In situ determination of total dissolved inorganic carbon by underwater membrane introduction mass spectrometry

Procedures have been developed for the determination of total dissolved inorganic carbon (DIC) in acidified seawater using an underwater mass spectrometer. Factors affecting the response of the membrane introduction mass spectrometer (MIMS) system were examined to optimize calibrations and enhance the accuracy of component ocean carbon system measurements. Laboratory studies examined the following influences on MIMS measurements of DIC: bicarbonate and carbonate contributions to the MIMS CO2 signal intensity, linearity of MIMS response over a wide range of carbon dioxide concentrations, influence of sample salinity on membrane permeability, and capability to use acidified solutions for calibrations of both DIC and CO2 fugacity. It was observed that (a) bicarbonate and carbonate contributions to carbon dioxide signal intensity were significant at slow flow rates, (b) MIMS response was linearly dependent on DIC within the concentration range of interest, (c) salinity has a discernable influence on membrane permeability that is, in turn, dependent on hydrostatic pressure, and (d) well calibrated MIMS measurements for both DIC and CO2 fugacity can be obtained using acidified DIC standards. High flow rates are required during CO2 fugacity measurements in circumneutral seawater to eliminate signal contributions from bicarbonate and carbonate.