Multiple reaction monitoring (MRM) is the most common method for the quantitation of analytes by LC/MS/MS. In MRM, ions are selected to make it through the first quadrupole and into the collision cell. These ions are referred to as the precursor, or parent, ion. These ions are fragmented in the collision cell. Certain fragment ions referred to as product, or daughter, ions are selected to make it through the second quadrupole. The transition from precursor/parent ion to product/daughter ions is referred to as an ion transition. For example, the precursor ion (quasimolecular ion) of morphine is 286. The most intense ions created by the fragmentation of 286 are 201, 181, and 165. This gives us three ion transitions for morphine: 286 → 201, 286 → 181, and 286 →165. For analysis, we might want to use 286 → 201 as the quantitative ion transition and 286 → 181 as the qualifier ion transition.
GC/MS and LC/MS/MS results are quantitative. A quantitative result can be reported or the results can be reported as positive or negative, depending on whether the analyte of interest is present or not present at or above a predetermined cut-off concentration.
