4-Bromomethyl-6,7-dimethoxycoumarin as a fluorescent label for carboxylic acids in chromatographic detection

Abstract

The UV absorbance, corrected fluorescence spectra, quantum yields and lifetimes of 4-bromomethyl-6,7-dimethoxycoumarin derivatives of carboxylic acids as well as of a few fatty esters are presented. The fluorescence quantum yields in methanol are not affected by the number of carbon atoms (C 2C 5) of the carboxylic acid. Among various solvents, water gives the highest quantum yield (0.64), whereas in non-hydrogen-bonding solvents the yields are less than 0.23. The yield from the C 2 derivative rises rapidly from 0.43 to 0.60 as 20% water is added to methanol, and from 0.23 to 0.61 if 50% water is added to acetonitrile. The fluorescence spectra and quantum yields are only slightly affected by the pH, ionic strength and the nature of electrolytes used in the mobile phase. The results show that fatty acid esters of 4-hydroxymethyl-6,7-dimethoxycoumarin have higher quantum yields and intrinsic fluorescence sensitivities than homologous 7-methoxycoumarin derivatives. Furthermore they offer a better possibility for gradient elution chromatography. Derivatization of fatty acids with 4-bromomethyl-6,7-dimethoxycoumarin is a very sensitive method for the evaluation of picomole amounts by liquid chromatography using fluorescence detection.