THERMAL-DESORPTION FROM MASS-SPECTROMETER FILAMENTS

In most mass spectrometers of the Residual Gas Analyser (RGA) or Partial Pressure Analyser (PPA) type, the gaseous sample is ionised by electron impact, using electrons emitted from a filament heated to incandescence. The hot filament may emit significant quantities of gas. This is especially noticeable with a new filament. The amount of gas liberated can be reduced by 'pre-conditioning' the filament before it is installed. The pre-conditioning consists of heating the filament in a controlled manner. In some cases, this has been found essential in order to obtain reliable performance. A computer-controlled RGA has been used to monitor the gaseous species evolved when a filament is heated to its operating temperature. Correlations have been observed between the species evolved during this operation, other physical parameters and the subsequent performance of the filament.