NMR observation revealed that bidirectional conversion occurred between 3-monochloropropane-1,2-diol (3-MCPD) and glycidol in the course of the analytical procedure of DFG standard method C-III 18 (09), option A; 3-MCPD was partly converted to glycidol at the transesterification step, and glycidol was converted partly to 3-MCPD at the derivatization step conducted at 80 °C under acidic condition in the presence of NaCl. Based on the proton numbers observed by 1H NMR, the degrees of the conversion were estimated to be 37 and >70%, respectively. In addition, epoxide ring-opening of glycidol and its esters was found to be ca. 90% by the acid treatment described in the method, option B. Thus, it was concluded that the standard method, option A, did not correctly give the combined amount of 3-MCPD esters and glycidyl esters in oils containing glycidyl esters, and the difference of the values obtained by options A and B did not correspond to the amount of glycidyl esters, either. In addition, derivatives of 3-MCPD with phenylboronic acid were not observed by NMR at the derivatization step, although they were detected by GC-MS in the organic phase at the following extraction step.
3-Monochloropropane-1,2-diol (3-MCPD)DFG standard methodGlycidolGlycidyl esterNaCl