Dissolved gas measurements in oceanic waters made by membrane inlet mass spectrometry

Abstract

A method is presented for the shipboard analysis of dissolved gases (O2, CO2, Ar, N2, H2S, dimethylsulfide) in oceanic waters using membrane inlet mass spectrometry (MIMS). Gases are extracted from seawater across a semipermeable membrane into the ion source of the mass spectrometer. The method can be used to simultaneously measure both major and trace gases in seawater samples in near real-time. In the case of DMS, low nmol L−1 detection limits can be achieved without any concentration step. Multiple calibration and method comparison exercises conducted in the subarctic Pacific Ocean show that the MIMS method provides gas measurements that are consistent with those obtained by standard techniques, with precision of replicate samples ranging from less than 1% to approximately 5% coefficient of variation (CV). To illustrate the usefulness of the MIMS approach, depth profiles are presented for O2, CO2, N2, and DMS at various coastal and open ocean stations in the subarctic Pacific. In addition to these discrete gas measurements, the MIMS system has also been used to continuously monitor gas concentrations along underway transects. High frequency gas measurements of O2, CO2, and DMS made along two coastal transects reveal a high degree of spatial and temporal variability in gas concentrations, likely resulting from the interplay of various biological, chemical, and physical forcings. The underway MIMS data provide insight into the biogeochemical controls on gas cycling in dynamic marine waters.