Spray-coated matrix application, imaging

To date, several reports have been published using MALDI imaging MS on whole-body (rat and mouse) tissue sections. Images of peptides, proteins, drugs, and metabolites have been presented. All of the published reports have used spray coating for application of matrix, but otherwise, they have differed in application and methodology. The earliest detailed report... 

Another approach to matrix application is to robotically deposit small (pL to nL) droplets of matrix on discrete areas of the sample surface. When the droplets are deposited as arrays over the entire tissue surface, mass spectra can be acquired at each matrix spot and reconstructed into an image. In this case, potential analyte delocalization is limited to the area under the matrix spot, typically -100-250 pm with commercially available spotters (e.g., Portrait 630 by Labcyte [23], ChIP by Shimadzu, TM iD by LEAP Technologies). This approach is fundamentally a trade-off between spatial integrity and resolution, because what limits resolution in most cases is the diameter of the laser beam, which is typically much smaller than the diameter of the matrix spot (on the order of 30-50 pm). Samples prepared by spray-coating, either manually or robotically, may be imaged at the diameter of the laser beam, but may not have maintained analyte localization throughout the spray process. 

High-resolution images with MALDI-IMS necessitate uniform coating by the matrix, no redistribution of surface analytes and linearity of MALDI signals after matrix application. In order to produce a homogeneous crystal layer over the tissue surface, spraying multiple coats of matrix across the surface of the tissue is recommended. 

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