Quantitative analysis of plasma caffeic and ferulic acid equivalents by liquid chromatography tandem mass spectrometry

A validated method was developed for the simultaneous determination of the hydroxycinnamates caffeic acid (CA), dihydrocaffeic acid (DHCA), ferulic acid (FA), dihydroferulic acid (DHFA), and isoferulic acid (IFA) in human plasma as metabolites derived from coffee consumption. The method includes a protein precipitation step prior to enzymatic hydrolysis of the conjugated metabolites (sulfate, glucuronide, and/or ester) back to their aglycone forms. After liquid–liquid extraction, the reconstituted extract was analysed by high-performance liquid chromatography coupled to negative electrospray ionisation tandem mass spectrometry. Calibration curves were constructed from spiked human plasma samples in the range of 0–4800 nM for each of the targeted analytes. Two internal standards, 3-(4-hydroxyphenyl)-propionic acid (500 nM) and 1,3-dicaffeoylquinic acid (200 nM), were spiked at the beginning of the sample preparation and before analysis, respectively. Good performance data were obtained with limits of detection and quantification of the five hydroxycinnamates ranging between 1–15 nM and 3–50 nM, respectively. Within and between-days precisions were respectively calculated between 8–18% and 8–30% (at 50 nM added initially), between 6–9% and 6–12% (at 200 nM), and between 5–9% and 5–9% (at 500 nM). Precision calculated from different analysts ranged from 18% to 44% (at 50 nM), from 8% to 16% (at 200 nM), and from 4% to 8% (at 500 nM). Using this method, we determined plasma levels in humans and measured the efficiency of deconjugation using our enzymatic cocktail.