Column liquid chromatography with diode laser-induced fluorescence detection of carboxylic acids after pre-column derivatisation

Several pre-column approaches to fluorescence derivatisation of carboxylic acids, compatible with diode laser-induced fluorescence (DIO-LIF) detection, were compared. For this purpose red-absorbing labels containing hydrazide, bromoacetamide and iodoacetamide functionalities were studied. The sensitivity of DIO-LIF detection is illustrated by the (3–10) × 10−12 M (75–250 amol) detection limit for various derivatised carboxylic acids. The real-life detection limit for the carboxylic acids is 3 × 10 −8 M, because derivatisation is not quantitative at lower concentrations. Derivatised n-alkylcarboxylic acids and non-steroidal anti-inflammatory drugs are easily separated by column liquid chromatography, despite the attachment of the bulky red-absorbing labels. The determination of the anti-inflammatory drug naproxen in saliva is shown as an example.