Atmospheric pressure matrix-assisted laser desorption/ionization in transmission geometry

In both atmospheric pressure matrix-assisted laser desorption/ionization (AP MALDI) and vacuum MALDI, the laser typically illuminates the analyte on the front side of an opaque surface (reflection geometry). Another configuration consisting of laser illumination through the sample backside (transmission geometry) has been used in conventional MALDI; however, its use and the number of reports in the literature are limited. The viability of transmission geometry with AP MALDI is demonstrated here. Such a geometry is simple to implement, eliminates the restriction for a metallic sample holder, and allows for the potential analysis of samples on their native transparent surfaces, e.g., cells or tissue sections on slides.