Precolumn fluorescence tagging reagent for carboxylic acids in high-performance liquid chromatography: 4-substituted-7-aminoalkylamino-2,1,3-benzoxadiazoles

Abstract

Four new 2,1,3-benzoxadiazole amine reagents having different functional groups at the 4- and 7-positions, [4-nitro-7-N-pipe-razino-2,1,3-benzoxadiazole (NBD-PZ), 4-(N,N-dimethylaminosulphonyl)-7-N-piperazino-2,1,3-benzoxadiazole (DBD-PZ), 4-(N,N-dimethylaminosulphonyl)-7-N-cadaverino-2,1,3-benzoxadiazole (DBD-CD) and ammonium 7-N-piperazino-2,1,3-benzoxadiazole-4-sulphonate (SBD-PZ)] were synthesized as fluorogenic tagging reagents for carboxylic acids in high-performance liquid chromatography. The reagents, except SBD-PZ, reacted with carboxylic acid at room temperature in the presence of activation agents to produce fluorescent adducts. The maximum wavelengths of arachidic acid tagged with DBD-PZ, DBD-CD and NBD-PZ were 569 nm (excitation, 440 nm), 561 nm (excitation, 437 nm) and 541 nm (excitation, 470 nm), respectively. Among various activation agents tested [diethyl phosphorocyanidate (DEPC), diphenyl phosphoroyl azide (DPPA), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC)-pyridine, 2,2′-di pyridyl disulphide-triphenylphosphine (Mukaiyama A) and 2-chloro-1-methylpyridinium iodide-triethylamine (Mukaiyama B)], DEPC and Mukaiyama A were more effective than the others. When the piperazino reagents (DBD-PZ and NBD-PZ) were used as the tagging reagents, the derivatization reaction in the presence of Mukaiyama A was faster than that in the presence of DEPC. Although the reaction in the presence of Mukaiyama A was completed after 30 min, an unknown peak derived from the activation agent appeared on the chromatograms. The fluorescence peak intensities were compared in the presence of DEPC. The order of the fluorescence peak areas obtained after reaction for 6 h in the presence of DEPC was DBD-PZ > DBD-CD > NBD-PZ. Thirteen saturated free fatty acids (FFAS) derivatized with DBD-PZ (or DBD-CD) and DEPC (or Mukaiyama A) in acetonitrile were separated completely by linear gradient elution on a reversed-phase ODS column. Eight drugs (ibuprofen, indomethacin, dinoprost, prostaglandin E 1, dehydrocholic acid, ursodesoxycholic acid, hydrocartisone succinate and prednisolone succinate) were also tagged with DBD-PZ in the presence of DEPC and separated by isocratic elution. The detection limits (signal-to-noise ratio = 3) of FFAs tagged with DBD-PZ were in the range 3.2–4.7 fmol, whereas those of drugs were in the range 3.9–14 fmol.