Maïté Bueno

Quantitative analysis of volatile selenium metabolites in normal urine by headspace solid phase microextraction gas chromatography-inductively coupled plasma mass spectrometry.

The combination of headspace-solid phase microextraction (HS-SPME) and gas chromatography-inductively coupled plasma mass spectrometry (GC-ICPMS) was evaluated for the determination of volatile selenium metabolites in normal urine samples, i.e. without selenium supplementation. HS-SPME operating conditions were optimised and a sampling time of 10 min was found to be suitable for simultaneous extraction of dimethylselenide (DMSe) and dimethyldiselenide (DMDSe). The amount of DMSe and DMDSe extracted onto fibre coating was calculated in clean matrix, i.e. Milli-Q water, on the basis of depletion experiments. When applied to normal urine samples, the developed method allowed the detection of four volatile selenium containing species, among which DMSe and DMDSe could be quantified by standard additions.

Optimisation of ICPMS collision/reaction cell conditions for the simultaneous removal of argon based interferences of arsenic and selenium in water samples.

The optimisation of ICPMS collision/reaction cell conditions for the simultaneous analysis of arsenic and selenium is described. A mixture of 3.8mL min(-1) of H(2) and 0.5mL min(-1) of He was found to be suitable for the removal of both ArAr(+) and ArCl(+) interferences. Detection limits down to 30ng (element) L(-1) in total analysis, and between 81 and 230ng (element) L(-1) in speciation analysis were achieved in chloride matrix (1gL(-1) NaCl). After validation, the method was applied to commercially available mineral waters.

An HPLC/ICPMS study of the stability of selenosugars in human urine: implications for quantification, sample handling, and storage

We report a study with HPLC/ICPMS on the long-term stability of the major selenium metabolite in human urine, namely methyl 2-acetamido-2-deoxy-1-seleno-β-D-galactopyranoside (selenosugar 1). Three separate experiments were performed of 4–28 weeks duration and incorporating various storage conditions: room temperature, 4 °C, −20 °C, −80 °C, lyophilisation, deoxygenation, or addition of a bactericide (NaN3). Triplicate samples of urine or water, spiked with selenosugar 1 at 200 μg Se L−1, were processed in each case. Selenosugar 1 was stable in water under all investigated conditions. For the urine samples, no significant degradation (<2%) was observed after 17 weeks frozen storage at −80 °C, or after lyophilisation and frozen storage at −20 °C, whereas small quantities of degradation products (ca. 3%) were recorded for frozen storage of wet samples at −20 °C. At 4 °C, the selenosugar was essentially unchanged after storage for up to 2 weeks, but clear losses were observed thereafter ranging up to 75% loss after 28 weeks. Several decomposition products were detected by HPLC/ICPMS, one of which was identified as dimethyl diselenide. Although present as only a trace constituent in the urine, dimethyl diselenide was recorded as a large HPLC peak, presumably because of a marked vapour enhancement effect due to more efficient transfer of volatile analytes to the plasma. In addition, total Se analyses revealed that Se was lost from the solutions during storage/handling, presumably as volatile species. Qualitative analysis of volatile species using head space sampling with solid phase microextraction followed by GC/MIP-AES and GC/MS revealed the presence of dimethyl selenide and dimethyl diselenide, based on comparison with standard compounds, and indicated the presence of dimethyl selenylsulfide based on comparison with literature data. The stability of selenosugar 1 and its isomer methyl 2-acetamido-2-deoxy-1-seleno-β-D-glucopyranoside (selenosugar 2), which occurs as a minor species in urine, was also investigated under room temperature storage in the presence and absence of light. Although both species were moderately stable when stored in the dark, their degradation was rapid in the light with clear losses recorded within three days. The work indicates that urine samples should be cooled immediately after collection, and that they may be stored at 4 °C (often the easiest way) for up to 2 weeks before analysis with no appreciable loss of selenosugar. For longer-term storage, urine samples should be kept at −80 °C or, when such facilities are not available, at −20 °C after lyophilisation. The study has also revealed potential quantification problems in Se speciation analysis resulting from different responses for Se species during ICPMS analysis.

Selenium speciation analysis at trace level in soils.

