Martine Potin-Gautier

A critical review of thermodynamic data for selenium species at 25°C

Abstract The accuracy of published selenium thermodynamic data was evaluated to obtain a reliable basis for modelling Se migration behaviour in contaminated areas. The Modified Bromleys Methodology was used to correct equilibrium constant to the standard state (zero ionic strength, 25°C and 1 bar pressure). Since redox reactions control the migration of selenium, equilibria between the various Se oxidation states and the associated reactions — acid–base, precipitation, and complexation — were studied. Although pKa values for H2SeO3 and H2SeO4 acid–base reactions are well known, an ambiguity exists for the second dissociation of H2Se. A value of 14 appears to be more suitable than a value close to 11.5. The most adequate standard potentials for the redox reactions of selenium are proposed. Because of the very low solubility of metal-selenides, most of their solubility product constants are derived from combination of formation thermodynamic data selected in the literature or extrapolated from those of the corresponding sulfides. Several values are reported for Se(IV) and Se(VI), but they are not always in agreement. In generally, few values have been determined recently and noticeable differences can be observed between old and recent values. For complexation reactions, there is a lack of data and for many equilibria only one constant is available.

Analytical methods for the speciation of selenium compounds: a review

Selenium, like sulphur, exists in the environment in several oxidation states and as a variety of inorganic and organic compounds. Dissolved inorganic selenium can be found in natural waters as selenide Se (−II), as colloidal elemental selenium Se (0), as selenite anions HSeO3− and SeO32− i.e. Se (+IV) and as the selenate anion (SeO42−) i.e. Se (+VI). Organic forms of selenium that may be found in organisms, air or in the aqueous environment, are volatile (methylselenides) or non volatile (trimethylselenonium ion, selenoamino acids and their derivatives). Knowledge of the different chemical forms and their environmental and biomedical distribution is important because of the dependence of bioavailability and toxicity on speciation. This paper reviews the different analytical methods used for the speciation of selenium compounds, with special attention to inorganic selenium and organoselenium species.

Speciation analysis of selenium in garlic by two-dimensional high-performance liquid chromatography with parallel inductively coupled plasma mass spectrometric and electrospray tandem mass spectrometric detection.

Speciation of selenium in garlic harvested in naturally seleniferous soil was investigated. The sample was leached with water and the aqueous extract was fractionated by preparative size-exclusion chromatography. Selenium was found in only one (low-molecular mass) fraction. The chromatographic purity of the fraction was verified by reversed-phase chromatography with inductively coupled mass spectrometric detection. Again, one major signal accounting for more than 95% of the total selenium was observed. The heartcut fraction containing this compound produced an intense peak in an electrospray mass spectrum with the selenium pattern centered at m/z 313 (80Se). Protonated molecular ions corresponding to the four Se isotopes gave rich fragmentation patterns by collision induced dissociation that allowed the identification of the selenium species to be γ-glutamyl-Se-methylselenocysteine without the need for an authentic standard.

An approach to the identification of selenium species in yeast extracts using pneumatically-assisted electrospray tandem mass spectrometry

An approach to the identification of unknown signals in selenium speciation analysis of yeast by reversed-phase chromatography with ICP-MS detection is described. The analytical strategy was based on: (i), heart-cutting of a Se-containing fraction in the reversed-phase chromatographic eluate followed by its lyophilization; (ii), pneumatically-assisted electrospray (ESI) MS and ESI tandem MS of the lyophilizate; and (iii) confirmation of the fragmentation pattern obtained using the sulfur analogue of the seleno compound that was expected to have been identified. The approach developed allowed the identification of Se–adenosylhomocysteine as the major selenium species in an extract of a selenized yeast sample.

Improved routine speciation of organotin compounds in environmental samples by pulsed flame photometric detection

The high toxicity of the organotin species requires sensitive analytical methods in order to understand the origins of pollution and perform monitoring programs in the water cycle. The optimisation of a new detection method, pulsed flame photometric detection (PFPD), is reported for the simultaneous determination of butyl-, phenyl- and octyltins. The methodology of the experimental designs at two levels was used. It allows the evaluation of the influence of the three gas flow-rates on the peak heights and resolution between the closest peaks obtained using two different wavelengths of detection (390 and 611 nm). The modelling of these two responses, according to second-order polynomials, leads to the precise adjustment of the operating conditions. PFPD has shown two significant improvements over conventional flame photometric detection: increased sensitivity (absolute detection limits: 0.07 to 2 pg as Sn) and greater selectivity, whatever the wavelength used. This new analytical process was validated by the analysis of certified reference material and spiked river water. It was used in routine analysis of environmental samples (wastewater, sludge, sand and oyster).

Rapid determination of organotin compounds by headspace solid-phase microextraction.

