Discrimination in Mass Spectrometer Ion Sources

Abstract

An analysis is made of the collection and definition of a beam of ions by an ion source. It is found that the first slit of the source has no discriminating effect but the second slit does have. This is due to the fact that ions of certain values of initial energy, angle of velocity vector, and position of striking the plane of the first slit cannot get through the second one. An efficiency of collection of a source for ions of a certain initial energy is defined as the ratio of the number with that initial energy that get through the second slit to the number with that initial energy which get through the first slit. An expression for this efficiency is derived and it is applied to rather typical data to show how an abundance ratio obtained by taking the ratio of peak heights may be in error several percent. The method of measuring isotopic abundances is recommended wherein the focusing conditions are changed only by varying the magnetic analyzer field.