are metabolized in mammals. The overview of the selenium sources takes a horizontal approach, which encompasses identification of new metabolites in yeast and food of plant and animal origin, whereas the survey of the mammalian metabolism takes a horizontal as well as a vertical approach. The vertical approach encompasses studies on dynamic conversions of selenium compounds within cells, tissues or whole organisms. New and improved sample preparation, separation and detection methods are evaluated from an analytical chemical perspective to cover the progress in horizontal speciation, whereas the analytical methods for the vertical speciation and the interpretations of the results are evaluated from a biological angle as well.

Metabolism of 210Po in rats: volatile 210Po in excreta

Polonium-210 ((210)Po) is one of the most toxic radionuclides and was used as a poison in the Alexander Litvinenko case. In this study of the metabolism of (210)Po in rats, volatile (210)Po in excreta was measured, filling a knowledge gap of the previous studies. Five rats were intravenously administrated with 2 kBq and another five with 10 kBq of (210)Po (citrate form). They were housed in a glass Metabowl system for 4 d following the administration. Volatile (210)Po from the excreta was collected in a trapping system filled with liquid scintillation cocktail and was measured by liquid scintillation counting. Results showed that the daily excretion of volatile (210)Po by the rats is in a very small percentage (0.002-0.009 %) of the administered amounts. However, if the administered amount is large, the excretion of volatile (210)Po can be significant.

Formation, Occurrence, Significance, and Analysis of Organoselenium and Organotellurium Compounds in the Environment

Among all environmentally-relevant trace elements, selenium has one of the most diverse organic chemistries. It is also one of the few trace elements that may biomagnify in food chains under certain conditions. Yet, the exact chemical forms of selenium involved in the uptake into organisms and transfer to higher trophic levels, as well as the biochemical mechanisms that lead to their subsequent metabolism in organisms, are still not well understood. This is in part due to the analytical challenges associated with measuring the myriad of discrete Se species occurring in organisms. While there are generalized concepts of selenium metabolism, there is a lack of conclusive analytical evidence supporting the existence of many postulated intermediates. Likewise, there is a disconnect between the major selenium species encountered in abiotic compartments (waters, soils, and sediment), and those found in organisms, which renders the qualitative and quantitative description of the bioaccumulation process uncertain. Here, we summarize the knowledge on important selenium and tellurium species in all environmental compartments, and identify gaps and uncertainties in the existing body of knowledge, with emphasis on problems associated with past and current analytical methodology.

Methylated Metal(loid) Species in Humans

While the metal(loid)s arsenic, bismuth, and selenium (probably also tellurium) have been shown to be enzymatically methylated in the human body, this has not yet been demonstrated for antimony, cadmium, germanium, indium, lead, mercury, thallium, and tin, although the latter elements can be biomethylated in the environment. Methylated metal(loid)s exhibit increased mobility, thus leading to a more efficient metal(loid) transport within the body and, in particular, opening chances for passing membrane barriers (blood-brain barrier, placental barrier). As a consequence human health may be affected. In this review, relevant data from the literature are compiled, and are discussed with respect to the evaluation of assumed and proven health effects caused by alkylated metal(loid) species.

Quantitative and qualitative trapping of volatile methylated selenium species entrained through nitric acid

Quantification and speciation of volatile selenium (Se) fluxes in remote areas has not been feasible previously, due to the absence of a simple and easily transportable trapping technique that preserves speciation. This paper presents a chemo-trapping method with nitric acid (HNO3) for volatile Se species, which preserves speciation of trapped compounds. The recovery and speciation of dimethylselenide (DMSe) and dimethyl diselenide (DMDSe) entrained through both concentrated nitric acid and hydrogen peroxide (H2O2) were compared by HPLC-ICP-MS and HPLC-HG-AFS analyses. It was demonstrated that trap reproducibility was better for nitric acid and a recovery of 65.2 +/- 1.9% for DMSe and 81.3 +/- 3.9% for DMDSe was found in nitric acid traps. HPLC-ES-MS identified dimethyl selenoxide (DMSeO) as the trapped product of DMSe. Methylseleninic acid (MSA) was identified to be the single product of DMDSe trapping. These oxidized derivatives have a high stability and low volatility, which makes nitric acid a highly attractive trapping liquid for volatile Se species and enables reconstruction of the speciation of those species. The presented trapping method is simple, quantifiable, reproducible, and robust and can potentially be applied to qualitatively and quantitatively study Se volatilization in a wide range of natural environments.

