Imaging Mass Spectrometry is a technology that combines advanced analytical techniques for the analysis of biological molecules with spatial fidelity. An effective approach for imaging biological specimens in this way utilizes Matrix-Assisted Laser Desorption Ionization Mass Spectrometry (MALDI MS). Briefly, molecules of interest are embedded in an organic matrix compound that assists in the desorption and ionization of compounds on irradiation with a UV laser. The mass-to-charge ratio of the ions are measured using a mass spectrometer over an ordered array of ablated spots. Multiple analytes are measured simultaneously, capturing a representation or profile of the biological state of the molecules in that sample at a specific location on the tissue surface.
Figure 1: Principles of MALDI-TOF Mass Spectrometry. a | Schematic outline of a typical separation of analytes in a linear matrix-assisted laser desorption ionization (MALDI)-time-of-flight (TOF) mass spectrometer based on their mass to charge ratio (m/z). b | Typical mass spectrum in the mass range between 2 and 20 kDa. c | A magnification of the mass range between 5 and 8 kDa. Nature Reviews Cancer 10, 639-646 (September 2010) | doi:10.1038/nrc2917
Generating an image using MALDI MS is accomplished by the systematic analysis of the entire sample area of interest. The laser is raster scanned across the tissue in order to collect molecular information from a regularly spaced array of positions. The molecular information encoded at each location is extracted and plotted to create ion images that can be directly correlated with the location of specific biological molecules.
Figure 2: MALDI Imaging Mass Spectrometry. Schematic outline of a typical workflow for fresh frozen tissue samples. Sample pretreatment steps include cutting and mounting the tissue section on a conductive target. Matrix is applied in an ordered array across the tissue section and mass spectra are generated at each x,y coordinate for protein analysis or tandem MS (MS/MS) spectra for protein identification. Further analytical steps include the visualization of the distribution of a single protein within the tissue (protein image) or statistical analysis to visualize classification images as well as database searching to identify the protein. The scale represents the relative intensity of the protein. IMS, imaging mass spectrometry; MALDI, matrix-assisted laser desorption ionization; MS, mass spectrometry; m/z, mass to charge ratio. Nature Reviews Cancer 10, 639-646 (September 2010) | doi:10.1038/nrc2917
To learn more about Imaging Mass Spectrometry, please refer to some of our recent review articles.