Determination of Se₈ in sediments by gas chromatography-mass spectrometry

A gas chromatography-mass spectrometry (GC-MS) method was developed to analyze cyclooctaselenium (Se₈) in sediments after extraction with carbon disulfide (CS₂). The method suffers from some analytical complications, most notably a poor peak shape for Se₈, manifesting in a broad peak (2 min) with strong fronting, and a decrease in Se₈ retention time with decreasing concentration. Detailed analysis of the mass spectral data suggests that (thermal) decomposition of Se₈ on the GC column is responsible for both of these phenomena. Despite these limitations, GC-MS with selected ion monitoring (SIM) yielded a sufficiently low detection limit, 10 µg/kg (dw), to analyze Se₈ in selenium-impacted sediments. The CS₂ extraction appears to be selective enough to allow the determination of Se₈ in sediments without further cleanup when GC-SIM-MS is used for analysis. One selenium-impacted sediment contained 7.1 mg/kg (dw) Se₈, corresponding to 41% of its total selenium and as much as 50% of its total elemental selenium. This is the first time that Se₈ has been positively identified in the environment. Preliminary results also indicated the presence of mixed Se-S rings in this sediment.