Molecular ion fragmentation and its effects on mass isotopomer abundances of fatty acid methyl esters ionized by electron impact

We have analyzed the isotopomer abundance ratios of an equimolar mixture of nine fatty acid methyl esters (decanoate, undecanoate, laurate, tridecanoa te, myristate, pentadecanoate, palmitate, heptadecanoate, and stearate) by selected-ion monitoring gas chromatography/electron impact/mass spectrometr y (GC/EI/MS). The abundance of the second lowest m/z isotopomer (I-M1) incr eased disproportionately compared with the abundance of the lowest mit isot opomer (I-M0) as a function of: (1) increasing sample size; (2) decreasing repeller voltage; and (3) decreasing alkyl chain length. We also compared t he abundance of the third lowest mit isotopomer (I-M2) and the abundance of the second lowest m/z isotopomer (I,1) of methyl palmitate and [4,4-H-2(2) ]methyl palmitate. We observed that the I (M2) / I-M1 for methyl palmitate was significantly lower than I-M2/I-M1 for [4,4-H-2(2)]methyl palmitate. Fr om these results, as well as a consideration of basic principles of ion che mistry and ion physics, we conclude that gas-phase chemistry, specifically proton (or deuteron) transfer from fragment ions to molecules, is a major c ontributor to the sample size dependence observed in mass isotopomer abunda nce measurements of fatty acid methyl esters ionized by EI. Our results and analysis do not support hydrogen abstraction as the reaction mechanism. In addition, we calculate that rearranged molecular ions are unlikely to cont ribute significantly to intermolecular proton transfer because of their rel atively brief lifetime. We also discuss alternative analytical techniques w hich might improve the precision and accuracy of isotopomer measurements by reducing molecular ion fragmentation. (C) 1999 American Society for Mass S pectrometry.