Quantification of eicosapentaenoic and docosahexaenoic acid geometrical isomers formed during fish oil deodorization by gas–liquid chromatography

Long-chain polyunsaturated fatty acids (LC-PUFAs) of the n-3 series and especially eicosapentaenoic and docosahexaenoic acids (EPA and DHA, respectively) have important biological properties. The main dietary sources of LC-PUFAs are fish and fish oil. Geometrical isomerization is one of the main reactions happening during the thermal treatment of polyunsaturated fatty acids. Refined fish oils are used to supplement food products in LC-PUFAs and the quality of these nutritional ingredients have to be controlled. In the present study, a suitable method for the quantification of EPA and DHA geometrical isomers in fish oils by gas–liquid chromatography (GC) is presented. A highly polar capillary column (CP-Sil 88, 100 m) operating under optimal conditions was used. Method selectivity was studied by GC–mass spectrometry. The performance characteristics of the quantification method were studied using samples of fish oil deodorized at 220 °C for 3 h. The linearity of the method was assessed by analyzing composite samples obtained by mixing fish oil deodorized at 220 °C with semi-refined fish oil (control). Precision was evaluated by analyzing the same samples in triplicate. Results showed that the validated method is suitable to quantify low amounts of geometrical (trans) isomers of EPA and DHA in refined fish oils. The limits of quantification of the EPA and DHA geometrical isomers are 0.16 and 0.56 g/100 g of fish oil, for EPA and DHA, respectively. Commercially available LC-PUFA oil samples were evaluated by using the validated method. The results show that the oils analyzed contain low amounts (<1% of total fatty acids) of geometrical isomers of EPA and DHA.