Characterization of the fluorescence properties of 4-fluoro-7-nitrobenzo-2-oxa-1,3-diazole derivatives of some primary and secondary sympathomimetic amines

In this study, 4-fluoro-7-nitrobenzo-2-oxa-1,3-diazole (NBD-F) was chosen as a fluorogenic reagent to derivatize methamphetamine and its metabolites to be detected by laser-induced fluorescence as its wavelength of maximum excitation (λex) is close to the argon-ion laser emission at 488 nm. In an early stage of this study, it was clearly observed that the fluorescence intensities of the secondary amine derivatives were <4% of that of primary amines. Accordingly, authentic NBD derivatives of 1-phenylethylamine, para-methoxyamphetamine, amphetamine, norephedrine, ephedrine and methamphetamine were synthesized and purified, and gave satisfactory mass spectrometric and elemental analyses. The electronic absorption and fluorescence spectra were studied in different organic solvents and buffered solutions. Bathochromic shifts of the absorption band maxima and increase in the molar absorptivity were observed as the solvent became more polar. All the compounds showed red shifts in the wavelength of maximum emission (λem) and decrease in the fluorescence intensities with increase in solvent polarity. The fluorescence intensities of primary and secondary amine derivatives were high and comparable in chloroform and extremely low in acetic acid. In pyridine, only the primary amine derivatives show high fluorescence intensity. The λex and λem of NBD derivatives were insensitive to changes in pH over the 2–9 range. Except for methamphetamine and p-methoxyamphetamine, the fluorescence intensities of all compounds decreased significantly at pH >9.