Multiply-charged Ion Formation by Sonic Spray

In sonic spray ionization technique, a solution from a fused-silica capillary is sprayed with a sonic gas flow coaxial to the capillary. The current for the ions produced and for the charged droplets is found to be dramatically enhanced when a voltage is applied to a surrounding metal piece, which is isolated from the solution by the fused-silica capillary. This can be ascribed to the induced concentration difference between positive and negative ions in the solution near the surface. Furthermore, multiply-protonated molecules produced in this way from protein solutions are analyzed with a quadrupole mass spectrometer. The ion intensity shows a strong dependence on the voltage but the diameter of the droplet produced by the spray is likely to be independent of the voltage. Thus, we conclude that the charge density of the droplet is regulated by the voltage.