Characterization of electron ionization mass spectral (EIMS) fragmentation patterns of chloropropanol esters of palmitic acid using isotope labeling technique

Abstract

Chloropropanol (CP) esters are a class of thermally-induced toxicants that are mainly formed in refined edible oils. The structural diversity of these esters presents significant analytical challenges which have often been overcome through analysis of their corresponding free alcohols after a hydrolysis step. Mass spectrometry-based methodologies incorporating characteristic fragmentation patterns of particular isomers of CP esters greatly facilitates their identification. The electron ionization mass spectra (EIMS) of various isomers of synthetic and commercially available 13C- and 2H-labeled CP ester standards of palmitic (C16) and other short chain fatty acids (C3 to C10) were generated and analyzed using GC/MS. Short chain CP esters were synthesized by reacting their respective acid anhydrides with the corresponding 3-chloro- and 2-chloro- propanediols in addition to 1,3-dichloro- and 1,2-dichloropropanols. Five fragmentation pathways were identified. Four of the five pathways, such as α-cleavage, McLafferty rearrangement, α-H rearrangement and cyclic acyloxonium ion formation, were characteristic of CP mono- and diesters. The remaining pathway generating chloronium ion was found only in dichlorinated isomers. The proposed fragmentation pathways for the palmitic acid esters were confirmed through the use of 13C- and 2H-labeled CP ester standards of palmitic acid, and the generality of identified fragmentation patterns was confirmed through the identification of equivalent ions in the mass spectra of short chain fatty acids (C3 to C16). Characteristic ions that were identified in this study retaining the chlorine atom in their structures can be considered as potential markers for the presence of CP esters.