Sensitivity Improvement of Nonvolatile Buffer Systems in Sheath Flow Rate-controlled Capillary Electrophoresis / Electrospray Ionization Mass Spectrometry

In CE analysis, nonvolatile buffer is often utilized to enhance resolution. When it hyphenates to mass spectrometer, nonvolatile salt will compete charge against analyte, and suppress the signal of analyte. Diluting the buffer by sheath liquid can lower the salt concentration and alleviate ion suppression, but the analyte is also diluted. In this study, a sheath flow rate-controlled CE/MS interface was developed. By controlling the flow rate of sheath liquid, we found the best dilution ratio of sheath liquid which affects the dilution of salt and analyte to improve the sensitivity of CE/MS using nonvolatile buffer. The dilution ratio of sheath liquid in ADS micelle and ammonium phosphate buffer systems was investigated. We found the dilution ratio that corresponds to the best analyte intensity in EOF infusion experiments, and the results were similar to the real CE/MS experiments. In addition, we studied the factors that cause the difference of analyte intensity between direct infusion and EOF infusion under the same sheath liquid dilute ratio. With the comparison of the experiment of sheath liquid interface infusion by syringe pump, the most influential factor was confirmed. In EOF infusion, the velocities of ADS micelle and phosphate group are lower than the EOF velocity because of their negative charge. Therefore, the concentrations of the solution entering to the MS are lower than the concentration of bulk solution.