Evaluation of using unfiltered seawater for underway measurement of dimethyl sulfide in the ocean by online mass spectrometry
Limnol. Oceanogr. Methods 11:549-560 (2013) | DOI: 10.4319/lom.2013.11.549
ABSTRACT: New, highly time-resolved techniques for measuring dimethyl sulfide (DMS) concentrations in seawater provide the opportunity to greatly increase the size of the DMS concentration database. However, lack of community-level quality control raises concerns about the quality of data obtained by these new techniques and their consistency with data obtained by existing techniques. One of the concerns is that the new techniques use a continuous flow of unfiltered seawater as a sample. Phytoplankton in unfiltered seawater might produce DMS in the sample, resulting in inaccurate DMS concentrations. Here, in our equilibrator inlet-"proton transfer reaction-mass spectrometry technique, unfiltered seawater is continuously supplied to the equilibrator, and dissolved DMS is extracted into a bubbling gas and detected by the mass spectrometry. To investigate the possible artifact from using unfiltered seawater and the magnitude of its effects, we conducted laboratory experiments in a closed system at equilibrium, focusing on the effects of sample filtration and gas bubbling on the temporal variation of DMS concentrations in coastal seawater samples dominated by diatoms. With N2 as the bubbling gas, DMS concentration increased with time in unfiltered seawater but not in filtered seawater. DMS concentration increased when unfiltered seawater was bubbled with N2 but not when it was bubbled with air, and the increase occurred just after dissolved O2 was depleted in the seawater in the equilibrator. These results indicate that O2 depletion artificially increased DMS concentrations in unfiltered seawater bubbled with N2. This artifact could be avoided by maintaining aerobic conditions in the equilibrator.