What is the role of an internal standard in LC-MS/MS analysis?

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The main idea is that an internal standard provides a constant reference that compensates for variability in the LC-MS/MS process. You add a known amount of a compound to every sample, calibrator, and quality control. This standard should behave similarly to the target analyte during extraction, chromatography, and ionization—often a stable isotope-labeled version—so it experiences the same losses and ionization effects as the analyte. By comparing the analyte’s signal to the internal standard’s signal (the ratio), you correct for matrix effects that can suppress or enhance ionization and for run-to-run instrument variations, injections, and recovery differences. This normalization enables accurate and precise quantification of the analyte in complex samples. The internal standard is not the calibration target itself, nor primarily a tool for reducing volume or preserving samples; its primary role is stabilizing measurement through consistent, co-eluting reference signals.