Imaging of Surface Charge and the Mechanism of Desorption Electrospray Ionization Mass Spectrometry

Distributions of charge deposited on surfaces in desorption electrospray ionization mass spectrometry (DESI-MS) were investigated using a static charge measurement apparatus, which gives an output voltage proportional to the local surface charge density. By scanning the probe along the surface and taking measurements at fixed intervals, a contour image of relative charge density reflecting the charge distribution on the surface can be plotted. Through the measured charge distribution and the derived charge density gradient, the motion of charged droplets in the DESI experiment can be inferred. Measurements taken under various DESI operating conditions, including spray pressure, angle, flow rate, and sprayer tip-to-surface distance, show that charge is spread over an area of a few square centimeters under typical conditions; effective desorption occurs from a much smaller area (∼1 mm2) of highest charge density. Higher sheath gas pressures and smaller sprayer tip-to-surface distances lead to concentration of charge distribution into a smaller area, whereas smaller spray angles favor charge distribution over a larger area. The appearance of the highest charge density in front of the DESI sprayer tip and near the MS inlet suggests that charged droplets are moved toward the MS inlet by pneumatic forces and by the vacuum suction, in agreement with results of earlier simulations. The present observations are consistent with previous studies using other techniques and support the accepted droplet splashing DESI mechanism.