Fig. 1 shows the representative chromatograms of the standard mixture including the nine types of fatty acids and the IS (Fig. 1a) or the human serum sample (Fig. 1b). Fatty acids in human serum were derivatized to their FAMEs and subsequently analyzed by GC–MS. The methyl ester derivatization of fatty acids and hexane extraction were performed according to procedures described by Wang et al. [7] with minor modifications, with the exception of the acid catalyst; while Wang et al. used 10% H2SO4 in MeOH as the catalyst [7], we failed to esterify the IS using this agent (i.e., the methyl-esterified IS could not be detected). On the other hand, methyl esterification using p-toluenesulfonic acid (PTSA) in MeOH has been reported in a previous study for the preparation of bile acid methyl esters [24]. Moreover, PTSA has been reported to be a useful acid catalyst [24] because it is less damaging to the reactants than mineral acids, such as HCl or H2SO4. Additionally, a recent study by Carmona et al. [25] revealed that PTSA yielded the highest conversion of free fatty acids in sunflower oil deodorizer distillate to methyl esters. Thus, in the present study, we used 10mM PTSA in MeOH for acid-catalyzed esterification instead of 10% H2SO4 in MeOH.