Mass Spectrometry Imaging (MSI) at IMSERC
MSI via MALDI-TOF Mass Spectrometry
With acquisition and implementation of the new Bruker Rapiflex tissue typer, the capabilities of mass spectrometry imaging (MSI) via MALDI were greatly expanded. The speed of the Rapiflex is not only about 70x that of our previous imaging platforms, but the spatial resolution is 10x smaller capable of routinely reaching 20 um resolution and even possibly tuned down to 5 or 10 um. This greatly expands IMSERC capabilities within imaging. There is essentially almost no limitation to what samples can be images, as long as it can be (1) sectioned or layered to a thin micron thickness (usually no greater than 20 um), (2) compounds of interest are ionizable via MALDI, and (3) it can fit on a standard microscope glass slide (however a specialized conductive ITO glass slide is used for analysis of the same dimensions). Within IMSERC, we have experience imaging chip-based polymers, microprinted or chemically reacted layers, and a variety of tissue samples. With substrate changes, only changes in sample preparation, matrix type, and matrix deposition are needed.
MSI results can be modeled via two methods:
(1) Targeted image creation: In this method, specific ions’ intensities are plotted in pixels throughout the sample to create an image.
(2) Untargeted image creation: In this method, statistical analysis software is used to classify or group each pixel’s spectrum into bins of significance via a variety of statistical tools. One example is hierarcheral clustering that examines each pixel spectrum and creates spectral similarity trees for image extraction. A real-life example is shown below using a self-assembled monolayer desorption ionization (SAMDI) film created by the Mrksich group for modeling.
In this image, hierarchical clustering analysis has automatically grouped corresponding spectra together correctly to image letters printed on the slide based on their spectral similarity. In this case, specifically, (1) red letters correspond to the binding of 2-mercaptopyrimidine with the self-assembled monolayer producing an ion of m/z 985.15, (2) blue letters correspond to the binding of 2-thiouracil to the self-assembled monolayer producing an ion of m/z 1001.15 and, and (3) yellow blank space corresponds to unbound self-assembled monolayer producing an ion of m/z 873.15. The summed spectra for the entire imaged area is shown below demonstrating the ability of hierarchical clustering (in this instance) to pull out small differences (in the case of the ion of m/z 1001.15) as unique similar features and cluster these pixels together in spatial organization as a possible image of interest without any specific direction in the analysis.