Derivatization of carboxylic acids with fluorescent reagents

This paper reports on the derivatization of carboxylic acids with fluoresce nt reagents after separation by thin-layer chromatography on normal- (silic a gel) and reversed-phase plates. Two of the most promising reagents were c ompared to determine which resulted in the highest sensitivity. Derivatization with 4-bromomethyl-7-methoxycoumarin (BrMmC) was performed i n the presence of 18-crown-6 as catalyst. The highly fluorescent esters cou ld be detected by fluorodensitometry at an excitation wavelength of 360 nm with a cut off filter for emission. To improve the results the thin-layer p lates were spray-coated with a solution of 10% poly(ethylene glycol) 4000 i n chloroform. Derivatization by esterification with panacyl bromide (p-(9-anthroyloxy)phe nacyl bromide, PAN), with the same crown ether as catalyst, was also invest igated. Because this reagent furnished more intensely fluorescent products, the reaction conditions were further optimized by use of palmitic, propion ic, and arachidonic acids as model substances. The limit of detection was i n the lower picomole range.