DETERMINATION OF DELTA-C-13 VALUES OF PLASMA LACTATE BY GAS CHROMATOGRAPHY/ISOTOPE RATIO MASS-SPECTROMETRY

Gas chromatography/isotope ratio mass spectrometry (GC/IRMS) allows tbe detection of low C-13 enrichment. However, this method needs to set up new metabolite derivatives suitable for GC/IRMS. In this work we describe a new derivative for plasma lactate analysis. After plasma extraction, diazomethane is used to methylate the lactate carboxyl group. The lactate methylester reacts with acetic anhydride in pyridine to give an acetylated methyl ester of lactate. The isotopic C-13 enrichment of this compound is determined by GC/IRMS. A correction method for the effects of the added derivative carbon atoms on measurements of the derivatized compounds is also described, This method appears reproducible and allows the detection of C-13 enrichment as low as 0.01 mol.%. A practical application for the measurement of C-13 plasma lactate enrichment after ingestion of an oral glucose load (0.5 g kg(-1)) enriched with 0.1% D-(U-C-13)glucose is shown.