Page 111 - Tandem Techniques

Page 92

Substituting for (v) from equation (2.4),

Rearranging,

It is seen that by scanning (H) over an appropriate range, the ions with increasing (m/z) ratios will be
sensed by the detector. This is a very simple treatment of the double focusing mass spectrometer and, in
fact, if the slit entering the magnetic sector was made sufficiently narrow to produce well-resolved mass
peaks, then the number of ions collected would be very small and the system relatively insensitive.
Nevertheless, the explanation given illustrates the function of the sector instruments and how it is
operated. The low sensitivity can be avoided by the choice of a suitable combination of electrostatic and
magnetic sectors such that the velocity dispersion in the two sectors is equal and opposite.
Consequently, a larger slit can be used and the resulting high sensitivity recovered.

Ionization Methods

There are number of ways of producing sample ions, many of which will be discussed when the actual
tandem interfaces are described. The most common form of ionization is by electron impact.

Electron impact ionization is a fairly harsh method of ionization and usually produces a range of
molecular fragments that in most cases helps to elucidate the structure of the molecule. However,
although molecular ions are often produced, which is important for structure elucidation, sometimes
only small fragments of the molecule are observed, with no molecular ion. Under such circumstances
alternative ionizing procedures