How does isotopic dilution work in LC-MS/MS quantitation?

How does isotopic dilution work in LC-MS/MS quantitation?

• A. External standards are used exclusively for calibrations.
• B. The mass spectrometer's performance is adjusted by isotope doping.
• C. An isotope-labeled internal standard is added; the analyte-to-IS ratio is used to calculate concentration.
• D. Calibration is not required when isotopic dilution is used.

Answer: (C)

Isotopic dilution uses an isotope-labeled version of the target analyte as an internal standard to correct for variability in every step of LC-MS/MS quantitation. By adding this labeled standard to the sample, it behaves identically to the analyte during extraction, chromatography, and ionization, so any losses or matrix effects affect both compounds similarly. The instrument then measures both the analyte and the labeled internal standard, and the ratio of their signals is used to calculate concentration. Because the two forms experience the same conditions, the ratio reliably reflects how much analyte is present, even when there are fluctuations in the run. A calibration curve built with standards containing the same internal standard links the signal ratio to known concentrations, enabling accurate quantitation of unknowns. This approach inherently mitigates matrix effects and instrument drift, though calibration is still required to translate ratios into concentrations. External standards alone don’t compensate for sample variability; isotope doping is not how this method works; and calibration is still needed even with isotopic dilution.

Isotopic dilution uses an isotope-labeled version of the target analyte as an internal standard to correct for variability in every step of LC-MS/MS quantitation. By adding this labeled standard to the sample, it behaves identically to the analyte during extraction, chromatography, and ionization, so any losses or matrix effects affect both compounds similarly. The instrument then measures both the analyte and the labeled internal standard, and the ratio of their signals is used to calculate concentration. Because the two forms experience the same conditions, the ratio reliably reflects how much analyte is present, even when there are fluctuations in the run. A calibration curve built with standards containing the same internal standard links the signal ratio to known concentrations, enabling accurate quantitation of unknowns. This approach inherently mitigates matrix effects and instrument drift, though calibration is still required to translate ratios into concentrations. External standards alone don’t compensate for sample variability; isotope doping is not how this method works; and calibration is still needed even with isotopic dilution.