TABLE OF CONTENTS With the exception of electron ionisation, all of the ionisation methods described in Section 3.2 lead to the formation of molecular ions (or pseudo-molecular ions) with little fragmentation. This is advantageous from the perspective of measuring the molecular weight of a compound, but a disadvantage if one wants to obtain structural information.
Tandem mass spectrometry was developed to derive structural detail originally through the use of two mass analysers between the ion source and the detector. A region between the two mass analysers is used to effect the dissociation of incoming ions. A schematic representation of a tandem mass spectrometry experiment is shown in Figure 4.1. The first mass analyser is used to transmit only ions of a particular m/z ratio into the dissociation region at any point in time. Within the dissociation region or chamber, ions are "excited" energetically through a number of different processes described within the next section. This leads to the production of fragments through bond cleavage of the precursor ions. The second mass analyser is scanned to pass, in turn, the products of the dissociation onto the detector. Since two mass analysis steps are involved, tandem mass spectrometry is often referred to as an MS/MS experiment and the resulting fragment or product ion spectra are known as MS/MS (or MS 2) spectra.
In practice the components of a sample are first surveyed in the MS-mode, usually...
