Individual volatile fatty acids determination by chromogenic derivatization coupled to multi-syringe chromatography

In this paper, a new multisyringe chromatography (MSC) system is proposed for a simple and accurate measurement of individual volatile fatty acids (VFA) in anaerobic treatment processes. The determination method is based on the derivatization of VFA with N-(1-naphthyl) ethylenediamine (EDAN) followed by the separation of VFA derivatives on an Onyx C18 monolithic column (25 mm×4.6 mm i.d.). Chromatographic separation conditions have been investigated and were found to be optimal with a mixture of acetonitrile and formic acid 0.1% (ratio 35/65), providing good separation of C2–C5 VFA in 8 min. Optimization of the derivatization reaction was also carried out with special attention paid to the buffering capacity of the reaction medium, so as to be able to deal with samples of various characteristics in terms of alkalinity or of VFA concentration range. Individual VFA could be quantified between 0.05–2.5 g L−1 with LOD of 0.01–0.02 g L−1. Overall procedure time was about 18 min for one analytical cycle, which fulfils the requirement of real-time monitoring of an anaerobic digester. Validation of the system developed has been assessed by application of the procedure to sludge samples from various origins, and comparative results with gas chromatography analyses showed satisfactory correlation (R²>0.98).