O⁻ and OH⁻ chemical ionization of some fatty acid methyl esters and triacylglycerols

Abstract

O(-) and OH(-) react with fatty acid methyl esters (FAMES) under chemical ionization conditions both as Bronsted bases to form [M - H](-) and as nucleophiles to form the carboxylate ion RCOO(-). O(-) shows a much greater tendency to react as a nucleophile than does OH(-). The [M - H](-) ions fragment by elimination of CH(3)OH, with unsaturation in certain positions in the fatty acid hydrocarbon chain promoting this elimination for unknown reasons. The reaction of O(-) and OH(-) with triacylglycerols leads to [M - H](-), characteristic of the molecular mass, and to carboxylate ions characteristic of the fatty acid(s) present in the lipid. The presence of the three ester functions in the lipids greatly enhances the formation of carboxylate ions compared to the FAMES.

Analysis of the seed oil of Heisteria silvanii (Olacaceae)—A rich source of a novel C18 acetylenic fatty acid

“…HTGC/NCI-MS using ammonia as a reagent gas yielded characteristic mass spectra for the TAG with abundant ions at [M − H] − and [RCOO] − (34,35). Base peak for all TAG was the quasi molecular ion [M − H] − .…”

Dissociation mechanisms of neutral methyl stearate and its hydrogen atom adduct formed from the respective positive ions by electron transfer

“…In addition, dissociations of the negative ion have also been reported 14–17. In the negative ion chemical ionization (NICI) mass spectrum, the predominant peaks are associated with [M—H] − , [RCOO] − , and [M—H—CH 3 OH] − ions 14. In the mass‐analyzed ion kinetic energy (MIKE) spectra of [M—H] − ions of FAMEs formed by NICI, the predominant peak was associated with the loss of CH 3 OH 15.…”

“…In EI mass spectra, metastable ion‐decay spectra, and CAD spectra of the molecular ion [M + ·], various peaks associated with the loss of a series of alkyl moieties are observed, and the associated dissociation mechanisms have been reported 8–13. In addition, dissociations of the negative ion have also been reported 14–17. In the negative ion chemical ionization (NICI) mass spectrum, the predominant peaks are associated with [M—H] − , [RCOO] − , and [M—H—CH 3 OH] − ions 14.…”

Identification of triacylglycerols containing two short‐chain fatty acids at sn‐2 and sn‐3 positions from bovine udder by fast atom bombardment tandem mass spectrometry

“…16 In contrast, the importance of TAGs in biological functions and nutrition have been well-known for some time, and therefore, many analytical methods involving mass spectrometry (MS) and tandem mass spectrometry (MS/MS), coupled with a variety of ionization methods, have already been used to determine their structures. [17][18][19][20][21][22][23][24][25][26][27][28] In fact, collision-induced dissociation (CID) of sodiumadducted molecules of glycerolipids including glycoglycerolipids, 29 phospholipids 30 and monoacetyldiglycerides (MADGs), 31 have provided practical and essential information on the regiospecificity of the fatty acyl groups on the glycerol backbone from our previous work. Furthermore, it has offered decisive evidence on the polar head group and fatty acid composition, as well as the double-bond position in the fatty acyl chains.…”