Free and total carnitine are measured by tandem mass spectrometry (MS/MS) stable isotope dilution analysis. Hydrolysis enables measurement of total carnitine, and esterified carnitine (acylcarnitine) is calculated as the difference between the total and free carnitine. Quantification is enabled using deuterium-labeled carnitine (d3-carnitine) added as internal standard. A selected reaction monitoring experiment is performed by MS/MS. The first mass spectrometer (Q1) detects carnitine and d3-carnitine precursors, and transmits them to a collision cell (Q2) within the mass spectrometer where they are fragmented. Specific fragments derived from the carnitine and internal standard are monitored in the second mass spectrometer (Q3).(Stevens RD, Hillman SL, Worthy S, et al: Assay for free and total carnitine in human plasma using tandem mass spectrometry. Clin Chem 2000;46:727-729)
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