Determination of apparent decomposition threshold energies of lithium adducts of acylglycerols using tandem mass spectrometry and a novel derived effective reaction path length approach

Abstract

Apparent decomposition threshold energies for the fragmentation pathways of lithiated acylglycerols were experimentally determined by collisional activation in a quadrupole-hexapole-quadrupole (QhQ) mass spectrometer. A previously developed 'derived effective reaction path length' approach for predicting bond dissociation energies (BDEs) of simple dissociations of electrostatic complexes such as alkali metal adducts (Li+), or halide adducts (Cl(-)) of acylglycerols, was extended to predict covalent bond apparent decomposition threshold energies of lithium adducts of a mono-acylglycerol, a 1,2-diacylglycerol, and a 1,3-diacylglycerol. The ability of the model to treat relatively large ionic systems (e.g. more than 100 atoms) represents a huge advantage of this approach. The model's calculated apparent decomposition threshold energies (Ea) are used in conjunction with the method of energy-resolved mass spectrometry, employing breakdown graphs, to give a more complete quantitative description of the fragmentation processes. Calculated Ea values allowed ranking of the 1,2-diacylglycerol as more reactive than the 1,3-diacylglycerol; the mono-acylglycerol was ranked the least reactive. The method was applied to the low molecular weight product ions generally associated with the hydrocarbon series CnH2n+1+, where two separate pathways are deduced as contributing to the production of the abundant m/z 81 fragment ion. The favored ranking of the neutral losses of fatty acyl substituents for the 1,2-diacylglycerol was determined as: loss of lithium fatty acetate > loss of fatty acid > loss of fatty acyl chain as ketene. For the 1,3-diacylglycerol, the descending order of ease of neutral loss was: loss of fatty acyl ketene > loss of lithium fatty acetate > loss of fatty acid. The results of this study demonstrate that the newly developed method is general in nature, and it can be used for the measurement of covalent bond decomposition threshold energies, as well as for the previously documented electrostatic (noncovalent) bond energies.