Microfungal alkylation and volatilization of selenium adsorbed by goethite

Selenium adsorbed in the oxyanionic form by Fe-oxides like goethite is considered of benefit for long-term stabilization of (79)Se under near field conditions of radionuclide waste disposal sites. However, microbe-mediated volatilization of the uranium fission product (79)Se has not yet been considered for risk assessment based on the use of the water-solid distribution coefficient K(D). We have performed incubation experiments in a ternary system selenium-microbe-goethite and show that mycobiota including the common black microfungi genera Alternaria alternata are capable of volatilizing the Se even if immobilized by goethite. The microfungi were incubated in a standardized nutrient broth suspension with 10 g L(-1) of the oxide target under defined conditions. Volatile organic selenium (VOSe) species formed in the head space of the culture flasks were sampled and measured directly by a cryotrapping cryofocusing gas chromatographic system coupled with ICP-MS detection (CT-CF-GC-ICP-MS). Alkylated VOSe species were found at the tens to hundreds ng m(-3) levels dominated by dimethyl selenide (DMSe) and dimethyl diselenide (DMDSe). The total amount of DMSe released into the 80-mL headspace volume within the 21 days of incubation was up to 1.12 +/- 0.17 nmol and 0.48 +/- 0.12 nmol for systems without and with goethite amendment, respectively. Alkylation rates of up to 0.1 mumol Se per day and g biomass cannot be neglected as a potential fission product mobilization pathway, unless the inherent radioactivity is proven to prevent any such microbial activity on the long-term. Otherwise it may lead to an onsite accumulation of (79)Se through evapoconcentration in the enclosed underground caverns.

Simultaneous separation and speciation of inorganic As(III)/As(V) and Cr(III)/Cr(VI) in natural waters utilizing capillary microextraction on ordered mesoporous Al2O3 prior to their on-line determination by ICP-MS

In this paper, a system of flow injection (FI) capillary microextraction (CME) on line coupled with inductively plasma mass spectrometry (ICP-MS) was proposed for simultaneous separation and speciation of inorganic As(III)/As(V) and Cr(III)/Cr(VI) in natural waters. Ordered mesoporous Al2O3 coating was prepared by sol-gel technology and used as CME coating material. Various experimental parameters affecting the capillary microextraction of inorganic arsenic and chromium species have been investigated and optimized. Under the optimized conditions, the limits of detection were 0.7 and 18 ng L(-1) for As(V) and Cr(VI), 3.4 and 74 ng L(-1) for As(III) and Cr(III), respectively, with an enrichment factor of 5 and a sampling frequency of 8h(-1). The relative standard deviations (R.S.D.) were 3.1, 4.0, 2.8 and 3.9% (C=1 ng mL(-1), n=7) for As(V), As(III), Cr(VI) and Cr(III), respectively. The proposed method was successfully applied for the analysis of inorganic arsenic and chromium species in mineral water, tap water and lake water with the recovery of 94-105%. In order to verify the accuracy of the method, two certified reference of GSBZ50027-94 and GSBZ50004-88 water samples were analyzed and the results obtained were in good agreement with the certified values. The ordered mesoporous Al2O3 coated capillary showed an excellent solvent and thermal stability and could be re-used for more than 30 times without decreasing extraction efficiency.

Consequences of Vapor Enhancement on Selenium Speciation Analysis by HPLC/ICPMS

Recent work has shown the presence of volatile selenium metabolites in human urine and suggested that these compounds could compromise quantitative selenium analyses by ICPMS. We show that with a commonly used sample introduction system (pneumatic nebulizer and spray chamber), two volatile selenium species recently identified in urine, namely, dimethyl selenide and dimethyl diselenide, gave greatly increased ICPMS responses (up to 58-fold) relative to selenite, an effect related to their volatilization in the spray chamber resulting in enhanced transport to the plasma. The quantitative consequences of this effect were demonstrated by measurement of total selenium and selenium species in certified reference material, NIES CRM 18 human urine. Direct flow injection analysis of the urine gave a total selenium concentration more than 2-fold higher than the certified value. These data suggested that NIES CRM 18 may contain part of its selenium as volatile species, and subsequent reversed-phase HPLC/ICPMS showed the presence of dimethyl selenide in addition to selenosugars and trimethylselenonium ion. Although the practice of quantifying unidentified chromatographic peaks against those of known compounds is common in speciation analysis, this approach when applied to NIES CRM 18 gave a value for the sum of selenium species which was twice the certified total selenium concentration. This work shows that the presence of volatile selenium species in urine precludes the use of flow injection analysis for total selenium measurements and imposes severe restrictions on the quantification of urinary selenium metabolites. In addition, it raises broader issues of the validity of the "dilute and shoot" approach to the determination of metals in clinical analysis of biological fluids.

Speciation of alkylated metals and metalloids in the environment

The analytical methodology for speciation of metals and metalloids associated with alkyl groups and biomacromolecules is critically reviewed. Alkylated metals and metalloids are not only known to be produced by microbial methylation within most anaerobic compartments in the environment, but also in the course of enzymatic transformations during human metabolism. Because of the toxicological relevance of these compounds present in trace to ultratrace concentrations, firm species identification and exact quantification are essential. While many instrumental techniques coupling chromatography (GC, HPLC, CE, GE) with plasma mass spectrometry (ICP-MS) are available for quantification, methods used for structural identification often suffer from inadequate sensitivity (EI-MS, ESI-MS, MALDI-MS, FT-ICRMS). Other problems encountered are sample derivatisation artefacts, lack of suitable standards for quantification, lack of equilibrium between spikes and sample, and the integrity of metal-protein association during separation, in particular during SDS-PAGE. Selected application examples with respect to mercury and arsenic speciation will be discussed critically.