Page 347 - Tandem Techniques

Page 331

usually formed by the secondary ion process, such as (M+H) and (M+Ag) , from thin layers of sample
+
+
deposited on a metal substrate such as silver. The use of other materials as a substrate, such as
nitrocellulose, can improve the ionization efficiency for very high molecular weight samples. To
employ this method of ionization in an LC/MS tandem instrument, the sample must be presented to the
ionization source as a dry film on a solid carrier, and therefore requires the use of a transport interface.

Fast Atom Bombardment (FAB)

Fast atom bombardment spectrometry is an extension of the secondary ion mass spectrometry, and the
principle of the ionization process is shown in Figure 9.1.

Figure 9.1
The Fast Atom Bombardment Ionization Source
A beam of energetic particles is generated and directed onto the sample, which is carried as a thin film
on a clean metal support. The secondary ions that are produced are extracted by a suitable ion optical
arrangement into the mass spectrometer analyzer. The impact of an ion striking the surface produces an
intense thermal spike. The energy from this thermal spike is then dissipated through the outer layers of
the sample. In the original device developed by Barber [1], neutral atoms of argon and xenon were
used, but eventually these were replaced by charged ions such as Ce and Xe . If a dry sample is used,
+
+
the surface becomes damaged by the intense incident beam, and the yield of secondary ions rapidly
decreases.