This paper describes the development of an analytical methodology to determine speciation of selenium present in soils at trace level (μg kg(-1)). The methodology was based on parallel single extractions and high performance liquid chromatography hyphenated to inductively coupled plasma mass spectrometry (HPLC-ICPMS). Two complementary chromatographic separations were used to confirm Se species identity. Different extractants, selected on the basis of sequential extraction schemes, were compared. Ultrapure water, 0.1 molL(-1) phosphate buffer (KH(2)PO(4)/K(2)HPO(4)) at pH 7 and 0.1 molL(-1) sodium hydroxide extractants were finally chosen owing to their efficiency in extracting Se and compatibility with Se species stability. These extractants allow also assessing respectively water-soluble Se (i.e. the most mobile Se fraction), exchangeable Se (i.e. sorbed onto soil component surface) and Se bound to soil organic matter. This methodology gives thus information on Se mobility related to its distribution in soil with preservation of original Se speciation. Detection limits range from 3 to 29ng(Se)L(-1) and from 0.1 to 10 μg(Se)kg(-1), allowing determination of Se species concentrations in extracts from soils containing native Se at trace level. The methodology was applied to three soils with total Se concentrations between 210 and 1560 μg(Se)kg(-1).

Operational optimisation of ICP—octopole collision/reaction cell—MS for applications to ultratrace selenium total and speciation determination

This paper describes the optimisation of inductively coupled plasma-mass spectrometry (ICPMS) equipped with collision/reaction cell (C/RC) technology for selenium analysis at ultratrace level. Gas flow rates for helium and hydrogen were carefully optimised using experimental design methodology in order to efficiently remove interferences (ArAr+ dimer) while keeping maximum sensitivity on 80Se. A mixture of both gases with 0.5 ml min−1 of He and 3.8 ml min−1 of H2 was selected as the best compromise. Voltage of the different ionic lenses disposed around the C/RC, allowing a better focalisation of the ion beam and kinetic energy discrimination, was also investigated and retained values were: −14 V for quadrupole bias, −13 V for octopole bias, −20 V for cell entrance, −20 V for cell exit and –45 V for plate bias. Detection limits in total analysis are close to 30 ng (Se) l−1 which represents a three times improvement to the ones obtained without C/RC. These conditions were tested with HPLC coupling for speciation of selenite (SeIV), selenate (SeVI), selenomethionine (SeMet) and selenocystine (SeCyst). Depending on selenium species, the detection limits achieved in C/RC mode monitoring 80Se are in the range 70–180 ng (Se) l−1 which represents a 2 times improvement in comparison with 82Se monitoring in standard mode. Accuracy of the method was tested by analysing spiked water and certified reference material (Rain water, TM-Rain95, National water research institute, Canada).

Distribution and speciation of ambient selenium in contrasted soils, from mineral to organic rich.

Selenium adsorption onto oxy-hydroxides mainly controls its mobility in volcanic soils, red earths and soils poor in organic matter (OM) while the influence of OM was emphasized in podzol and peat soils. This work aims at deciphering how those solid phases influence ambient Se mobility and speciation under less contrasted conditions in 26 soils spanning extensive ranges of OM (1-32%), Fe/Al oxy-hydroxides (0.3-6.1%) contents and pH (4.0-8.3). The soil collection included agriculture, meadow and forest soils to assess the influence of OM quality as well. Trace concentrations of six ambient Se species (Se(IV), Se(VI) and 4 organo-Se compounds) were analyzed by HPLC-ICP-MS in three extractants (ultrapure water, phosphate and sodium hydroxide) targeting Se associated to different soil phases. The Kd values determined from ultrapure water extraction were higher than those reported in commonly used short-term experiments after Se-spiking. Correlations of ambient Se content and distribution with soil parameters explained this difference by an involvement of slow processes in Se retention in soils. The 26 Kd values determined here for a wide variety of soils thus represent a relevant database for long-term prediction of Se mobility. For soils containing less than 20% OM, ambient Se solubility is primarily controlled by its adsorption onto crystalline oxy-hydroxides. However, OM plays an important role in Se mobility by forming organo-mineral associations that may protect adsorbed Se from leaching and/or create anoxic zones (aggregates) where Se is immobilized after its reduction. Although for the first time, inorganic Se(IV), Se(VI) and organo-Se compounds were simultaneously investigated in a large soil collection, high Se proportions remain unidentified in each soil extract, most probably due to Se incorporation and/or binding to colloidal-sized OM. Variations of environmental factors regulating the extent of OM-mineral associations/aggregation may thus lead to changes in Se mobility and bio-availability.

