Comparison of two derivatizing agents for the simultaneous determination of selenite and organoselenium species by gas chromatography and atomic emission detection after preconcentration using solid-phase microextraction

Sensitive analysis of selenium speciation in natural seawater by isotope-dilution and large volume injection using PTV-GC-ICP-MS

Although oceans play a key role in the global selenium (Se) cycle, there is currently very little quantitative information available on the distribution of Se concentrations and Se speciation in marine environments. In general, determining Se concentration and speciation in seawater is highly challenging due to very low Se levels ((sub)ng⋅L-1), whereas matrix elements interfering Se pre-concentration and detection are up to the g⋅L-1 levels. In this study, we established a sensitive method for the determination of the various Se chemical fractions present in natural seawater, i.e. selenite (SeIV), selenate (SeVI), organic Se-II + Se0 and total Se, using species-specific isotope dilution gas chromatography coupled to inductively coupled plasma mass spectrometry (ID-GC-ICP-MS). We compared different derivatization reagents and optimized specific pre-treatment protocols, including a microwave assisted oxidation protocol for the determination of total Se and organic Se-II + Se0 using H2O2. To increase sensitivity, we developed an online pre-concentration method based on large volume injection (LVI) using a programmed temperature vaporization (PTV) inlet. Eventually, the developed method achieved low absolute and methodological detection limits, i.e., respectively, 0.1-0.3 pg and 0.9-3.1 ng.L-1 for the different fractions. The accuracy of our method was of 2% for a certified reference material (CRM) diluted in artificial seawater while the precision was better than 4% for a freshwater CRM in artificial seawater matrix as well as two common seawater CRMs certified for trace elements excluding Se. As a proof-of-concept, we quantified the various Se fractions in a large number of natural water samples from the Baltic and North Seas, encompassing a wide range of salinity (7-35 psu), which shows that its detection limits are sufficient to determine total Se, SeIV, SeVI and organic Se-II + Se0 concentrations in brackish and marine systems.

Quantification of volatile-alkylated selenium and sulfur in complex aqueous media using solid-phase microextraction

Biologically produced volatile-alkylated Se and S compounds play an important role in the global biogeochemical Se and S cycles, are important constituents of odorous industrial emissions, and contribute to (off-)flavors in food and beverages. This study presents a fully automated direct-immersion solid-phase microextraction (DI-SPME) method coupled with capillary gas chromatography-mass spectrometry (GC/MS) for the simultaneous quantification of 10 volatile-alkylated Se and S compounds in complex aqueous media. Instrumental parameters of the SPME procedure were optimized to yield extraction efficiencies of up to 96% from complex aqueous matrices. The effects of sample matrix composition and analyte transformation during sample storage were critically assessed. With the use of internal standards and procedural calibrations, the DI-SPME-GC/MS method allows for trace-level quantification of volatile Se and S compounds in the ng/L range (e.g. down to 30 ng/L dimethyl sulfide and 75 ng/L dimethyl selenide). The applicability and robustness of the presented method demonstrate that the method may be used to quantify volatile Se and S compounds in complex aqueous samples, such as industrial effluents or food and beverage samples.

Determination of Se(IV) in natural waters by adsorptive stripping voltammetry of 5-nitropiazselenol

The high tendency of 5-nitropiazselenol for self-accumulation on thin mercury film electrode was used innovatively for determination of Se(IV) in natural waters. 5-Nitropiazselenol was formed by reaction between Se(IV) and 4-nitro-1,2-phenylenediamine in acidic solution and self-accumulation process was carried out directly from reaction media. The adsorbed 5-nitropiazselenol was stripped in HCl solution by DP cathodic potential scan. All parameters influencing the measurement were optimized and evaluated. Detection limit of this method is 0.06 ng mL(-1). Interferences of various cations and anions were studied. The adsorption tendency of some other piazselenols made by some aromatic ortho-diamines was also investigated. The problems arising from applying potential during accumulation process in natural waters analysis were discussed. This method was applied for determination of Se(IV) in natural waters collected from some internationally registrated lagoons south of Caspian Sea. The obtained results were compared with the results of ICP-AES and DPCSV after electrochemical preconcentration.

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.

Headspace solid-phase microextraction for the determination of volatile organic sulphur and selenium compounds in beers, wines and spirits using gas chromatography and atomic emission detection

A rapid and solvent-free method for the determination of eight volatile organic sulphur and two selenium compounds in different beverage samples using headspace solid-phase microextraction and gas chromatography with atomic emission detection has been developed. The bonded carboxen/polydimethylsiloxane fiber was the most suitable for preconcentrating the analytes from the headspace of the sample solution. Volumes of 20 mL of undiluted beer were used while, in the case of wines and spirits, sample:water ratios of 5:15 and 2:18, respectively, were used, in order to obtain the maximum sensitivity. Quantitation was carried out by using synthetic matrices of beer and wine, and a spiked sample for spirits, and using ethyl methyl sulphide and isopropyl disulphide as internal standards. Detection limits ranged from 8 ng L(-1) to 40 ng mL(-1), depending on the compound and the beverage sample analyzed, with a fiber time exposure of 20 min at ambient temperature. The optimized method was successfully applied to different samples, some of the studied compounds being detected at concentration levels in the 0.04-152 ng mL(-1) range.