Accurate measurement of enrichment by GC/MS is critical for many studies of fatty acid metabolic kinetics. Traditional approaches to determine the tracer:tracee ratio (TTR) often fail to account for day-to day variations in instrument response and concentration-dependent processes which affect measured ion abundance ratios; these artifacts invalidate the traditional assumption that measured ion abundance ratios equate to true isotopomer ratios. We have observed a 68% range in measured ion abundance ratios on the same instrument for 2H2-palmitate standards of known TTR; concentration dependencies introduce an additional 10-20% bias in ion abundance ratios. We have developed a new approach for determining TTR which incorporates concentration dependencies into the framework of a standard curve to account for instrument response variability. Arterialized blood samples were obtained during steady state conditions from a human volunteer infused with 2H2-palmitate during postabsorptive basal conditions. Palmitate TTR was measured on two separate occasions separated by 3 months on the same instrument. The rate of appearance of palmitate in plasma differed by 45% (1.48 vs. 2.13 μmol/kg/min) for the two runs when the TTR was determined by a traditional approach directly from the measured ion abundance ratios, but was virtually identical (1.29 vs. 1.30 μmol/kg/min) after correction for instrument response and concentration effects. We conclude that an approach which accounts for variability in instrument response and concentration effects improves the accuracy of TTR determinations by GC/MS. Supported by NIH CA62177 and DK47899.
|State||Published - Dec 1 1997|