Relative quantification in imaging of a peptide on a mouse brain tissue by matrix-assisted laser desorption...

It is well-known that the number of analyte ions generated by matrix-assisted laser desorption ionization (MALDI) is not directly proportional to the analyte concentration at the irradiated spot. This is an obstacle to acquiring quantitatively meaningful maps for materials in a tissue by MALDI imaging. The problem worsens as the matrix suppression due to contaminants in the sample increases. In this work, we use a peptide as an example and show that we can overcome this problem by utilizing three guidelines derived from our recent studies on the generation of reproducible MALDI spectra. First is to acquire MALDI spectra under a temperature-controlled condition. Second is to keep the matrix suppression below an experimentally determined limit, and the third is to construct the image map using the peptide-to-matrix ion abundance ratio rather than the peptide ion abundance. The strategy works well for contaminated tissue samples and generates quantitatively meaningful maps. Also, it is demonstrated that a preposterous map can be generated when the peptide ion abundance is used in the construction of the map.