Imaging with MALDI-MS

In addition to frozen tissue sections, tissue blotting and laser-capture microdisection methodologies can also be used. In the former method, the proteins are transferred by blotting freshly dissected tissues onto an active Cig-coated surface. Another surface of choice for blotting is a carbon-filled polyethylene membrane. In the laser-capture microdisection method, a specific population of cells from a stained tissue is transferred onto an ethylene-vinyl acetate transfer film. A matrix is applied to the isolated cells through a narrow capillary. 

Application of a matrix to tissue sections is a critical step in MALDI imaging. As mentioned above, the target analytes should not change their in vivo position and should not spread or smear during use of the MALDI matrix. The following procedures have been developed to achieve a uniform coating of a matrix  

Airspray coating, in which the matrix solution is sprayed over the sample with a pneumatic airbrush. Several coatings are applied and the sample is dried between applications. 

In electrospray deposition, the matrix solution is electrosprayed through a needle that is attached to a two-dimensional plotter. The needle is moved across the sample in a regular pattern. This procedure provides better control of the amount of the matrix deposited and yields a homogeneous coating. 

In the sliding-drop method, a large volume of matrix is deposited along a side of the tissue sample and is distributed over the tissue using a thin spatula. In this procedure there is a possibility of migration of analytes from their in vivo position. 

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