Laboratory inter-comparison of dissolved dimethyl sulphide (DMS) measurements using purge-and-trap and solid-phase microextraction techniques during a mesocosm experiment

We compare dissolved dimethyl sulphide (DMS) measurements made by our independent laboratories during a mesocosm study of marine phytoplankton under different CO2 regimes in a Norwegian fjord. Sample preparation and analyses were conducted using headspace solid-phase microextraction (SPME) with gas chromatography-mass spectrometry (Max-Planck Institute for Chemistry, MPIC), and purge-and-trap extraction (P&T) with gas chromatography and flame photometric detection (University of East Anglia, UEA). The two analytical systems were calibrated independently. During the evolution of the bloom (22 days) DMS concentrations ranged from 1-35 nM and 90 pairs of data were available for comparison. We found a small systematic difference between the two methods, with UEA measuring on average 8% more DMS than MPIC. Overall, there was good correlation between the datasets (r(2)=0.997, P=0.01), with higher correlation for concentrations greater than 5 nM (r(2)=0.998, P=0.01) and increased scatter at lower concentrations (r(2)=0.833, P=0.01). We discuss potential reasons for the differences between the measurements and address the treatment of natural samples for DMS analysis. We recommend SPME be considered for wider use and encourage full analytical comparisons in the low concentration range.