Determination of ephedrine, pseudoephedrine, and norephedrine in mixtures (bulk and dosage forms) by proton nuclear magnetic resonance spectroscopy.

A simple, specific, and accurate 1H nuclear magnetic resonance (NMR) spectroscopic method has been developed for quantitative determination of the Ephedra alkaloids (-)-ephedrine, (+)-pseudoephedrine, and (+/-)-norephedrine, either singly or in mixtures with each other. Determination of individual alkaloids was carried out in D2O solution, with acetamide as internal standard. Although calculations were based on integrals for the C-CH3 protons, those for the N-CH3 and -CH-O- protons may also be useful, depending on the compound. Determination of diastereomeric cross-contamination of ephedrine and pseudoephedrine--or of the concentrations of these alkaloids in the presence or absence of (+/-)-norephedrine--was feasible by using the integrals for the -CH-O- protons after addition of a trace of DCl. Mean recoveries for ephedrine and pseudoephedrine from their respective synthetic mixtures with the internal standard (acet- amide) were > or = 99.9 +/- 0.6% (n = 10) and 99.6 +/- 0.8% (n = 10) of the amount added. Recovery for pseudoephedrine from diastereomeric mixtures with ephedrine was > 99.4 +/- 0.7% (n = 10) of the amount added, with as little as 1.92% still being measurable. Mean recovery of (+/-)-norephedrine from mixtures with ephedrine and pseudoephedrine was > 99.7 +/- 2.5% (n = 4) of the amount added, with about 1% still being measurable. Application of the proposed NMR spectroscopic method to commercial dosage forms, including ephedrine sulfate injections and pseudoephedrine hydrochloride tablets, yielded assay results ranging from 97.8 to 100.2% (mean, 99.2%) and from 98.7 to 100.5% (mean, 99.7%) of declared, respectively.