MALDI imaging is the use of matrix-assisted laser desorption ionization as a mass spectrometry imaging technique in which the sample, often a thin tissue section, is moved in two dimensions while the mass spectrum is recorded.
- The peptides are mixed with a matrix solution on a plate. This matrix is a small organic molecule that has strong absorbance at the laser wavelength and is highly sublimable.
- A laser beam is introduced to the plate so that ions are produced at every pulse. This is advantageous as it produces shot-to-shot reproducibility.
- The ionic peptides travel in the tube towards the detector. The peptides that have lower molecular weight will travel faster and vice versa and will be detected by the detector at the end of the tube.
MALDI imaging mass spectrometry involves the visualization of the spatial distribution of proteins, peptides, drug candidate compounds and their metabolites, biomarkers or other chemicals within thin slices of sample such as animal tissue or plant. It is a promising tool for putative biomarker characterisation and drug development. Initially scientists take tissue slices mounted on microscope slides and apply a suitable MALDI matrix to the tissue, either manually or now automatically. Next, the microscope slide is inserted into a MALDI mass spectrometer. The mass spectrometer records the spatial distribution of molecular species such as peptides, proteins or small molecules. Suitable image processing software can be used to import data from the mass spectrometer to allow visualisation and comparison with the optical image of the sample. Recent work has also demonstrated the capacity to create three-dimensional molecular images using the MALDI imaging technology and co-registration of these image volumes to other imaging modalities such as magnetic resonance imaging (MRI).
Currently, the following vendors in the field of MALDI imaging mass spectrometry offer commercial instruments:
There are some free software packages available for visualization and mining of MALDI imaging data, as Biomap (Novartis), Datacube Explorer (AMOLF), EasyMSI, Mirion, MSiReader (free, NCSU). In addition the following commercial software suites for MALDI imaging are available: Quantinetix, MALDIVision, SCiLS Lab
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