Advantages of hydride generation interface for selenium speciation in waters by high performance liquid chromatography–inductively coupled plasma mass spectrometry coupling

This paper focuses on the analytical performance improvement of the coupled technique HPLC-ICPMS using on-line collision/reaction cell technology for selenium elemental and speciation analyses at the ng (Se) l(-1) level in aquatic environment. Collision/reaction cell operating parameters were optimised, resulting in selected conditions of 5.5 ml min(-1) H(2) and 0.5 ml min(-1) He mixture. The detection limits obtained were around 5 ng (Se) l(-1) for total analysis, and between 7 and 15 ng (Se) l(-1) depending on the species for speciation analysis. The capability of UV irradiation-hydride generation interfacing to increase detector sensitivity was also evaluated for speciation analysis. The detection limits obtained were in the range 2-8 ng (Se) l(-1) depending on the species. Moreover, such interface allowed to prevent bromine introduction to the ICPMS which is particularly convenient for selenium trace analysis in natural waters as (80)Se is preserved free from BrH interferences. The developed method was validated using certified water with low selenium content (TM Rain 95, NWRI, Canada) and applied to the analysis of different waters.

Analytical advances in butyl-, phenyl- and octyltin speciation analysis in soil by GC-PFPD.

The development and validation of a method for organotin analysis in soils, including butyl-, phenyl- and octyl-tins, are described in this study. The influence of pretreatment step based on sample lyophilization was first studied. Different solid-liquid extraction techniques including mechanical stirring (MSAE), accelerated solvent (ASE), microwave (MAE) and ultrasound (UAE), were compared. MSAE gave the best recoveries and repeatability and was thus chosen for OTC extraction from soils. Then, ethylation/extraction step before GC-PFPD analysis was investigated and the best derivatisation conditions were assessed in order to achieve a simple, non-expensive and reliable routine procedure. Finally, the robustness of the method was tested by the analysis of several soils with different organic matter content allowing the validation of developed protocol. The method appears to be reliable and accurate for the OTC determination in a broad range of soils.

Kinetic degradation processes of butyl- and phenyltins in soils.

The degradation of organotin compounds (OTC) in agricultural and forest soils is studied in sandy soil samples. Individual experiments involving the three butyl- and the three phenyltins were carried out during 90 d in controlled conditions (darkness, 28 degrees C, aerobic conditions, 13% moisture) and with spiking concentration representative of environmental levels (20-50 micrg(Sn) kg(-1)). After the validation of first-order degradation kinetic model, mechanisms involved throughout the study were considered. Degradation pathways are proposed for butyl- and phenyltins and discussed according to literature data. The degradation of mono- (MBT, MPhT), di-organotins (DBT, DPhT) and TBT is clearly identified as a single successive loss of an organic group whereas TPhT is directly degraded to MPhT. The half-life times were dependent on their substitution degree, ranging from 24 (TPhT) to 220 (MBT) d. The less substituted the OTC is, the more persistent it is. In the range 4.3-5.7, pH does not seem to influence OTC degradation under the present operating conditions. Finally this study shows the significant persistence in soil samples in our experimental conditions for most of studied organotins and highlights the potential impact on soil quality.

Organotin speciation in French brandies and wines by solid-phase microextraction and gas chromatography—Pulsed flame photometric detection

An analytical method devoted to organotin compounds (OTC) determination in brandy and wine was developed. It is based on solid-phase microextraction (SPME) of ethylated organotins. The following operating factors were examined: SPME mode/nature of fibre coating, sample volume/dilution, and sampling time. The optimisation work led to dilute the sample in an aqueous buffer (1/11, v/v ratio) in order to satisfactorily decrease the matrix effects due to competitive sorption of non-OTC species onto/into fibre coating. The optimised operating conditions consist of polydimethylsiloxane (PDMS) coated fibre used in headspace mode for 30 min. In wines, the limits of detection (LOD) and quantification (LOQ) ranged from 1 to 40 and 3 to 80 ng(Sn)L(-1) respectively, according to the species. The analytical validation was made by evaluating the accuracy of OTC determination in spiked samples with various concentrations over the whole calibration range, i.e. from LOQ to 1000 ng(Sn)L(-1). Recovery was around 80-110% and precision (relative standard deviation, RSD) was between 12% and 25%. Despite the presence of two chromatographic peaks corresponding to sulphur compounds during brandy analysis, the selectivity of the method is adequate. The analysis confirmed the analytical performances and applicability of the method to wine and brandy samples. The obtained results emphasise the contamination of brandy and wine by organotins, the storage in plastic container seeming to be confirmed as the main OTC source.