Differentiation of regioisomeric ring-substituted fluorophenethylamines with product ion spectrometry

The electron ionization (EI) of aromatic ring-substituted isomers gives virtual identical mass spectra which seriously affects their analysis. Especially regioisomeric meta- and para-ring-substituted compounds cannot show any ortho-effect reactions making their differentiation by mass spectrometry impossible. Furthermore o-, m- and p-substituted compounds can only be separated insufficiently by chromatography due to their very similar retention that do not allow univocal identification. Product ion mass spectrometry has proved to be a useful tool to differentiate structurally closely related fluorophenethylamines even in the case of the meta- and para-isomers. A series of N-alkylated o-, m- and p-fluoroamphetamines and 1-(4-fluorophenyl)butan-2-amines have been synthesized in microscale and studied by product ion spectrometry. The combination of chemical ionization (CI) and product ion spectrometry of hydrogen fluoride loss ions [M+H−HF]+ allows a univocal differentiation of all studied fluoro-substituted phenethylamines without prior derivatization. This method with submicrogram detection limits provides great advantages for the differentiation between aromatic regioisomeric fluorophenethylamine designer drugs where other methods such as nuclear magnetic resonance (NMR) spectrometry lack sufficient sensitivity or might fail because complex mixtures have to be analyzed.