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Desorption Electrospray Ionization (DESI) coupled to a high resolution mass spectrometer: fundamental studies and improvements of analytical performance

Final Report Summary - HIGHRESDESI (Desorption electrospray ionisation (DESI) coupled to a high resolution mass spectrometer: fundamental studies and improvements of analytical performance)

Introduction

The objective of this project was to improve analytical performance of Desorption electrospray ionisation (DESI) coupled to a high resolution Fourier transform mass spectrometer (FT-MS) and gain more insight into the DESI mechanism and underlying processes. DESI surface imaging ion source is not currently commercially available for high-end FT-ICR instruments and its construction and optimisation for biological samples was one of the important parts of the project. DESI is a novel ionisation method that combines features of Electrospray ionisation (ESI) with those of the family of Desorption ionisation (DI) methods. The DESI experiment allows molecules present on surfaces to be analysed by Mass spectrometry (MS) without requiring the sample to be introduced into the vacuum system of the MS. DESI differentiates itself from other DI methods in MS because the sample can be analysed outside the MS in an open laboratory environment.

DESI mechanism

An important part of the project was the investigation of the DESI mechanism. Electron scanning microscopy was used to visualise surfaces that underwent DESI process with ferromagnetic nanoparticles solvated in DESI mobile phase mixture. It seems that nanoparticles adopt certain pattern on the surface that is a direct result of the force equilibrium on the surface. This provides a new view into the DESI mechanism.

DESI imaging on commercial FT-ICR-MS

There are two novel aspects of the DESI imaging coupled to the 9.4T FT-ICR-MS. Ones is the utilisation of such ultrahigh resolution with ambient ionisation. The other novel aspect is in the integration of the atmospheric pressure ionisation imaging into the existing software for Matrix-assisted laser desorption / ionisation (MALDI) imaging, which allows the user of this commercial dual source MS to perform MALDI-MS and different ambient MS imaging from the same user interface and to utilise the same software tools. DESI and Desorption atmospheric pressure photoionisation (DAPPI) were used to test the ambient surface imaging capabilities of this new ionisation platform. Results of DESI imaging experiments performed on brain tissue sections were in agreement with the previous MS imaging reports obtained by DESI imaging but due to the high resolution and mass accuracy of the FT-ICR instrument it was possible to resolve several ions at the same nominal mass in the DESI-MS spectra of brain tissue. These isobaric interferences at low resolution are due to the overlap of ions from different lipid classes with different biological relevance.

Nanoassisted laser desorption-ionisation (NALDI) studies

One of the often investigated aspects of ambient ionisation is the influence of the surface that functions as a substrate for the sample. It was previously reported that commercially available Nanostructure-assisted laser desorption / ionisation (NALDI) for ...surfaces are advantageous as substrates for DESI ionisation as well. Further investigation of this phenomena resulted in the discovery of NALDI imprinting imaging technique. This novel imaging technique has several advantages compare to standard MALDI and DESI imaging. The work of the researcher during the second part of the project revealed that NALDI surfaces can also be used for a simple determination of a double bond position by a laser DI MS. The procedure is based solely on the catalytic properties of nanostructured NALDI surfaces. These surfaces can induce oxidation of analytes, which results in a mass shift that can be detected by MS after the analyte is ionised and transported into the MS.

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