Analysis of ephedra‐alkaloids using sweeping and cation‐selective exhaustive injection and sweeping micellar electrokinetic chromatography methods

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Abstract
Two stacking methods of capillary electrophoresis (CE) were developed for the separation of very dilute solutions of ephedra‐alkaloids, namely ephedrine, pseudoephedrine, methylephedrine, methylpseudoephedrine, norephedrine, and norpseudoephedrine. A sweeping method which uses a carrier comprised of phosphoric acid, sodium dodecyl sulfate (SDS), diethylamine and acetonitrile permits the detection of the alkaloids down to the 10−1 μg/mL level, and the cation‐selective exhaustive injection and sweeping micellar electrokinetic chromatography (CSEI‐Sweep‐MEKC) method using phosphoric acid, SDS, and acetronitrile as electrolytes can detect down to the 10−3 μg/mL level. The former requires the conductance of the sample solution to be adjusted beforehand, and only five peaks were observed, two of which were overlapped. The latter is capable of separating the six alkaloids but has a somewhat poorer reproducibility. Using an optimized injection time, it was found that the more diluted a solution is, the greater the sweeping effect will be. The CSEI‐Sweep‐MEKC method with a 600 s injection time and a 10−1 μg/mL solution concentration provides an amplification effect of ∼ 104. The method is suitable for analyses of dilute herb drug extracts and mouse sera. The effect of buffers on the separation and validation of the methods in this study are also discussed.