Separation of isomers of dienoic acids by electromigration techniques

Different capillary electromigration techniques were employed to resolve geometrical isomers of sorbic acid, decadienoic acid, and ethyl sorbate. Since these substances differ in their polarity, shape, and size, various electromigration approaches were investigated to separate the four geometrical isomers of each compound. With capillary electrophoresis (CE) modified with a cyclodextrin (beta-CD) the four isomers of sorbic acid were separated using a buffer that consists of 60 mM tetraborate and 8 mg/mL beta-CD. The separation of decadienoic acid geometrical isomers was not possible, even at elevated tetraborate and cyclodextrin concentrations. The four isomers of decadienoic acid were successfully separated using micellar electrokinetic chromatography (MEKC) with a buffer consisting of 30 mM tetraborate and 100 mM SDS and microemulsion electrokinetic chromatography (MEEKC). Ethyl sorbate is the least polar of all the studied substances and its isomers could not be separated by MEKC or MEEKC. The resolution was improved and isomers were fully separated using capillary electrochromatography (CEC) with ODS stationary phase and a mobile phase consisting of 10 mM boric acid in 50% acetonitrile. Minor differences in the polarity and the shape of isomers and high resolving power of the applied techniques were sufficient for separation of very similar compounds. We have shown that versatile electromigration techniques can be applied for separation of geometrical isomers of dienoic acids and their esters.