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 (RSD) 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 RSD of the peak areas obtained in UV mode ranged from 1.2 to 3.1% at the 500 ng level (n=4). Non-linear behavior 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 (RSD between 0.5 and 1.8% at the 100 ng level, n=4) except for caffeic acid (RSD 7.5% at the 50 µg level, n=4).

Abstract: Aliphatic hydrocarbons (C7 to C36), PAH and nitro-PAH were analyzed using a HP 1050 instrument coupled to a Model 5989 A quadrupole MS via a particle beam interface. Samples were applied to a 10 µm LiChrosorb Si-60 column (25 cm x 4.6 mm i.d.). Isocratic elution with heptane was carried out for 5 min followed by a linear gradient of 0-50% THF in heptane over 30 min with a final hold for 5 min. He was the nebulizing gas at 241 kPa and 70°C. For nitro-PAH the best results were obtained by negative-ion CI detection with CH4 as reagent gas. In flow injection experiments maximum ion intensities were obtained in THF/heptane (3:7); detection limits were 1-700 pg nitro-PAH. Calibration graphs were linear over two orders of magnitude.