Optimised Desorption Electrospray Ionisation Mass Spectrometry Imaging (DESI-MSI) for the Analysis of Proteins/Peptides Directly from Tissue Sections on a Travelling Wave Ion Mobility Q-ToF

Mark W Towers,Tamas Karancsi,Emmanuelle Claude,Steven D Pringle,Emrys A Jones

doi:10.1007/s13361-018-2049-0

Abstract

Desorption electrospray ionisation mass spectrometry imaging (DESI-MSI) is typically known for the ionisation of small molecules such as lipids and metabolites, in singly charged form. Here we present a method that allows the direct detection of proteins and peptides in multiply charged forms directly from tissue sections by DESI. Utilising a heated mass spectrometer inlet capillary, combined with ion mobility separation (IMS), the conditions with regard to solvent composition, nebulising gas flow, and solvent flow rate have been explored and optimised. Without the use of ion mobility separation prior to mass spectrometry analysis, only the most abundant charge series were observed. In addition to the dominant haemoglobin subunit(s) related trend line in the m/z vs drift time (DT) 2D plot, trend lines were found relating to background solvent peaks, residual lipids and, more importantly, small proteins/large peptides of lower abundance. These small proteins/peptides were observed with charge states from 1+ to 12+, the majority of which could only be resolved from the background when using IMS. By extracting charge series from the 2D m/z vs DT plot, a number of proteins could be tentatively assigned by accurate mass. Tissue images were acquired with a pixel size of 150 μm showing a marked improvement in protein image resolution compared to other liquid-based ambient imaging techniques such as liquid extraction surface analysis (LESA) and continuous-flow liquid microjunction surface sampling probe (LMJ-SSP) imaging.Graphical ᅟ

Highlights

Mass spectrometry imaging [1] (MSI) allows for the spatial mapping of a wide range of molecules from a sample surface
The spectra generated for peptides were similar to what would be expected via electrospray analysis, i.e. multiply charged ions were generated by DESI
This stems from issues regarding the desorption of protein related ions directly from the tissue or differences in desorption time scales compared to glass or Teflon [15,16,17,18,19]

Summary

Introduction

Mass spectrometry imaging [1] (MSI) allows for the spatial mapping of a wide range of molecules from a sample surface. Towers et al.: DESI-MSI of Peptides/Proteins from Tissue involves the formation of charged droplets, as the formation of multiply charged ions is thought to exclude gas phaseionisation mechanisms. They reported optimised parameters for the ionisation of peptides from spotted samples; these included a short sprayer tip to surface distance with high gas and liquid flow rates. The ionisation method described to be most applicable for proteins is the Bdroplet pickup^ method [15,16,17,18,19] In this process, an initial spray generates a pool of liquid on the surface to be analysed, into which the analyte molecules are extracted. All other DESI based protein work performed to date being from glass, PTFE or PMMA spotted surfaces [6,7,8,9,10,11]

Methods

Results

Conclusion