Comparative investigation of UV, electrochemical and particle beam mass spectrometric detection for the high-performance liquid chromatographic determination of benzoic and cinnamic acids and of their corresponding phenolic acids

Abstract

The capabilities of different detection techniques, UV, controlled-potential coulometry and particle-beam electron-impact mass spectrometry (PB-EI-MS) for the HPLC analysis of phenolic acids were studied; fifteen benzoic and cinnamic acid derivatives were considered. For the electrochemical detector (ED) a reversed-phase LC method was set up, whereas normal-phase partition chromatography, on a CN column, was used for UV and MS. Library-searchable EI mass spectra were obtained using the PB-MS technique with flow-injection analysis. UV detection was performed at 280 nm, whereas measurements with the LC-coulometric system were carried out using a porous graphite electrode. The detector responses were compared in terms of linearity, precision and limits of detection; for this purpose, the mass spectrometer was operated under selected-ion monitoring conditions. A linear dynamic range of at least 10 3 was found for the HPLC method with electrochemical detection, with detection limits ranging from 1 to 5 pg injected; the relative standard deviation (R.S.D.) was typically 0.6–3.0% at the 0.1 ng level ( n=4). Using UV or PB-EI-MS detection, minimum amounts in the 5–50 and 2–5 ng ranges, respectively, could be detected. Calibration curves were linear from the limit of detection to at least 15 μg for most of the analytes detected by UV; the R.S.D. of the peak areas obtained in UV mode ranged from 1.2 to 3.1% at the 500 ng level ( n=4). Non-linear behaviour over the entire amount range studied (from 10 ng to 10 μg) was observed using the LC-PB-MS technique, so that two different calibration fittings at low and high levels were calculated. Precision of the LC-PB-MS system was generally good (R.S.D. between 0.5 and 1.8% at the 100 ng level, n=4) except for caffeic acid (R.S.D. 7.5% at the 50 μg level, n=4).