Atmospheric pressure molecular imaging by infrared MALDI mass spectrometry

An atmospheric pressure (AP) MALDI imaging interface was developed for an orthogonal acceleration time-of-flight mass spectrometer and utilized to analyze peptides, carbohydrates, and other small biomolecules using infrared laser excitation. In molecular imaging experiments, the spatial distribution of mock peptide patterns was recovered with a detection limit of approximately 1 fmol/pixel from a variety of MALDI matrixes. With the use of oversampling for the image acquisition, a spatial resolution of 40 microm, 5 times smaller than the laser spot size, was achieved. This approach, however, required that the analyte was largely removed at the point of analysis before the next point was interrogated. Native water in plant tissue was demonstrated to be an efficient natural matrix for AP infrared laser desorption ionization. In soft fruit tissues from bananas, grapes, and strawberries, potassiated ions of the most abundant metabolites, small carbohydrates, and their clusters produced the strongest peaks in the spectra. Molecular imaging of a strawberry skin sample revealed the distribution of the sucrose, glucose/fructose, and citric acid species around the embedded seeds. Infrared AP MALDI mass spectrometric imaging without the addition of an artificial matrix enables the in vivo investigation of small biomolecules and biological processes (e.g., metabolomics) in their natural environment.