Multiple gas measurements at trace levels with a quadrupole mass spectrometer: gas phase calibration

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Abstract
Measurement of the steady-state elimination of 6 nonrespiratory gases in trace concentrations allows estimation of the functional distribution of pulmonary ventilation-perfusion (.ovrhdot.VA/.ovrhdot.Q) ratios. A mass spectrometer system to obtain the ratios of inert gas partial pressures required by the .ovrhdot.VA/.ovrhdot.Q theory was developed. The components of a sensitive and versatile quadrupole mass spectrometer was assembled in order to test its feasibility for rapid, nearly continuous in vivo measurements of trace amounts of foreign gases in both blood and expired gas. Investigations were done on a number of gases for this application and consequently a set of 6 were selected that were compatible with both .ovrhdot.VA/.ovrhdot.Q distribution theory and quadrupole analyzer characteristics. In gas phase calibration experiments, determinations of the detection limits (at signal-to-noise ratio .gtoreq. 10:1) for SF6 (0.5 ppm at a mass-charge ratio (m/e) = 127), Kr (0.8 ppm at m/e = 84), Freon 12 (2 ppm at m/e = 85), enflurane (2 ppm at m/e = 117), diethyl ether (9 ppm at m/e = 59) and acetone (15 ppm at m/e = 58) were completed. The quadrupole analyzer responded linearly from a concentration of at least 1000 ppm (vol/vol) in air down to the detection limit for each gas. The short-term reproducibility (SD) of the mass spectrometer was .+-. 1.4% at output levels greater than 40 pA and was .+-. 0.8 pA at outputs below that level. At predicted mixed expired concentrations, the SD of an inert gas measurement ranged from 1% of the mass spectrometer output for enflurane and acetone to 8% of the output for SF6. The state-of-the-art quadrupole mass spectrometer may provide the measurement capability to implement the multiple inert gas .ovrhdot.VA/.ovrhdot.Q technique, and modification of the inlet system to make the required blood phase measurements is planned.