Construction of a Desorption Electrospray Ionization Source

Desorption ElectroSpray Ionization is an ionization technique for mass spectrometers invented in 2004. It involves directing an electrospray onto a surface containing an ionizable substance. Impacting charged particles from the electrospray produce gaseous ions of the substance on the surface. The presence of the sample on the surface at atmospheric pressure with little preparation allows for innovative new applications, none of which are viable with conventional ESI. These applications stem from DESI’s ability to ionize a trace (tens of ng/mm^2) sample directly from a surface with little preparation or other constraints such as pressure . In vivo sampling of tissue surfaces is an example of an application of DESI. Our project was to build an apparatus that allowed us to use DESI as an input source for a mass spectrometer. The first thing we did was to construct our DESI emitter. The electrospray is formed from solvent flowing through a fused silica tubing threaded through a tee-piece. Nebulizing gas is connected to the tee piece at a 90 degree angle to the solvent flow. The gas flows between the fused silica capillary and a larger, concentric piece of PEEK tubing leading up to the electrospray point. The DESI emitter was held by an insulated clamp on a stage that can be translated in two dimensions. DESI spectra of four compounds, including monensin, leucine enkephalin, rhodamine and reserpine, were obtained using this set up. Conditions for optimum DESI performance were also found.