Variation of Appearance Potentials of Alkane Fragment Ions with the Repeller Voltage of the Mass Spectrometer Ion Source

Abstract

The differences in the mass spectrometric appearance potentials for parent molecule ions and the ion fragments which are formed from the parent by hydrogen abstraction have been measured for propane, n‐butane, and n‐pentane as a function of the repeller voltage in the ion source. With pentane, the potential difference varies from close to zero for no repeller voltage to 1.1 volts for 0.5 volt repeller voltage or higher. There is a similar but smaller increase of 0.6 volt with n butane, while with propane the changes in appearance potentials are well within the experimental error. Of the various explanations considered, the most likely is that zero repeller voltage leads to relatively longer ion residence times in the source and that, as a result, some low‐probability de‐excitation processes have time to occur. This implies that appearance potentials which are obtained for this type of process with use of the usual repeller voltages can be considerably in excess of the minimum threshold energies.