Flow-gated capillary electrophoresis (CE) takes the format of the single-cross microchip by using a flow gate and a silica capillary for rapid sample injection and separations (Ref. Zhang and Gong, Journal of Chromatography A 2014, 1324, 231-237). The obvious advantages of flow-gated CE over its counterpart microchip include replaceable capillaries, fresh separation buffer, and compatibility with various detectors such as mass spectrometry and a sheath flow cuvette for laser-induced fluorescence. However, electrokinetic injection is the monopoly until recently a hydrodynamic injection method was introduced by Opekar and Tuma in the article published in Journal of Chromatography A (2017, 1480, 93-98).

Their flow-gated CE system used a syringe pump to deliver sample solutions to the flow gate, and a gravity pump was used to provide gating flow and separation buffer. For sample injection, the gating flow was temporarily stopped and the sample filled up the capillary gap; and then a vacuum was a vacuum was applied to an airtight reservoir that contained buffer waste. This under-pressure sucked sample solution into the separation capillary. The injection volume was well controlled by setting up appropriate vacuum and a time interval. After sample injection, the gating flow was resumed and the separation was carried out.

This hydrodynamic injection method is valuable for the wide applications of flow-gated CE. Specifically, it will reduce injection bias commonly associated with electrokinetic injection; and it may facilitate sample stacking or other sample pre-concentration strategies that welcome hydrodynamic injections.