Alex Mikulich Final.pdf

Use of laser desorption ionization (LDI) for mass spectrometry requires careful deposition of the analyte onto the laser target. Specifically, performing quantitative abundance measurements of biomarkers or abiotic organic materials (with or without matrices promoting ionization of unfragmented ions) requires reduction of variations in the mass spectrum intensities resulting from spatially inhomogeneous targets. Additionally, coupling of LDI to liquid based separation methods (e.g. chromatography or electrophoretic) can be done by continuously spotting the concentrated eluate onto a moving substrate and, later, extracting retention times of these analytes by examining the spatial variation of the composition of the deposit. Uncontrolled wetting of the substrate leads to poor definition of retention time while deposition of well-defined spots improve calculated retention time. Direct deposition of eluate using a capillary or pipette, the dried droplet method, is sensitive to the substrate surface roughness, substrate-solution interaction, and gravity. In particular, the formation of a ring during drying is found to occur regardless of surface chemistry and results in the so-called “coffee stain” effect. For these reasons, airbrushing through stencil masks presents an attractive method for improving both the spatial homogeneity of the deposition and reducing the spot size of the deposited analyte over the simpler dried-droplet approach. Initially, the researchers tried using a micropipette to deposit small amounts of rhodamine onto a stainless steel plate. Then, several variables were altered, including solute concentration, solvent type, plate temperature, and the delivery method. It was found that if the solvent were quickly evaporated after the droplets hit the surface, the solute molecules did not have enough time to migrate to the edges. Thus, isopropanol as a solvent helped reduce coffee staining. This was further improved by heating the sample plate to a temperature between 100 and 150 degrees Celsius. The tiny droplets that were atomized with the airbrush vaporized on contact, and thus a homogeneous deposition could be achieved. The use of an airbrush also allowed for smaller spot sizes. Even with a 0.5 microliter droplet from the pipette, the spot size was still several millimeters in diameter. An airbrush, coupled with a stencil, was able to give clearly defined spots measuring as low as 200 microns. Therefore, when performing mass spectrometry with LDI, airbrushing the solvent onto a hot plate through a stencil can offer significant improvement over a dried-droplet method.