Direct infusion mass spectrometric analysis of ephedrine and pseudoephedrine

For thousands years Ephedrae herba has been used in traditional Chinese herbal medicine. Its main bioactive constituents are ephedrine and pseudoephedrine. Nowadays, these alkaloids have found application in various clinical treatments, as sports-enhancing drugs and for the illicit production of amphetamine-like drugs. Ephedrine and pseudoephedrine are diastereomers, and because of their high polarity and similar pK_a values, their chromatographic separation may be a challenge. In this study a possible application of direct infusion mass spectrometry and direct infusion tandem mass spectrometry has been proposed for differentiation of ephedrine and pseudoephedrine as well as the relative determination of the contents of these compounds in a mixture. At low collision energy condition, the ratio of relative abundances of ions [M+H–H₂O]⁺ and [M+H]⁺ permit distinction of the analyzed diastereomers, and enable evaluation of their relative content in a mixture with no need of chromatographic separation. The performed quantum chemical calculation have shown that protonated pseudoephedrine contains stronger hydrogen bond than protonated ephedrine, which can justify the observed higher relative abundance of ion [M+H–H₂O]⁺ in the mass spectrum of pseudoephedrine, in comparison to that of ephedrine.