Calibration of an in situ membrane inlet mass spectrometer for measurements of dissolved gases and volatile organics in seawater.

Use of membrane inlet mass spectrometers (MIMS) for quantitative measurements of dissolved gases and volatile organics over a wide range of ocean depths requires characterization of the influence of hydrostatic pressure on the permeability of MIMS inlet systems. To simulate measurement conditions in the field, a laboratory apparatus was constructed for control of sample flow rate, temperature, pressure, and the concentrations of a variety of dissolved gases and volatile organic compounds. MIMS data generated with this apparatus demonstrated thatthe permeability of polydimethylsiloxane (PDMS) membranes is strongly dependent on hydrostatic pressure. For the range of pressures encountered between the surface and 2000 m ocean depths, the pressure dependent behavior of PDMS membranes could not be satisfactorily described using previously published theoretical models of membrane behavior. The observed influence of hydrostatic pressure on signal intensity could, nonetheless, be quantitatively modeled using a relatively simple semiempirical relationship between permeability and hydrostatic pressure. The semiempirical MIMS calibration developed in this study was applied to in situ underwater mass spectrometer (UMS) data to generate high-resolution, vertical profiles of dissolved gases in the Gulf of Mexico. These measurements constitute the first quantitative observations of dissolved gas profiles in the oceans obtained by in situ membrane inlet mass spectrometry. Alternative techniques used to produce dissolved gas profiles were in good accord with UMS measurements.