Partial Pressure Measurement

The quadrupole mass spectrometer is now well established as the instrument used almost universally for partial pressure measurement. This spectrometer can be considered as a hot-cathode extractor ionisation gauge with the addition of an electrostatic mass filter between the ion source and the ion collector. This filter consists of a square array of circular rods approximately 6 mm diameter and between 100 to 150-mm long. Positive ions are injected parallel to the central axis towards the ion collector. A filtering action is achieved by applying combined radio frequency and dc potentials between opposite pairs of rods. The characteristics of the filter can be controlled by small changes in the relative values of the radio frequency and dc voltages and the mass range by changes in the frequency of the ac signal. Care must always be taken when operating this mass spectrometer in an analytical mode because of non-linearities caused by space charge and other effects which are important, particularly in the injection region between the ion source and filter. Also the filter is subject to loss of sensitivity over the long term, generally caused by contamination from hydrocarbon impurities in the vacuum system. This is not normally a problem in ultra-high vacuum operation. Although almost all filters have been designed to operate with relatively low voltages applied to the rods, alternative modes are being considered which require distinctly high voltages or lower frequencies. There are hopes that these will have advantages for general operation.