High sensitivity tracer detection using high-precision gas chromatography-combustion isotope ratio mass spectrometry and highly enriched [U-13C]-labeled precursors.

The use of highly enriched, uniformly labeled fatty acid ([U-13C]) with analysis by high-precision gas chromatography-combustion isotope ratio mass spectrometry (GCC-IRMS) has been evaluated as a metabolic tracer technique. 13C/12C ratios are routinely determined to precisions (SD) of less than 0.00001 (delta PDB less than 1/1000) for greater than 10 ng of fatty acid, and less than 0.001 (delta PDB less than 100/1000) for samples of 30 pg of fatty acid, the latter corresponding to a 100-fmol sample. Baseline fatty acid 13C/12C in human plasma fractions is shown to fluctuate not more than 0.000 04 (delta PDB 4/1000) over 10 h. 13C/12C enrichments greater than 0.001 (delta PDB 100/1000) are obtained in a fatty acid plasma fraction subsequent to a 10-mg dose of 42% 13C-labeled stearic acid to a 78-kg adult. Biokinetics are discerned over an 13C/12C enrichment range of less than 0.0002 (approximately 13/1000 in delta PDB units) in plasma. A means for correction of isotope ratio contamination due to carbon-containing derivatives is presented. High-precision GCC-IRMS used in concert with highly enriched tracers is shown to possess advantages versus organic GC/MS for stable isotopic tracer detection and is superior to radiotracer methods in terms of dose sizes and analysis efficiency.