Liquid chromatography–UV determination and liquid chromatography–atmospheric pressure chemical ionization mass spectrometric characterization of sitosterol and stigmasterol in soybean oil

A narrow-bore HPLC–UV method was developed for the analysis of two of the more abundant naturally occurring phytosterols in vegetable oils: sitosterol and stigmasterol. The method enabled detection of the compounds at a concentration of 0.42 μg/ml and quantitation at concentrations of 0.52 and 0.54 μg/ml for sitosterol and stigmasterol, respectively. An excellent linearity was determined over two orders of concentration magnitude (r2 0.999–1.000) and verified by applying the Mandel fitting test (p>0.099) and the lack-of-fit test (p>0.057) performed at the 95% confidence level. A good intra-day precision ranging from 0.15 to 1.16% was calculated at two concentration levels (2 and 100 μg/ml). The inter-day reproducibility was verified on 3 different days by performing an homoscedasticity test and analysis of variance. A solid-phase extraction method was developed on silica cartridges for the isolation of phytosterols from soybean oil providing recovery values of 101±9 and 106±7% for sitosterol and stigmasterol, respectively. Good accuracy of the method was statistically demonstrated since no matrix effect was found for both the analytes. The developed method was applied to the quantitative assay of phytosterols in a soybean oil sample (61±5 mg/100 g of stigmasterol and 118±4 mg/100 g sitosterol). The HPLC–atmospheric pressure chemical ionization MS technique enabled the identification of stigmasterol, sitosterol and campesterol in the oil sample.