Advantages of High Resolution Ion Mobility on the Cyclic IMS for DESI Mass Spectrometry Imaging

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Susan Slade, Emmanuelle Claude
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DESI is a powerful and sensitive MS ionisation technique for the profiling and imaging of metabolites and lipids direct from unmodified complex biological samples. However, the direct analysis of small molecules can be challenging due to the structural diversity and isobaric nature of these types of compounds. Ion mobility separation (IMS) has proven to enhance system peak capacity, improve specificity and separate structural isomers.

Here we describe the implementation of DESI mass spectrometry imaging (MSI) where ion mobility separation was improved using a multi-pass cyclic IM travelling-wave device where the number of passes around the device can be scaled to improve resolution.

Data were acquired using a SELECT SERIES Cyclic IMS instrument (Q-cIM-oa TOF) mounted with either an ESI source for the IM optimization of the bile acid conjugate standard and DESI XS source (Waters, UK) for imaging experiments and the identification experiments.

Initial DESI imaging experiment was carried out at 100 x 100 microns pixel size on mouse brain tissue section with the cIM performing a single pass of ions from m/z 50-1,200 in positive mode. In a second experiment, the cIM conditions were optimised to allow the maximum of passes for the separation of phospholipids m/z 750-850 which was 7 passes. Interrogation of the resulting data highlighted numerous examples where the power of the increased ion mobility separation enabled partial or baseline separation of isobaric species and showed  different and distinct localisation in the mouse brain.

Finally, we have investigated the separation of truly isobaric bile acid conjugates such as TDCA and TCDCA. Standards were analysed in electrospray in IMS-MS and IMS-MS/MS to evaluate their separation. Further experiments were performed by DESI imaging directly on liver tissue section. With 13 passes, at least two peaks could be observed in the mobilogram and specific ion images were generated.

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