We have developed a highly sensitive and robust method for the analysis of delta(34)S in individual organic compounds by coupled gas chromatography (GC) and multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). The system requires minimal alteration of commercial hardware and is amenable to virtually all sample introduction methods. Isobaric interference from O(2)(+) is minimized by employing dry plasma conditions and is cleanly resolved at all masses using medium resolution on the Thermo Neptune MC-ICPMS. Correction for mass bias is accomplished using standard-sample bracketing with peaks of SF(6) reference gas. The precision of measured delta(34)S values approaches 0.1 per thousand for analytes containing >40 pmol S and is better than 0.5 per thousand for those containing as little as 6 pmol S. This is within a factor of 2 of theoretical shot-noise limits. External accuracy is better than 0.3 per thousand. Integrating only the center of chromatographic peaks, rather than the entire peak, offers significant gain in precision and chromatographic resolution with minimal effect on accuracy but requires further study for verification as a routine method. Coelution of organic compounds that do not contain S can cause degraded analytical precision. Analyses of crude oil samples show wide variability in delta(34)S and demonstrate the robustness and precision of the method in complex environmental samples.

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Oceanic emissions of volatile dimethyl sulfide (DMS) represent the largest natural source of biogenic sulfur to the global atmosphere, where it mediates aerosol dynamics. To constrain the contribution of oceanic DMS to aerosols we established the sulfur isotope ratios ( 34 S/ 32 S ratio, δ 34 S) of DMS and its precursor, dimethylsulfoniopropionate (DMSP), in a range of marine environments. In view of the low oceanic concentrations of DMS/P, we applied a unique method for the analysis of δ 34 S at the picomole level in individual compounds. Surface water DMSP collected from six different ocean provinces revealed a remarkable consistency in δ 34 S values ranging between +18.9 and +20.3‰. Sulfur isotope composition of DMS analyzed in freshly collected seawater was similar to δ 34 S of DMSP, showing that the in situ fractionation between these species is small (<+1‰). Based on volatilization experiments, emission of DMS to the atmosphere results in a relatively small fractionation (−0.5 ± 0.2‰) compared with the seawater DMS pool. Because δ 34 S values of oceanic DMS closely reflect that of DMSP, we conclude that the homogenous δ 34 S of DMSP at the ocean surface represents the δ 34 S of DMS emitted to the atmosphere, within +1‰. The δ 34 S of oceanic DMS flux to the atmosphere is thus relatively constant and distinct from anthropogenic sources of atmospheric sulfate, thereby enabling estimation of the DMS contribution to aerosols., a metabolite of marine phytoplankton, is one of the major organosulfur compounds produced in the oceans (1). One of its degradation products, dimethylsulfide (DMS), is volatile and supersaturated in all marine surface waters. Large amounts of DMS (0.55-1.1 Tmol S·y −1 ) are released from the ocean to the atmosphere (2), where it contributes to the formation and growth of aerosol particles (3). Over remote oceans, distant from anthropogenic sulfur inputs, DMS is the major source of nonsea-salt sulfate aerosol (4, 5). The oxidation of DMS to submicron sulfate aerosols was suggested to increase cloud-condensation nuclei and the albedo of clouds, leading to a potential climate feedback loop operating through the DMS-producing biota in the surface ocean (6). Although evidence for the direct, local climate feedback via DMS is limited (7), DMS likely contributes, together with organic and sea-spray aerosol sources, to the complex dynamics of climate-active aerosols in the lower atmosphere (3, 7).Sulfur isotope measurements ( 34 S/ 32 S ratio, δ 34 S) may offer a way to constrain the contribution of ocean-derived DMS to global sulfur cycling and aerosol budgets. Recently, Oduro et al.(8) used a multistep extraction of large seawater volumes (e.g., 50 L) coupled with Raney nickel dehydrosulfurization and subsequent fluorination for the S isotope analysis of DMSP in intertidal macroalgae (+17.3 to +19.3‰) and estuarine phytoplankton blooms (+19 to +20‰). However, there are still no direct DMS and DMSP δ 34 S data in oligotrophic oceans owing to their typically low concentrations of 0.5-5 and 5-80 ...

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