Enantiomeric separation and quantification of ephedrine-type alkaloids in herbal materials by comprehensive two-dimensional gas chromatography

Abstract

The separation of ephedrine-type alkaloids and their enantiomers in raw herbs and commercial herbal products was investigated by carrying out enantioselective separation in the first-dimension column (containing β-cyclodextrin as the chiral selector) of a comprehensive two-dimensional gas chromatography (GC × GC) system, whereas a polar polyethylene glycol capillary column was used for separation in the second dimension. Naturally occurring ephedrine-type alkaloids and their synthetic analogues (enantiomeric counterparts) were adequately resolved from each other, as well as from potential interference species in the sample matrix using GC × GC, whereas single column GC analysis was unable to separate all the alkaloids of interest. Detection limits in the order of 0.1–1.3 μg/mL and linearity of calibration with R 2 ≥ 0.999 over approximately the range of 0.5–100 μg/mL for the quantitative determination of various ephedrine-type alkaloids were obtained. The commercial herbal products tested contained mostly (−)-ephedrine, (+)-pseudoephedrine, (−)- N-methylephedrine and (−)-norephedrine, with concentrations in the range of 40–2100, 0–1300, 15–300 and 0–30 μg/g of the product, respectively, and repeatability of analysis was generally in the range of 1–5%. The present GC × GC method is effective and useful for the determination of the dosage levels of the principle ephedrine-type alkaloids in commercial health supplements and complex raw herb formulations, as well the differentiation of ephedrine-containing products that were derived from natural plant or synthetic sources, e.g., simply by visualizing the presence or absence of the enantiomeric pairs of (±) ephedrine and (±)- N-methylephedrine in the GC × GC chromatograms.