Two-step matrix application for the enhancement and imaging of latent fingermarks

Matrix deposition is a crucial aspect for successful matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI MSI) analysis. The search for more efficient protocols over the years has resulted in the devising of "dry matrix methods" in which the matrix is solely or preliminarily deposited as powder and acts in most cases as a seeding agent. Although not fully embraced by the MALDI MSI community, these methods have proven to be more efficient in terms of ion intensity, ion abundance, and ion images in the experimental circumstances they were employed. Here we report a novel two-step matrix application method, that we have named the "dry-wet" method, where the matrix is dusted onto the sample followed by solvent spray using a robotic device. The new method has been successfully applied to the detection and mapping of several analyte classes within latent fingermarks. Dusting the matrix generated the added advantage of enhancing the latent fingermarks which are invisible. This allows not only for an optical image to be taken of the fingermark in situ but also bridges the gap in the application of MALDI MSI technology in this field; with the use of the methodology reported, fingermark enhancement, recovery, and analysis from different surfaces is now compatible with subsequent MALDI MSI analysis thus allowing visual and chemical information to be obtained simultaneously.