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 a fter alkaline hydrolysis, and DMSO is reduced to DMS using a combinati on of sodium borohydride and hydrochloric acid. The DMS produced is st ripped, 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 mt 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 less than or equal to 10%, Mean yields of the sequential analysis of D MS + DMSP + DMSO (1.5-25 nM) standard mixtures in seawater were >90% f or the three species. Filtered seawater samples stored frozen (-20 deg rees C) showed no significant changes in DMSO concentration, Since DMS P is the only compound, other than DMSO, that gives rise to DMS upon r eduction with NaBH4, tests were performed to ensure that DMSP is quant itatively removed before the DMSO analysis. Boranes were identified as the substances that produced two major peaks in the chromatogram of t he reduction products, Adequate chromatographic conditions were establ ished to avoid their coelution with DMS, This entire protocol allows t he sequential determination of DMS, DMSP, and DMSO in natural waters a nd 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.