Cyclodextrin-modified microemulsion electrokinetic chromatography for separation of α-, γ-, δ-tocopherol and α-tocopherol acetate

Different forms of tocopherols, together with tocotrienols, are collectively named as vitamin E, and each possesses different degree of medical, biological and physiochemical significance. The main difficulty of separating different forms of tocopherols lay in their highly structural similarities and hydrophobicities. Microemulsion electrokinetic chromatography (MEEKC), claimed to attain high peak efficiency with great solubilization power, has not previously been applied to the separation of tocopherols. The effects that various parameters, such as buffer system, type and concentration of cyclodextrins, temperature, and sample matrix, have on the separation of tocopherols by MEEKC have been investigated. By using a buffer mixture of 4% (w/w) sodium dodecyl sulfate (SDS), 6.6% (w/w) 1-butanol, 0.8% (w/w) n-octane, 20% (w/w) 2-propanol, 68.6% (w/w) phosphate (25 mM, pH 2.5), and 25 mM heptakis(2,6-di-O-methyl)-β-cyclodextrin (DM-β-CD), the separation of α-, γ-, and δ-tocopherol, α-tocopherol acetate, as well as the antioxidant butylated hydroxytoluene (BHT) at −26 kV, 25 °C was completed within 35 min. The practical potential of the present approach has been further validated by the determination of tocopherols in a vitamin E preparation, with the result of 132.63 (RSD 1.25%), 176.51 (RSD 0.29%), and 64.32 mg (RSD 3.34%) per 500 mg capsule for α-, γ- and δ-tocopherol, respectively.