Abstract
This thesis investigates the possibility of using ion beam analysis (IBA) with mass spectrometry imaging (MSI) techniques for multimodal elemental and molecular imaging on single sample. This was found to be possible through the careful consideration of suitable substrates to mount the samples which would compensate for the different requirements with using IBA and MSI techniques. Appropriate workflows were also investigated to determine the effects of one analysis to the corresponding information obtained from the subsequent analysis.
In this thesis, three techniques were used. The elemental imaging tool was particle induced X-ray emission (PIXE) which is a widely used IBA technique for mapping elements. The two MSI techniques tested here were matrix assisted laser desorption ionisation (MALDI) and desorption electrospray ionisation (DESI).
Suitable substrates to mount tissue homogenates were investigated for the compatibility with IBA and MSI techniques. Investigated substrates included standard glass slides (used for MSI), polyethylene (PET) films, carbon film and highly ordered pyrolytic graphite (HOPG). For PIXE, the PET substrate was the most compatible due to the thin nature of the film (4 µm) and had low elemental impurities compared with the other investigated substrates. PET was also found to be compatible with DESI, where the intensity of molecular endogenous compounds and maps were comparable to standard glass slides. For MALDI analysis, the carbon foil and HOPG were more suited. PET was not compatible with the laser, causing holes to form in the substrate.
Using the chosen candidate substrates for each technique, appropriate workflows for IBA with MSI on a single sample were investigated. Two workflows were adopted: ‘Workflow 1’ where the MSI technique (DESI or MALDI) was carried out prior to IBA, the other was ‘Workflow 2’ where IBA irradiations were carried out prior to the MSI technique. For Workflow 1, redistribution of elements in the tissue homogenate due to the MSI analysis was explored. It was found that whilst MALDI does not appear to have an adverse impact on trace element distribution, DESI redistributed Cl and K, but not Fe. This work explored whether prior PIXE analysis (involving 2.5 MeV proton beam irradiation (at low, medium and high fluence)) caused changes to the observed molecular compounds. The results presented here show irradiating the sample at medium and high fluence with the ion beam leads to some compounds (such as lipids) to change in intensity. It was found that beam damage can be reduced by application of a matrix prior to ion beam analysis. Finally, it was found that ion beam induced chemical changes are also less pronounced on the carbon foil substrate. This thesis concludes that this is due to the higher thermal conductivity of the substrate, and therefore that beam heating plays a role in the observed damage to tissue.