Analysis of marine biogenic sulfur compounds using Raman spectroscopy: dimethyl sulfide and methane sulfonic acid

摘要

Biogenic sulfur compounds, such as dimethyl sulfide (DMS), dimethyl sulfoniopropionate (DMSP), dimethyl sulfoxide (DMSO), and methane sulfonic acid (MSA), are important components of the global sulfur cycle and, as contributors to the formation of non-anthropogenic aerosols, impact global climate. In general, these chemicals are found in the marine environment in extremely low concentrations (nM), but under certain circumstances, e.g. intracellular accumulations and inclusions in glacial ice, they can be found with concentration orders of higher magnitude (μM or greater). Current analytical methods, which are designed to measure concentrations in the nM range, do so by sacrificing detailed knowledge of the chemical environment and spatial distribution of these compounds. An alternative approach to the quantitative measurement of biogenic sulfur compounds directly (i.e. without pre-processing) has been investigated in de-ionized water and artificial seawater. The detection limits for the measurement of DMSO and MSA using Raman spectroscopy with both visible and UV excitation have been measured. For DMSO, a sensitivity of <10 mM has been determined. For MSA, concentrations as low as 3 mM can be detected. It has been determined that, for aqueous solutions, the use of water as an internal standard for quantitative measurement appears to be adequate for measurement with high linearity over several decades of concentration. For DMSO and MSA, the measured sensitivities for quantitative Raman detection are adequate to address important issues in the biogeochemistry of these compounds. Moreover, for MSA it was determined that, based on measured band positions, previous studies of MSA in glacial ice have incorrectly attributed the species to solid salt rather than the, more likely,free ion in concentrated aqueous solution. In addition to sensitivity measurements, resonance/pre-resonance enhancement of the Raman spectrum of DMSO and MSA, respectively, has been found with 248 nm excitation. Finally, the previously unreported Raman spectrum of Mg(MSA)_2 has been measured.