Abstract
An interface for capillary electrophoresis/mass spectrometry (CE/MS) using sonic spray ionization (SSI) was developed. In the SSI technique, 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 can be used with a wide range of buffer solutions regardless of the conductivity of the solutions. SSI is now being used as an interface in semi-micro and conventional liquid chromatography/mass spectrometry (LC/MS). 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 with the sample-introduction capillary. To prevent this in CE/MS, the author has set a buffer reservoir between the sample-injection capillary and the electrophoresis capillary. Using this interface, the author has demonstrated CE/MS analysis 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.
