Junior professor for Food Safety and Quality Friedrich-alexander-Universität Erlangen-Nürnberg Erlangen, Germany
Abstract: Lipids in food and other biological matrices are often present as a highly complex fraction. This structural complexity is due to the different lipid classes (such as sterol lipids or acyl lipids) and the vast number of structurally different fatty acid chains. Accordingly, despite constant improvements in chromatographic and mass spectrometry methods, the comprehensive analysis and confident structural identification of lipids still poses significant challenges. A promising addition to the analytical toolbox is ion mobility spectrometry (IMS), which, coupled to chromatography and mass spectrometry, adds an additional analytical dimension that can be used for lipid annotation. In addition to this, IMS can be effectively used for the separation of isobaric lipid species such as cis/trans fatty acids isomers or 1,2-and 1,3-diacylglycerols.
Here, we show the potential and benefit of IMS for the structural analysis of triacylglycerols and their epoxy derivatives. For this purpose, positional isomer pairs of triacylglycerols were synthesized from 1,2-dipalmitate and 1,3-dipalmitate through a Steglich esterification with fatty acids with 16 to 20 carbon atoms and 0 to 5 double bonds. Epoxy-modified triacylglycerols were synthesized in analogous manner using epoxy fatty acids, created from unsaturated fatty acids through the Prilezhaev reaction with 3-chloroperbencoic acid. The lipids were analysed using a combined trapped ion mobility spectrometry/high resolution time of flight mass spectrometer (timsTOF MS) with positive electrospray ionisation, both using direct infusion and hyphenated to ultrahigh performance liquid chromatography. Using slow ramp times, maximizing resolution for the ion mobility, small differences of ca 1% in collision cross sections (2-3 Å2) could be detected and the isomers were partially resolved. We subsequently applied this for the detailed structural analysis of plant extracts and oxidized fish oils high in oxidized lipids, highlighting the value of ion mobility spectrometry in combination with chromatography and mass spectrometry in lipid analysis
