A sample introduction method based on negative pressure in flow injection-capillary electrophoresis system and its application to the alkaline-earth metal cation separation

A new sample introduction method based on negative pressure has been developed in the flow injection (FI) - capillary electrophoresis (CE) system with contactless conductivity detection (CCD). In the system, the sample was carried to the interface by background electrolyte (BGE), which was carried by a dual-channel peristaltic pump. Sample introduction into capillary was performed through the negative pressure at the outlet of capillary. Duration of the injection was controlled by a precise homemade time circuit connected to the voltage supply of vacuum pump. When the sample was transported to the inlet of capillary, the vacuum pump was started and a small volume sample was injected within 1.0 - 2.0 s. Negative pressures from 20 to 25 kPa are appropriate and other important injection parameters were studied. The RSD values ( n = 8) of the negatively pressurized injections, within the range of 0.8 - 1.1% for migration time, are comparable with those of gravity injection. The performance of the whole system was further demonstrated with satisfactory results in the separation of four alkaline - earth metal cations ( Mg2+, Ca2+, Sr2+, Ba2+) with CCD.