What is the basic principle of mass spectrometry-based quantification in clinical chemistry, and why is it used for confirmatory testing?

• A. It measures optical density; used for screening.
• B. It measures mass-to-charge ratio of ions with high specificity and sensitivity; used to confirm results and distinguish isobaric interferences.
• C. It measures pH; used to quantify acids.
• D. It detects radioactive decay; used for isotopic tracing.

Answer: (B)

Mass spectrometry-based quantification relies on measuring the mass-to-charge ratio of ions with high specificity and sensitivity. In practice, the analyte is ionized and selected by its exact m/z (often using tandem MS to monitor specific transitions), then quantified against a calibration curve with an internal standard. Because exact masses and characteristic fragment ions are unique to a compound, MS can distinguish isobaric interferences—substances with the same nominal mass but different composition or fragmentation patterns. This precision makes it ideal for confirmatory testing after screening, providing definitive identity and accurate concentration and reducing false positives. Often, chromatography precedes MS to separate compounds and improve selectivity, and stable-isotope–labeled internal standards help correct for matrix effects and instrument variability.