Analysis of vitamin E isomers in seeds and nuts with and without coupled hydrolysis by liquid chromatography and coulometric detection

A method is described for the determination of vitamin E isomers [α-, (β+γ)- and δ-tocopherols] in seeds and nuts by reversed-phase HPLC with coulometric detection. Three methods of sample treatment were compared. The first method included alkaline hydrolysis, extraction of analytes from unsaponifiable and injection into the chromatographic system. In the second method, alkaline hydrolysis and later continuous membrane extraction of isomers were coupled with the HPLC system. The third method involved direct extraction of the analytes through a silicone membrane coupled on-line with the chromatographic system. The three methodologies used for the determination of vitamin E isomers in these samples afforded good results for α-tocopherol. However for (β+γ)- and δ-tocopherols the best results were obtained with the third method. The method without hydrolysis was the simplest one to carry out and analysis took no longer than 40 min from weighing of the samples. Accordingly, it is the method proposed.