Measurement of DMS, DMSO, and DMSP in natural waters by automated sequential chemical analysis

Dimethyl sulfide (DMS), dimethylsulfoniopropionate (DMSP), and dimethyl sulfoxide (DMSO) are key components of the marine sulfur cycle. The concentrations of these compounds exhibit large spatial and temporal variability in the surface ocean, creating a need for high resolution sampling. Existing automated underway measurement systems for DMS do not measure DMSP or DMSO, so their spatial variability is less well-characterized. We present an accurate and robust method for the automated, high throughput sampling and measurement of DMS, DMSO, and DMSP (DMS/O/P) in a single water sample. The method is based on a three-step sequence of purge and trap gas chromatography, where DMS analysis is followed by the enzymatic reduction of DMSO to DMS and the alkaline hydrolysis of DMSP to DMS. The system, which we call the Organic Sulfur Sequential Chemical Analysis Robot (OSSCAR), includes automated calibrations and blank determinations. OSSCAR can be used as a front-end system for any sulfur detector and is suited for continuous underway analysis or the measurement of discrete water samples. The system described here has a minimum detection limit of ∼ 0.1 nM of DMS/O/P in a 2.5 mL sample. Assessment of liquid standards and intercalibration against independent analytical systems demonstrate good precision and accuracy of our method. Shipboard analysis of surface water DMS/O/P concentrations on a transect from Ocean Station Papa (50°N, 145°W) to Vancouver Island demonstrates the utility of OSSCAR for mapping variability in reduced sulfur compounds across dynamic and contrasting oceanographic conditions.