Desorption electrospray ionisation (DESI) provides a useful alternative to matrix assisted laser desorption ionisation (MALDI) for mass spectrometry imaging (MSI) with the ambient nature of the technique increasing the scope of samples that can be analysed whilst reducing the sample preparation requirements.  Furthermore DESI offers the possibility to generate multiply charged ions for biomolecules like peptides and proteins directly from a surface. Here we  investigate sample analysis parameters and conditions to optimise the ionisation/desorption of biomolecules directly from glass surface and tissue sections.

Analyses were performed on a SYNAPT G2-Si Q-TOF-IMS-MS,positive MS mode with integrated tri-wave ion guide optics used to separate ions by ion mobility in the gas phase. The DESI (Prosolia) stage was mounted directly using an electrospray inlet.

Initial optimisation and experiments were performed on single peptides/proteins. Extracted proteins were also trypically digested  spotted onto a Teflon coated glass slide. Various  DESI solvent compositions were tested using methanol, Water, acetonitrile, formic acid. The MS spectra produced were in large comparable with those generated via a supporting ESI-MS analyses. However, it was noted that a number of  peaks observed in the ESI spectrum were absent for the DESI data.  In this case acetonitrile was found to be marginally better as the organic component than methanol. Furthermore the spray solution containing 0.2% formic acid was found to improve ionisation.  In the case of the tryptic digest samples, peptide species with charge states of up to 4+ and 5+ were detected and were baseline separated using ion mobility. 

Direct analysis of larger m/z molecules directly from tissue has shown to be more difficult because of the other endogenous molecules like lipids present within the tissue that ionise preferentially compare to larger endogenous molecules.  Subsequent to these analyses, the focus turned to digested tissues. Trypsin , was reconstituted in 50 mM ammonium bicarbonate, 0.5% Octyl β-D-glucopyranoside with concentration of 20 µg/mL.