Cyclic Acyloxonium Ions as Diagnostic Aids in the Characterization of Chloropropanol Esters under Electron Impact (EI), Electrospray Ionization (ESI), and Atmospheric Pressure Chemical Ionization (APCI) Conditions

Abstract

During mass spectrometric analysis of various lipids and lipid derivatives such as the chlorinated counterparts of triacylglycerols, the detailed structure of the characteristic and common ions formed under electron impact (EI), electrospray ionization (ESI), and atmospheric pressure chemical ionization (APCI) conditions by the loss of a single fatty acid remains ambiguous. These ions are designated in the literature as "diacylglyceride ions" and are frequently depicted with a molecular formula without showing any structural features and sometimes represented as cyclic acyloxonium ions. Characterization of these ions is of considerable importance due to their utility in structural identification of lipid derivatives. This study provides complementary evidence on the cyclic nature of "diacylglyceride ions" through the use of the simplest 3-monochloropropanediol diester as a model and the use of isotope labeling technique. Tandem MS/MS studies have indicated that the ion at m/z 135.6 generated from 1,2-bis(acetoyl)-3-chloropropane through the loss of an acetyl group was identical to the ion at m/z 135.6 generated from 4-chloromethyl-2,2-dimethyl-1,3-dioxolane, the latter being generated from a cyclic precursor through the loss of a methyl radical, keeping the dioxolane ring structure intact, thus confirming the cyclic nature of these ions. The corresponding cyclic oxonium ions generated from longer chain chloropropanol diesters, such as the ion at m/z 331.2 originating from 3-monochloropropanediol (3-MCPD) diesters containing palmitic acid(s), could serve as chemical markers for the presence chloropropanol esters.

Characterization of Electron Ionization Mass Spectral (EIMS) Fragmentation Patterns of Chloropropanol Esters of Palmitic Acid Using Isotope Labeling Technique

“…Ions in series M can undergo pathway c to produce mainly the cyclic acyloxonium ion at m/z 135.53 through the displacement of the ester group at sn-1 position. The loss of the ester group at sn-1 positions 15 rather than at sn-2 supports a formation mechanism 12 involving the preferential displacement of the ester group from the less hindered position through an S N 2 mechanism. Furthermore, these studies have also demonstrated the following leaving group priority under electron ionization conditions; esters halides alcohols.…”

“…The same ion series has also been observed under various soft ionization conditions 1,12 and has been referred to in the literature as diacylglyceride ions . This ion series could have great diagnostic utility during the analysis of CP esters 12 . Furthermore, isotope labeling studies 14 using CP acetates R CH 3 have confirmed the elemental composition of this ion.…”

“…Table 2 lists the selected commercial standards used to verify the retention times and EI mass spectra of the synthesized CP palmitates. As in the case of CP acetates 12 , isotope labeling was used to confirm the proposed fragmentation pathways of CP palmitates.…”

“…tation pathways of CP palmitates using isotope labeling approach The previously studied 12 mass spectral fragmentations of CP acetates were used as a guide to predict the general behavior of CP palmitates and other esters under electron ionization conditions. This study allowed the identification of characteristic fragmentation patterns of the CP esters which can generate the most abundant peaks in their mass spectra.…”

“…Increased commercial availability of CP ester standards has led to the resurgence of interest in direct ed 3-CP dipalmitate, as an acyclic protonated fragment, whereas, MacMahon, Begley, and Diachenko 7 using a similar approach and identical precursor characterized the ion at m/z 331 as a cyclic acyloxonium ion generated upon elimination of a fatty acid group. Only recently, Rahn and Yaylayan 12 established not only the cyclic nature of these ions M-RCO 2 but also their intrinsic cationic character. Furthermore, the use of a single ion series such as M-RCO 2 is not sufficient for proper characterization of CP esters, the need to properly establish the structures of all specific ions originating from various isomers of CP esters is becoming increasingly important especially for the development of direct methods of detection.…”