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 instrument 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 water sample contribute to high signal stability and reproducibility. Dissolved gas concentrations are determined from intensities of the mass spectrometer signals in the multiple ion detection mode. Precision (coefficient of variation) is <0.5% for N-2, O-2, and Ar concentrations and <0.05% for N-2/Ar and O-2/Ar ratio data. Deviation from expected values was bem een 0.5 and 1.5% for air-equilibrated water of widely ranging temperature and salinity. Advantages of the instrument over existing methods include rapid throughput (similar to 20-30 samples h(-1)), lack of sample water preparation (e.g., no degassing step), small sample size (<10 mL), and high-precision measurement of both concentration and gas ratio data. A limitation of the present instrument is the difficulty in measuring water with significantly supersaturated gas concentrations. An example of the utility of the instrument is described for the measurement of denitrification in estuarine sediment.