Modification of a Time‐of‐Flight Mass Spectrometer for Investigation of Ion‐Molecule Reactions at Elevated Pressures

Extensive modifications of a Bendix time‐of‐flight mass spectrometer have been accomplished to permit investigation of ion‐molecule reactions at ion source pressures up to about 1 Torr. In contrast with previous attempts to operate a TOF mass spectrometer, the present method employs a continuous electron beam and a dc extraction field in an ion source similar to those used in magnetic mass spectrometers. The master pulse initiating the timing cycle for mass analysis is applied to a focus grid in the low pressure region external to the source chamber and an auxiliary positive blocking pulse is applied to limit the sampling region during the ``on'' cycle. This modification eliminates degradation of instrument performance resulting from charge exchange and other secondary processes during ion acceleration, thereby extending the pressure range over which the ion source is operable by about two orders of magnitude and simultaneously increasing significantly resolution at moderate pressures. Additionally, a nov...