Abstract
Saturated branched chain fatty acids (BCFA) are present as complex mixtures in numerous biological samples. The traditional method for structure elucidation, electron ionization (EI) mass spectrometry, sometimes does not unambiguously enable assignment of branching in isomeric BCFA. Zirrolli and Murphy (Zirrolli , J. A. , and R. A. Murphy. 1993. Low-energy tandem mass spectrometry of the molecular ion derived from fatty acid methyl esters: a novel method for analysis of branched-chain fatty acids. J. Am. Soc. Mass Spectrom. 4: 223-229.) showed that the molecular ions of four BCFA methyl ester (BCFAME) yield highly characteristic fragments upon collisional dissociation using a triple quadrupole instrument. Here, we confirm and extend these results by analysis using a tabletop 3-D ion trap for activated molecular ion EI-MS/MS to 30 BCFAME. iso-BCFAME produces a prominent ion (30-100% of base peak) for [M-43] (M-C₃H₇), corresponding to the terminal isopropyl moiety in the original iso-BCFAME. Anteiso-FAME yield prominent ions (20-100% of base peak) corresponding to losses on both side of the methyl branch, [M-29] and [M-57], and tend to produce more prominent m/z 115 peaks corresponding to a cyclization product around the ester. Dimethyl and tetramethyl FAME, with branches separated by at least one methylene group, yield fragment on both sides of the sites of methyl branches that are more than 6 C away from the carboxyl carbon. EI-MS/MS yields uniquely specific ions that enable highly confident structural identification and quantification of BCFAME.
Highlights
Saturated branched chain fatty acids (BCFA) are present as complex mixtures in numerous biological samples
We report here spectra from BCFA methyl ester (BCFAME) from these natural mixtures to compare and contrast spectra, and we present evidence that the technique applies to a broad range of BCFAME
BCFAME were separated on a BPX70 capillary to normal, iso, or anteiso BCFA with internal branching were made by applying rules for interpretation developed for the knowns
Summary
High energy collisionally activated dissociation (CAD) in a tandem time-of-flight (TOF-TOF) mass spectrometer initiated by MALDI has been applied to locate branching in a single BCFAME recently [9]; this method has not been further developed. CAD spectra of the molecular ion yielded a novel series of peaks with intensities depending on CAD energy. All spectra were collected under the identical instrument settings, including collision energy (excitation amplitude) and mass isolation window. In our hands, these conditions provided suitable fragments intensities across all BCFAME without the need to customize parameters. Using vernix and lanolin, which contains some BCFA that are not present in vernix, we performed MS/MS on molecular ions of BCFAME using a tabletop internal ionization ion trap mass spectrometer. We report here spectra from BCFAME from these natural mixtures to compare and contrast spectra, and we present evidence that the technique applies to a broad range of BCFAME
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