Mass Transport in a Micro Flow-Through Vial of a Junction-at-the-Tip Capillary Electrophoresis-Mass Spectrometry Interface

When coupling capillary electrophoresis with postcolumn detection methods, such as mass spectrometry, the presence of postcolumn band broadening must be considered. The band broadening effects introduced by junction-at-the-tip CE-MS interfaces using a postcolumn micro flow-through vial are investigated by studying the hydrodynamic flow patterns and mass transport process inside the micro vial at the end of the CE separation capillary. Simulation results obtained by solving the Navier-Stokes and mass balance equations provide insights into the velocity field and concentration distribution of the analytes in the micro vial and demonstrate that, with a low flow rate of chemical modifier solution, the laminar flow streams confine the analyte molecules to the central part of the micro vial and thus maintain major features of the peak shapes. Peaks detected by UV and MS under similar experimental conditions were compared to verify the numerical prediction that the main features of the UV peak can be retained in the MS peak Experiments also show that band broadening can be minimized when an appropriate chemical modifier flow rate is selected.