MEASUREMENT OF C-13 ENRICHMENT OF PLASMA LACTATE BY GAS CHROMATOGRAPHY/ISOTOPE RATIO MASS-SPECTROMETRY

An application of a gas chromatography/isotope ratio mass spectrometry (GC/IRMS) method for stable carbon isotope analysis of blood plasma lactic acid is presented. The method involves a simple extraction procedure followed by derivatization with diazomethane, It is shown that derivatization is by single methylation, thus minimizing the dilution of the derivative's C-13 content, yet still ensuring good chromatographic behavour on a polar capillary column. This ensures a high sensitivity of the isotopic analysis, Repeatability, expressed by the coefficient of variation, varied from 0.3% to 19%, depending on sample enrichment. Reproducibility was 2.3% over a 10 day period. The detection limit, defined as 2SD, was about 0.0004 atom % excess (APE), equivalent to 0.001 mol % excess, when based on a measured precision of about 0.2 parts per thousand in delta notation. A comparison is made between enrichments obtained using a calibration curve and those obtained using a correction for the added methyl carbon. The two methods agreed well, with a relative difference (Delta APE/APE x 100%) of less than 0.5% for samples enriched with between 0.004 and 1,28 APE. It is concluded that the method provides simple and precise isotope analysis of picomole quantities of blood lactate.