Importance of Utilizing Natural Isotopologue Transitions in Expanding the Linear Dynamic Range of LC-MS/MS Assay for Small-Molecule Pharmacokinetic Sample Analysis - A Mini-review.

Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is a widely used quantitative method in small-molecule pharmacokinetic sample analysis. The linear dynamic range of mass analyzers, typically spanning 3 orders of magnitude, is usually insufficient for this purpose. Utilization of multiple isotopologues has been proposed as a compelling approach to expand the linear dynamic range of LC-MS/MS assays, particularly when the detector is saturated. Isotopologues are a statistical mixture of molecules of the same compound but of different exact masses due to the presence of natural chemical isotopes. While the concept of isotopologues is widely recognized in large-molecule bioanalysis and small-molecule metabolite profiling, it has not been commonly implemented in small-molecule targeted quantification. To increase the awareness of the value of isotopologues in small-molecule LC-MS/MS analysis, this minireview provides the basis of isotopologue distribution in MS/MS and summarizes published studies as well as our own experience in utilizing multiple isotopologues to expand the linear dynamic range of small-molecule LC-MS/MS assays. Considering that utilizing natural isotopologue transitions in the LC-MS/MS assays represents an easy, straightforward, and robust way to expand the linear dynamic range, we believe this method deserves wide application in small-molecule pharmacokinetic sample analysis and can particularly benefit people working in pharmacokinetic labs as well as the GLP bioanalytical labs in pharmaceutical industry.