Adaptation of desorption electrospray ionization to a transmission mode (TM-DESI) entails passing an electrospray plume through a sample that has been deposited onto a mesh substrate. A combination of mass spectrometry and fluorescence microscopy studies is used to illustrate the critical role material composition, mesh open space, and mesh fiber diameter play on the transmission, desorption, and ionization process. Substrates with open spaces less than 150 m and accompanying minimal strand diameters produce less scattering of the plume and therefore favor transmission. Larger strand diameters typically encompass larger open spaces, but the increase in the surface area of the strand increases plume scattering as well as solvent and analyte spreading on the mesh. Polypropylene (PP), ethylene tetrafluoroethylene (ETFE), and polyetheretherketone (PEEK) materials afford much better desorption than similarly sized polyethylene terephthalate (PETE) or nylon-6,6 (PA66) substrates. Ultimately, the manner in which the electrospray plume interacts with the mesh as it is transmitted through the substrate is shown to be critical to performing and optimizing TM-DESI analyses. In addition, evidence is presented for analyte dependent variations in the desorption mechanisms of dry and solvated samples. . In DESI, ions are produced by directing charged solvent droplets from an electrospray source toward a sample that is either a bulk material in its native state (e.g., pharmaceutical tablet) or one that has been deposited from solution onto a sampling surface. Analytes present at the surface are desorbed and ionized by the incoming plume and subsequently transferred to the mass spectrometer inlet by the influence of the applied potential and the pressure differential between atmospheric pressure and the low-pressure region of the mass analyzer. To date, DESI has found many applications including forensic analysis [2][3][4][5] Recent adaptations of DESI, including geometry independent DESI in gas tight enclosures [35] and transmission mode desorption electrospray ionization (TM-DESI), have been developed to reduce the geometry dependence of DESI experiments [36]. In the transmission DESI mode the sample is not deposited onto a continuous solid surface but rather onto a sampling mesh. In this adaptation, the incident spray angle and collection angle are fixed at 0°and the spray is transmitted through the sample (Figure 1). Along with the simplification of the experimental geometry, the transmission mode also allows convenient analysis of both dry (i.e., following evaporation of the deposition solvent) and wet (i.e., solvated) samples with similar performance characteristics to those achieved using traditional DESI [36].Surface variables including the chemical composition, porosity, texture, and electrical conductivity of the substrate have been reported to affect DESI analyses [2,24,[37][38][39]. Dramatic reductions in response have been noted for high conductivity surfaces due to neutralization of the incoming ion plume at the su...