The use of stable isotope tracer methodology to study fatty acid kinetics requires accurate measurement of tracertracee ratio (TTR) by gas chromatography/mass spectrometry (GC/MS). Current approaches for measuring TTR have limitations because of the influence of day-to-day instrument variability and sample quantity on measured values. We have developed a new approach which accommodates concentration dependencies within a standard curve approach to improve the accuracy and reproducibility of methyl palmitate TTR measurements made by GC/MS. The new approach was applied to plasma samples obtained from a human subject infused with 2H2-palmitate. When samples were analyzed on separate occasions, a conventional approach to measure the TTR of 2H2-palmitate produced a 45% day-to-day variability and a within-day variability of approximately 10% for different quantities of sample analyzed. The new approach decreased both sources of error such that the TTR values obtained on the t wo occasions differed by only 0.3%. We conclude that correcting for concentration dependency biases within a standard curve approach markedly improves the accuracy and reproducibility of methyl palmitate TTR measurements. Therefore, this approach should be considered when using stable isotopically labeled tracers to evaluate in vivo fatty acid kinetics.
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