Scheme 2. Selected Reaction Monitoring.
Clozapine (blue) and other compounds have been desorbed and ionized prior to mass analysis. In selected reaction monitoring (SRM), the first round of mass analysis removes all ions outside the mass window of 325 Da – 329 Da that allows isolation of precursor ions for subsequent fragmentation. The remaining ions are then fragmented. For the second round of mass analysis, ions outside of the 268 Da – 272 Da are removed, leaving only the signature fragment of clozapine (blue) to be imaged.