Pressure measurement in the chemical ionization source of a Du Pont 21 490-B mass spectrometer

A pressure measuring probe has been built for the chemical ionization source of a Du Pont 21 490-B mass spectrometer. The probe fits exactly in the place of the solid sample probe and, when connected to a Baratron manometer, enables pressure measurement directly in the source. This can be used to calibrate an ionization gauge connected to the source housing or a manometer in the reagent gas inlet line. The probe has been used to determine the maximum source pressure attainable for various reagent gases in the Du Pont 21 490-B mass spectrometer.