TABLE OF CONTENTS As described in Section 2.1, the traditional method of ion production in mass spectrometry is electron impact or electron ionisation (EI) in which gaseous sample molecules are bombarded with a stream of electrons (equation 2.1). Other processes by which ions can be formed during electron ionisation include dissociative ionisation (equation 3.1), ion pair formation (equation 3.2) and electron capture in which negatively charged ions are produced (equation 2.2). The electron ionisation technique is widely used for the study of relatively volatile organic molecules by mass spectrometry.
The electrons are produced by heating and passing a current through a thin ribbon of metal (such as ruthenium) known as the filament or cathode. The electrons are projected across the ion source by their attraction to an anode on the opposite side of the chamber (Figure 3.1). The energy of the electrons depends on the difference in the potentials applied to the cathode and anode. If a voltage difference of 70V is maintained, the electrons have energies of 70 electron volts (eV), or the equivalent of 6.8 10 3 kJ mol ?1. A small magnetic field is usually applied across the ion source to cause the electrons to follow a helical path in order to increase the probability that they interact with a gaseous sample molecule. A pressure of typically 10 ?6 Torr or 1.3 10 ?4 Pa is maintained in...
