Lipid separation and structural characterization using travelling wave cyclic ion mobility

Abstract

The analysis and structural characterization of lipids remain challenging due to the chemical structure diversity and isobaric nature. In recent years, liquid chromatography coupled to ion mobility-mass spectrometry (LC-IM-MS) for lipidomics has shown advantages in lipid identification. In particular, collision cross section (CCS) obtained from the IM measurements represents a physical property that can be used to enhance the confidence of lipid identification. Data were collected on a hybrid quadrupole cyclic IM (cIM) orthogonal acceleration time-of-flight instrument. It provides the option to perform either a single pass, or multiple passes until the desired resolution is achieved. MS and CID fragmentation data were obtained on precursor IM separated lipids followed by TOF mass measurement. Using the advanced travelling WAVE technology, a portion of the IMS separation can be selected and stored in a Pre-Array trap region. The stored ions can be re-injected to enable ion mobility analysis and by repeating this IMS to the “n” experiments can be performed. Ion mobility provides additional separation dimension that allows the separation of isobaric and isomeric compounds. The separation and structural characterization of different lipid classes using cIM is currently under study. Different lipid classes with positional isomer (Sn1/Sn2 vs Sn2/Sn1), different double bond positions, cis and trans isomers, glucosyl and galactosyl ceramide isomers, PIP and ganglioside isomers were investigated. Some of the isomers were baseline separated only after 1 pass (approximately at 65 IMS resolution) and others with 50 passes (approximately at 450 IMS resolution). In summary, cyclic IMS provides novel, scalable ion mobility resolution and the increased resolution is useful to resolve and separate isobaric and isomeric lipids species. Advanced modes of operation with ion activation followed by ion mobility separation offers new insights into lipid structural characterization.