Solid-phase microextraction method for carbon isotopic analysis of volatile carboxylic acids in human plasma by gas chromatography/combustion/isotope ratio mass spectrometry

A new analytical method is described for the determination of the physiological concentration and low-level enrichment of (13)C-short-chain volatile organic acids (SCVAs) (e.g. (13)C-acetate and (13)C-butyrate) in human plasma. This two-step method involves solid-phase microextraction (SPME) coupled to gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) without any organic solvents or derivatizing agents. Two SCVA extraction methods were compared using a carboxen/polydimethylsiloxane fiber: headspace sampling (HS) and liquid sampling (LS) SPME. The influences of extraction temperature and time were tested to optimize the adsorption of SCVAs onto the fiber. The comparison of the peak area responses of the acids in the two adsorption methods showed better sensitivity in the human physiological concentration range in the LS mode than in the HS mode.The accuracy of isotopic enrichment measurement was determined using plasma spiked with (13)C-acetate and (13)C-butyrate solution from 0 to 1 mol percent excess (MPE). The linearity and repeatability (RSD < 5%) were measured in LS mode. Plasma SCVA concentrations were also determined relative to 3-methylvalerate (internal standard). Linearity and repeatability were observed from 0 to 400 microM for acetate, from 0 to 20 microM for propionate, and from 0 to 10 microM for butyrate. This method was also used to determine plasma acetate production obtained from lactulose (an undigestible disaccharide) fermentation in one healthy volunteer over 3 h. The acetate concentration increased twofold, 2 h after oral lactulose intake. These results are in agreement with the data obtained by GC/MS in healthy volunteers and obese adults following a lactulose intake by using higher amounts of labelled tracers.SPME coupled with GC/C/IRMS can be used to analyze (13)C-SCVAs at low enrichment (<0.5 MPE) within the physiological concentration measured in human plasma.