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

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 mu g 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 mu 8) 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. 75% at the 50 mu g level, n=4).