Specificity is one of the hallmarks of LC-MS/MS, but what does that mean? It means that when something is detected using tandem mass spectrometry we can be highly certain that it is the analyte we seek.
By detecting a parent-to-fragment transition that is specific to an analyte, known as multiple reaction monitoring (MRM), we can be quite certain that the peak is the analyte we seek, and that background interference is minimal. There are cases in which two analytes have the same MRM transition, so we may still need good LC separation, but typically the appropriate MRM for each analyte is not common to other compounds in a given method.
To be doubly certain that the MRM peak is the compound we seek we can analyse two MRMs per compound—the ratio will be characteristic for each compound. Extra MRMs are easily added as some instruments can analyse up to 300 MRMs in a single scan.
To unequivocably confirm the presence of an analyte we can momentarily stop the instrument from scanning MRM transitions and generate complete MS/MS spectra before resuming MRM analysis, whenever an analyte is detected. We then have MRM specificity, MRM ratio specificity and a full fragmentation pattern to prove our analyte is the one we seek.
Contributed by Applied Biosystems (europe.appliedbiosystems.com)