A substrate for use in providing an ionized target comprising a structured substrate has a plurality of recesses, at least a portion of the plurality of recesses containing an initiator, the substrate being capable of having a target loaded on it. In one methods, irradiation of the substrate can cause the initiator to restructure, releasing it from the recesses and thereby desorbing and ionizing the target. The target so desorbed and ionized can be detected by mass analyzers. The mass of the targets at a given point on the surface can be recorded to provide a spatial mapping of the targets on the surface.