Sample preparation is a critical step in imaging spectroscopy. Scientists take thin tissue slices mounted on conductive microscope slides and apply a suitable MALDI matrix to the tissue, either manually or automatically. Next, the
microscope slide is inserted into a MALDI
mass spectrometer. The mass spectrometer records the spatial distribution of molecular species such as peptides, proteins or small molecules. Suitable image processing software can be used to import data from the mass spectrometer to allow visualization and comparison with the optical image of the sample. Recent work has also demonstrated the capacity to create three-dimensional molecular images using MALDI imaging technology and comparison of these image volumes to other imaging modalities such as
magnetic resonance imaging (
MRI).
Tissue preparation The tissue samples must be preserved quickly in order to reduce molecular degradation. The first step is to freeze the sample by wrapping the sample then submerging it in a cryogenic solution. Once frozen, the samples can be stored below -80 °C for up to a year. The mounted tissue section thickness varies depending on the tissue. Tissue sections can then be thaw-mounted by placing the sample on the surface of a conductive slide that is of the same temperature, and then slowly warmed from below. Washing with varying grades of ethanol removes lipids in tissues that have a high lipid concentration with little delocalization and maintains the integrity of the peptide spatial arrangement within the sample.
Matrix application The matrix must absorb at the laser wavelength and ionize the
analyte. Matrix selection and solvent system relies heavily upon the analyte class desired in imaging. The analyte must be soluble in the solvent in order to mix and recrystallize the matrix. The matrix must have a
homogeneous coating in order to increase sensitivity, intensity, and shot-to-shot reproducibility. Minimal solvent is used when applying the matrix in order to avoid delocalization. One technique is spraying. The matrix is sprayed, as very small droplets, onto the surface of the sample, allowed to dry, and re-coated until there is enough matrix to analyze the sample. The matrix is placed in a sublimation chamber with the mounted tissue sample inverted above it. Spatial resolution highly impacts the molecular information gained from analysis. == Applications ==