MEMBRANE INLET MASS-SPECTROMETER FOR RAPID HIGH-PRECISION DETERMINATION OF N-2, O-2, AND AR IN ENVIRONMENTAL WATER SAMPLES

A membrane inlet mass spectrometer was modified to perform rapid, high -precision measurements of dissolved N-2, O-2, and Ar in water. The in strument pumps water at <1 mL min(-1) through semipermeable microbore silicone tubing positioned inside an inlet vacuum line of a quadrupole mass spectrometer. Precise pumping and temperature control of the wat er sample contribute to high signal stability and reproducibility. Dis solved gas concentrations are determined from intensities of the mass spectrometer signals in the multiple ion detection mode. Precision (co efficient of variation) is <0.5% for N-2, O-2, and Ar concentrations a nd <0.05% for N-2/Ar and O-2/Ar ratio data. Deviation from expected va lues was bem een 0.5 and 1.5% for air-equilibrated water of widely ran ging temperature and salinity. Advantages of the instrument over exist ing methods include rapid throughput (similar to 20-30 samples h(-1)), lack of sample water preparation (e.g., no degassing step), small sam ple size (<10 mL), and high-precision measurement of both concentratio n and gas ratio data. A limitation of the present instrument is the di fficulty in measuring water with significantly supersaturated gas conc entrations. An example of the utility of the instrument is described f or the measurement of denitrification in estuarine sediment.