Matrix assisted laser desorption ionization (MALDI) mass spectrometry imaging (MSI) coupled with ion mobility separation has demonstrated significant utility over the last decade for the separation of matrix ion species from endogenous ion species in the gas phase. Since its introduction ten years ago, desorption electrospray ionization (DESI) has been gathering momentum as a complementary MSI technique to the more traditional SIMS and MALDI-MS approaches, proving especially beneficial for the analysis of metabolites/ lipids localization in tissue. In this study, we compare and contrast DESI imaging with MALDI imaging on the same ion mobility enabled mass spectrometer, with variety of samples. We demonstrate that additional classes of molecules are ionized by DESI which are clearly defined using ion mobility.

Different tissue samples including mouse brain sections and human tumor sections, have been analyzed using the same mass spectrometer by both MALDI and DESI.  By keeping the parameters for the ion mobility and mass analyzer constant between the different techniques, the ion distribution overlap could be studied in detail. One advantage of MALDI imaging using an ion mobility enabled MS is ability to differentiate clustered matrix peaks from tissue derived analytes (e.g. lipids) as two distinct nested trendlines are observed in the m/z vs drift time plot. As DESI does not require a matrix compound for the ionization of molecules on the surface of tissue section, it could be expected that DESI MS spectra and 2D-plot m/z vs drift time would be cleaner. However, the DESI spectra show similar strong lipid peaks as observed in MALDI, but also intense fatty acid species were detected at the lower mass range. Closer inspection of the ion mobility dimension revealed further trendlines in the 2D-plot , corresponding to either different classes of molecules or different charge states of ions, present at much lower abundance.Additional information highlighted by the use of the ion mobility separation when DESI imaging was compared to MALDI imaging.