Application of purge and trap-atmospheric pressure chemical ionization-tandem mass spectrometry for the determination of dimethyl sulfide in seawater

We describe a method for measuring trace concentrations of dimethyl sulfide (DMS) in seawater using a commercial tandem mass spectrometer configured for atmospheric pressure chemical ionization (PT-APCI-MS/MS), coupled with a custom-built purge and trap gas extraction system. DMS was ionized through proton transfer, generating abundant [M + H](+)ions. The semiautomated method analyzes samples in under 6 min, and is capable of processing up to 10 samples in a single batch. A detection limit of 0.9 pmol L(-1)was determined for the analysis of 5 mL sample volumes, with a precision of 3.9% between replicates. Practical performance was evaluated during two oceanographic research cruises within the coastal waters around Vancouver Island, British Columbia. To demonstrate method utility, a series of DMS depth profiles were obtained along two transects extending from the west coast of Vancouver Island into deep water off the continental shelf. Additional depth profile sampling was conducted in Saanich Inlet, a coastal anoxic fjord with active chemotrophic sulfur cycling. This method enabled us to capture the deep-water accumulation of subnanomolar DMS in the anoxic water of Saanich Inlet, providing evidence of cryptic sulfur cycling. The method was also leveraged to facilitate stable isotope rate measurement experiments, in which the consumption of isotopically labeled DMS, dimethylsulfoxide, and dimethylsulfoniopropionate tracers was monitored in the low picomolar range. These measurements enable metabolic rate determinations using low-level tracer additions that do not perturb in situ microbial activity. Our sensitive, high throughput method helps to improve understanding of the natural marine cycling of volatile sulfur compounds.