Separation and quantification of the triacylglycerols in evening primrose and borage oils by reversed-phase high-performance liquid chromatography

Abstract

Evening primrose and borage oil are used frequently in nutritional and clinical studies where an impaired Δ 6-desaturase enzyme activity may be bypassed by supplementation with γ-linolenic acid (GLA, 18:3 n − 6). The separation and quantification of the triglycerides of borage oil and evening primrose oil has been carried out using reversed-phase HPLC with UV detection. Borage oil was found to have 34 UV-detectable fractions and evening primrose 22. The TG fractions were collected manually, their fatty acid composition determined and quantified with an internal standard. The probable identify of the individual TG fractions was deduced using the fatty acid composition of the TG fractions, calculated theoretical carbon numbers (TCN) for the various TG species and the predicted probability of occurrence. Correction factors, U i, for GLA (18:3 n − 6), gadoleic acid (20:1 n − 9), erucic acid (22:1 n − 9) and nervonic acid (24:1 n − 9) were estimated to be 0.3–0.4, 0.6, 0.4 and 0.3, respectively, and are used along with other known U i correction factors for unsaturated fatty acids to calculate TCN values for all the TG species. These U i values represent the loss in affinity of the unsaturated fatty acid for the reversed-phase C-18 stationary phase. The reversed-phase HPLC trace of borage oil is much more complex compared to evening primrose oil. Apart from differences in the total fatty acid composition there are substantial differences in the quantity of individual TG species present in the two oils. The clinically important fatty acid, γ-linolenic acid, is distributed much more widely throughout the TG species of borage oil compared to evening primrose oil. Over 90% of the GLA present in evening primrose occurs in the first 9 eluting TG species whereas only about 65% is found in these TG species of borage oil.