Sensitive analysis of selenium speciation in natural seawater by isotope-dilution and large volume injection using...

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 (Se^IV), selenate (Se^VI), 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, Se^IV, Se^VI and organic Se^-II + Se⁰ concentrations in brackish and marine systems.