ASSAY OF THE CONCENTRATION AND STABLE-ISOTOPE ENRICHMENT OF SHORT-CHAIN FATTY-ACIDS BY GAS-CHROMATOGRAPHY MASS-SPECTROMETRY

Isotope dilution gas chromatographic mass spectrometric assays were de veloped for the concentration of short-chain fatty acids and of lactat e in plasma. The assays involve spiking the samples with H-2- and/or C -13-labeled internal standards and direct derivatization with 2,4-difl uoroaniline, using 1,3-dicyclohexylcarbodiimide as a coup ling agent. The assay can be used to determine the isotopic enrichment or mass iso topomer distribution of the fatty acids and of lactate. Blanks by ubiq uitous formate, acetate and lactate are minimized by the one-step deri vatization procedure without deproteinization, using reagents dissolve d in hydrocarbons. The assays were applied to experiments conducted in live dogs, pigs and monkeys, and in perfused rat livers. These techni ques, in conjunction with appropriate stable isotope models, can be us ed to study carbohydrate fermentation in humans, ruminants and non-rum inant mammals.