Direct injection of edible oils as microemulsions in a micellar mobile phase applied to the liquid chromatographic determination of synthetic antioxidants

Abstract

A simple and quick procedure for analysis of hydrophobic samples by direct injection in a liquid chromatograph, without previous extraction, has been developed. The sample is solved in a water/sodium dodecyl sulphate/ n-pentanol microemulsion without destroying the microemulsion structure, and injected. A micellar mobile phase containing 0.1 M SDS, 2.5% n-propanol and 10 mM phosphate of pH 3 is used. The procedure is applied to the determination of synthetic antioxidants (propyl gallate, tert-butylhydroquinone, 2,4,5-trihydroxybutyrophenone, nordihydroguaiaretic acid, octyl gallate, 3- tert-butyl-4-hydroxyanisole and dodecyl gallate) in sunflower, corn and olive oils. Linear calibration curves ( r>0.9999) with the same sensitivities are obtained by injecting the antioxidants as micellar solutions and as oil containing microemulsions. Sensitivities are also independent from the contents and nature of the oil in the microemulsion. By injecting 20 μl microemulsion containing 5% oil, limits of detection range from 2.3 to 5.9 ng; this range corresponds to concentrations which are well below the values permitted in the US and European countries. Repeatabilities at a level of 10 μg antioxidant per gram of microemulsion range from 1.1% to 2.5%.