The development of a hydrodynamic flow assisted double junction interface for signal improvement in capillary electrophoresis–mass spectrometry using positively charged nonvolatile additives

• A hydrodynamic flow assisted double junction CE/MS interface was proposed.
• Hydrodynamic flow was used to tune the direction of analytes and additives.
• Only the positively charged analytes flow toward the ESI source.
• Ion suppression due to the positively charged additives can be overcome.

To alleviate signal suppression resulting from nonvolatile positive ion additives, a hydrodynamic flow assisted double junction capillary electrophoresis–mass spectrometry (CE–MS) interface was proposed. The double junction interface which could alleviate the ion suppression due to nonvolatile negative ion additives was modified so that a hydrodynamic flow could be introduced to the interface. Using this setup, the apparent velocity of the ions was determined based on its electrophoretic mobility, electroosmotic flow in the transfer capillary, and hydrodynamic flow introduced by the syringe pump. CE–MS analysis of positively charged triazines was performed to demonstrate the practical value of this approach by using cetyltrimethylammonium ion (CTA+) as an additive. Because the separation column was dynamically coated with CTA+, the EOF was reversed and the separation was performed under counter EOF mode. Under an appropriate hydrodynamic flow, the analytes (triazines) could be propagated toward the MS, whereas the additive (CTA+) ion was retained in the interface. Consequently, the problem of signal suppression by CTA+ was alleviated, and the signals were enhanced more than 20-fold.