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-pot ential coulometry and particle-beam electron-impact mass spectrometry (PB-EI-MS) for the HPLC analysis of phenolic acids were studied; fifte en benzoic and cinnamic acid derivatives were considered. For the elec trochemical detector (ED) a reversed-phase LC method was set up, where as 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 perfor med at 280 nm, whereas measurements with the LC-coulometric system wer e carried out using a porous graphite electrode. The detector response s were compared in terms of linearity, precision and limits of detecti on; for this purpose, the mass spectrometer was operated under selecte d-ion monitoring conditions. A linear dynamic range of at least 10(3) was found for the HPLC method with electrochemical detection, with det ection limits ranging from 1 to 5 pg injected; the relative standard d eviation (R.S.D.) was typically 0.6-3.0% at the 0.1 ng level (n=4). Us ing UV or PB-EI-MS detection, minimum amounts in the 5-50 and 2-5 ng r anges, respectively, could be detected. Calibration curves were linear from the limit of detection to at least 15 mu g for most of the analy tes detected by UV; the R.S.D. of the peak areas obtained in UV mode r anged 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 obse rved using the LC-PB-MS technique, so that two different calibration f ittings 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 leve l, n=4).