A sonic spray interface for capillary electrophoresis/mass spectrometry

Abstract

We have developed a sonic spray ionization interface that enables coupling of capillary electrophoresis (CE) and mass spectrometry (MS). In sonic spray ionization, a sample solution is sprayed at any solution-flow rate from a sample-introduction capillary with a high-speed gas flow coaxial to the capillary, and ions are formed at atmospheric pressure. Therefore, it is used with a wide range of buffer solutions regardless of the conductivity of the solutions. However, the pressure around the tip of the sample-introduction capillary is reduced by the high-speed gas flow, so the solution is pumped into the capillary at a flow rate above 0.1 µL/min due to the difference of pressure between the two ends of the capillary. Since the solution-flow rate in CE is much lower than this pumping rate, the resolution of CE separation is expected to be decreased by the pumping effect when an electrophoresis capillary is connected directly to the sample-introduction capillary. To avoid this in our CE/MS interface, we added a buffer reservoir between the sample-introduction capillary of the ion source and the electrophoresis capillary. We confirmed that this prevents the solution in the electrophoresis capillary being pumped by the pressure difference. Furthermore, we have demonstrated analysis by CE/MS with a mobile-phase buffer containing 15 mM of phosphate by filling the buffer reservoir with an acetic-acid solution as a substitute for the mobile-phase buffer. This increased the ion intensity 100-fold by enhancing the evaporation of charged droplets produced by the spray. Copyright © 1999 John Wiley & Sons, Ltd.