The Structure of Graphite Fibers Utilized in Field Ionization

Abstract

Field ionization mass spectrometry has become an increasingly useful analytical tool because of the absence of mass spectra cracking patterns normally associated with electron impact methods. However a rather trouble some experimental difficulty has been the development of an ionization source with ionization efficiency sufficient to produce detectable signals of trace components.A graphite filament ionizer was recently developed at our laboratory and consists of a graphite whisker mounted taut between two tungsten posts. The exposed length of filament and its diameter are nominally 8 mm and 10-3 mm (1 micron) respectively. Ion yields of 10-11 amp at 10-6 torr (10-5 amp/torr) with ion energies of 6-7 Kev are readily obtainable. The filaments have also been used for field emission and Cox has used the Fowler-Nordheim equation modified for cylinderical geometry to obtain an estimated emitter diameter of 40 Å from his experimental data.