Mass spectrometric investigation of field ionisation with a sharp edge

Abstract

Field-ionisation mass spectra have been measured for a number of compounds. The sharp edge of a razor blade was used for producing the strong electric field. Usually the mass spectrum is simple with the parent ion predominating and with only slight formation of fragment ions. Ions of mass heavier than the parent ion are sometimes formed by addition to the parent ion of a proton or a hydrated proton. The ease of ionisation of various compounds can be correlated in two ways with their ionisation potentials.

The macroscopic electric field at the sharp edge of an idealised razor blade can be calculated when the blade is above a slotted plane. A similar calculation is possible for a thin wire. By two separate arguments which use these results the true electric field effective in producing positive ions with a razor blade edge is found to be about seven times greater than the macroscopic field. This intensification of the field is ascribed to surface roughness.

An electric field to retard the ion beam was needed in the ion source. A simple analysis of its operation shows that it leads to a loss of efficiency and to possible mass discrimination.