Sequential method for the field determination of nanomolar concentrations of dimethyl sulfoxide in natural waters

A procedure for the determination of dimethyl sulfoxide (DMSO), along with dimethyl sulfide (DMS) and dimethyl sulfoniopropionate (DMSP), at nanomolar levels in natural waters has been developed. After removal of DMS by purge and cryotrapping, DMSP is removed by the same method after alkaline hydrolysis, and DMSO is reduced to DMS using a combination of sodium borohydride and hydrochloric acid. The DMS produced is stripped, cryotrapped, and analyzed by gas chromatography. Detection of 3 pmol of DMSO was achieved, resulting in a detection limit of 0.05 nM for a 50 mL sample. Mean yield for standards in the range 0.7-130 nM (n = 31) was 95%, and mean precision (as coefficient of variation) was 14%. Precision for replicates of natural seawater samples was always ≤10%. Mean yields of the sequential analysis of DMS + DMSP + DMSO (1.5-25 nM) standard mixtures in seawater were >90% for the three species. Filtered seawater samples stored frozen (-20 °C) showed no significant changes in DMSO concentration. Since DMSP is the only compound, other than DMSO, that gives rise to DMS upon reduction with NaBH(4), tests were performed to ensure that DMSP is quantitatively removed before the DMSO analysis. Boranes were identified as the substances that produced two major peaks in the chromatogram of the reduction products. Adequate chromatographic conditions were established to avoid their coelution with DMS. This entire protocol allows the sequential determination of DMS, DMSP, and DMSO in natural waters and is suitable for field work, so it should be very useful in studies of dimethyl sulfur biogeochemistry. Some examples of DMSO (along with DMS and DMSP) measurements in the Mediterranean Sea are presented.