A recommended esterification method for gas chromatographic measurement of conjugated linoleic acid

The effect of the methylation method on isomerization of conjugated linoleic acid (CLA) in gas chromatographic analysis was studied. Among methylation methods examined, the magnitude of isomerization of CLA was greatest with BF3 catalyst, followed by HCl and H2SO4 catalyst. Short-time methylation did not extensively change the CLA composition in ail methods, and c,t and t,c isomers were essentially maintained, while the appearance of t,t isomers and unknown peaks was practically restricted. After 120 min of methylation, there was essentially no conversion in the H2SO4, method, in contrast to a marked change in the BF3 method. The antioxidants butylated hydroxytoluene, ascorbic acid, beta-carotene, and alpha-tocopherol did not suppress conversion, while dimethylsulfoxide (DMSO) and dimethylformamide (DMF) attenuated the changes in CLA composition. Suppression was more effective in the H2SO4 method than in the BF3 method. Thus, methylation with H2SO4 in the presence of a proper amount of DMSO or DMF is recommended for esterification of CLA.