Headspace solid-phase microextraction (SPME) followed by gas chromatography (GC) coupled to pulsed flame photometric detection have been investigated for the simultaneous speciation analysis of 14 organotin compounds, including methyl-, butyl-, phenyl-, and octyltins compounds. The analytical process (sorption on SPME fibre and thermal desorption in GC injection port) has been optimised using experimental designs. Six operating factors were considered in order to evaluate their influence on the performances of a SPME-based procedure. The evaluation of accuracy, precision and limits of detection (LODs) according to ISO standards and IUPAC recommendations has allowed the method to be validated. The LODs obtained for the 14 studied organotins compounds are widely sub-ng(Sn) l(-1). The precision evaluated using relative standard deviation ranges between 9 and 25% from five determinations of the analytes at 0.25-125 ng(Sn) l(-1) concentrations. The accuracy was studied throughout the analysis of spiked environmental samples. These first results show that headspace SPME appears really as attractive for organotins determination in the environment and the monitoring of their biogeochemical cycle.

Analysis for selenium speciation in selenized yeast extracts by two-dimensional liquid chromatography with ICP-MS and electrospray MS-MS detection

An analytical approach allowing the detection of previously unreported glutathione S-conjugates with selenocompounds in water extracts of nutritional yeast supplements was developed. The procedure was based on the use of two-dimensional size-exclusion and reversed-phase HPLC for the separation of selenospecies, ICP-MS for the monitoring of the eluting selenium, and electrospray MS-MS for the identification of the eluted species. The presence of six compounds with molecular masses of 197, 603, 562, 584, 372 and 432 (in the elution order from reversed-phase HPLC) was identified on the basis of the selenium isotopic pattern. The identity of the Mr 197 and 432 species was confirmed, by collision induced dissociation MS, to be selenomethionine and Se-adenosylhomocysteine, respectively. The compounds with larger Mr (562, 584, 604) were demonstrated, by reaction with dithiothreitol followed by HPLC-ICP-MS and ES-MS-MS, to contain a Se–S bridge between glutathione (γGlu–Cys–Gly) and a selenocompound that did not respond in the positive ES-MS mode. The MS-MS analysis of the original compound confirmed the presence of a selenium-containing fragment that could be fragmented only at high fragmentation energies. A similar Se-containing moiety (m/z 227), resistant to collision induced dissociation, was found in the 372 compound attached to a glutamine (Gln) residue as demonstrated by MS-MS.

Optimization of solid-phase microextraction for the speciation of butyl- and phenyltins using experimental designs

Abstract This paper deals with the optimization of solid-phase microextraction (SPME) for organotin speciation in water. The analytical method consists of an in situ ethylation, simultaneous solid-phase microextraction of the derivatives, followed by a gas chromatographic analysis with flame photometric detection. Experimental design methodology was used to evaluate the influence of six analytical parameters on the mean peak area (Smean). The adsorption of the compounds on the SPME fibre was found to be the most important parameter and two other factors are positively significant: the adsorption time and the sample volume. The adsorption profiles and the optimal operating conditions were determined from the modelling of Smean. The detection limits range from 2 to 4 ng l−1 (monophenyltin excepted: 18 ng l−1) and linearity is from 50 to 600 ng l−1. The relative standard deviations are 7–10% for five determinations. Water samples were analysed in order to verify the accuracy of the optimized method by comparing results with those obtained using a conventional solvent extraction of the ethylated organotins.

Speciation of organic selenium compounds by high-performance liquid chromatography–inductively coupled plasma mass spectrometry in natural samples

The importance of selenium in the environment and in biological systems is now well recognized. Its biochemical functions, e.g., its anti-oxidant role and therefore cell protecting function, as an essential constituent of the enzyme glutathione peroxidase, has provoked a growing interest in the determination of this element. Selenoamino acids are essential for the understanding of the biogeochemical cycle of Se and TMSe+ is a known urinary metabolite present at high levels when Se is taken in excess. In this work we present a hyphenated technique (HPLC–ICP-MS) available for the separation of the organic Se compounds (selenocystine, selenomethionine and trimethylselenonium ion). The choice of column and solvents has been a critical parameter. Good repeatability and excellent detection limits (less than 1 µg l–1 for each species) have been reached for standard solutions and the application to some natural samples (enriched yeast, human serum and urine) has shown quite promising results.

Speciation of seleno compounds in yeast aqueous extracts by three-dimensional liquid chromatography with inductively coupled plasma mass spectrometric and electrospray mass spectrometric detection

A three-dimensional liquid chromatographic purification protocol based on sequential size-exclusion, anion-exchange and cation-exchange separation mechanisms was developed for the mapping of seleno compounds in aqueous yeast extracts. The method allowed the demonstration of the presence of more than 30 different seleno compounds. Semi-preparative size-exclusion and anion-exchange chromatography were optimized for maximum resolution using electrospray-compatible buffers in order to purify the compounds for mass spectrometric analysis. Molecular masses were attributed to many of the compounds on the basis of the selenium isotopic pattern in the electrospray mass spectra and of the collision-induced fragmentation patterns. Limitations preventing the ultimate identification of the selenium species detected are discussed.