Tandem mass spectrometry involves the process of selecting and separating a product ion(s) (or daughter ion(s)) and fragmenting it in a second mass analyzer. This is commonly referred to as MS/MS or MS2. Additional tandem processes can be applied to ions in the MS/MS data to create MS3 data, and so forth.
The metabolites A and B share the same exact mass, and as such, cannot be differentiated by the MS data alone (see Part 1 of this series). The MS/MS data, described in Part 2, offers fragment information that can assist in eliminating one of the candidates. Taking it a step further and thus verifying the candidate metabolite A, ESI+ MS3 data is presented herein.
The fragmentation scheme below shows both the Parent and Metabolite A with the fragment at 121 Da, in bold, fragmenting to create a fragment at 93 Da. This is supported by the nearly-identical MS3 data displayed below.
On the right-hand side of the fragmentation scheme, the fragments 106 and 108 Da are expected for the MS3 data of the Parent and Metabolite A, respectively.