Analysis of fatty acid methyl esters with high accuracy and reliability. V. Validation of theoretical relative response factors of unsaturated esters in the flame lonization detector

Abstract Because unsaturated fatty acid methyl esters (FAME) are subject to autoxidation, it is virtually impossible to obtain and maintain high purity standards. Accordingly, it is not possible to determine flame ionization detector response factors by the usual technique of analyzing standard mixtures of known composition. In an alternative approach, the response factors of methyl oleate, methyl linoleate, methyl linolenate, methyl arachidonate and methyl 4,7,10,13,16,19‐docosahexaenoate relative to methyl stearate were estimated by determining the peak areas before and after quantitative hydrogenation in the presence of an internal standard. The estimates showed excellent agreement in all cases with the theoretical factors predicted by Ackman and Sipos and thus constitute an independent and unambiguous proof that the theoretical factors are highly accurate for all olefinic unsaturated FAME. Whereas it is common practice to determine an empirically derived correction factor for each FAME by analyzing standard mixtures of known composition, the thesis is now proposed that, for both saturated and olefinic unsaturated FAME, the proper approach to accurate analysis requires that peak areas be corrected using the theoretical response factors as the only correction factors. If the correct result cannot be obtained when analyzing a primary standard of saturated FAME, it is an indication of faulty technique or equipment, and the only acceptable resolution of the problem is to locate and correct the fault(s).