Quantitative analysis of gas mixtures from sources external to a mass spectrometer (MS) requires vacuum hardware that systematically introduces the gas to the ionization region of the MS and calibration procedures that reflect the MS response to time dependent partial pressures in the gas source. Formalism for describing the gas flow, fractionation processes, and the ionization process is presented. Long-term sensitivity data for D2 is presented for three types of mass spectrometers: a quadrupole, a cycloid, and a magnetic sector mass spectrometer. The use of appropriate standard gas mixtures to monitor the accuracy of the MS calibration by control chart is advocated. Bias data from control charts can be used as the basis for minor calibration adjustments rather than total recalibration.

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