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

Philippe D Tortell

Limnol. Oceanogr. Methods 3:24-37 (2005) | DOI: 10.4319/lom.2005.3.24

ABSTRACT: A method is presented for the ship-board analysis of major and trace gases (O2, CO2, Ar, N2, H2S, DMS) in oceanic waters using membrane inlet mass spectrometry (MIMS). The key feature of the analytical system is a large area membrane interface through which gases are extracted from seawater into the ion source of the mass spectrometer. The large size of the membrane significantly increases analytical sensitivity, enabling DMS concentrations to be measured down to several nmol L-1 without any concentration step. Multiple calibration and method comparison exercises conducted in the Subarctic Pacific Ocean show that the MIMS system provides gas measurements that are consistent with those obtained by standard methods, with precision of replicate samples ranging from < 1 to ~ 5% C.V. The method enables multiple gases to be measured simultaneously in seawater samples in near real-time. As an illustration of this, depth profiles are presented for O2, CO2, N2, and DMS at various coastal and open ocean stations in the Subarctic Pacific. In addition to discrete gas measurements, the MIMS system can also be utilized 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.