Rapid measurement of 13C-enrichment of acetic, propionic and butyric acids in plasma with solid phase microextraction coupled to gas chromatography–mass spectrometry

An analytical procedure based on solid phase microextraction (SPME) has been developed to quantify [1-13C]-labelled short-chain fatty acids (SCFAs)—mainly acetic, propionic and butyric acids—in a small volume (120 (mu)l) of deproteinised plasma (corresponding to 200 (mu)l of raw plasma) by gas chromatography–mass spectrometry (GC–MS) analysis. Simultaneous SCFA extraction was optimal after 5 min using a 75 (mu)m Carboxen/polydimethylsiloxane-coated fiber. The base peak of the three analytes has been characterised by middle-resolution mass spectrometry (R>6000). All these data allowed the specificity reinforcement of the measure. The validation of the method also considered the linearity and the repeatability of the [13C]SCFA measurements by SPME–GC–MS. Results were linear in a range from 5 to 100 mol% of [13C]SCFA enrichment and the method provided a good intra-day (R.S.D.<1.7%, n=8) and inter-day repeatability (R.S.D. <3.2%, 5 days). The method was applied to the measurements of isotopic enrichment of butyric acid in the plasma of rat which received the tracer ([1-13C]butyric acid) by cecal infusion before blood sampling in portal vein. Results of [1-13C]butyric acid enrichment showed an excellent correlation (r^2=0.9832; n=30) with data obtained on the same samples using a previously published procedure based on diethyl extraction and derivatisation before GC–MS analyses. SPME coupled to GC–MS appears to be a powerful analytical tool for the direct isotopic measurements of low deproteinised plasma volume avoiding consequently preliminary treatment such as extraction or derivatisation. The presented method could be of great interest for real time [13C]SCFA plasma determination of in metabolic in vivo studies in small animal models.