Simultaneous separations of cations and anions by capillary electrophoresis with contactless conductivity detection employing a sequential injection analysis manifold for flexible manipulation of sample plugs

Different schemes for simultaneous separations of both cations and anions were implemented in capillary electrophoresis employing a sequential injection analysis manifold for flexible manipulation of the background electrolyte and samples. The utilization of contactless conductivity allowed sensitive detection in narrow capillaries of 10 μm internal diameter, which in turn enabled the use of hydrodynamic pumping without significant penalty in dispersion otherwise caused by the laminar nature of the flow. This allowed to position sample plugs at a desired place along the entire length of the capillary prior to electrophoretic separation or to carry out concurrent pumping during the separation itself for optimization of resolution. Also implemented was dual single-end injection, the injection of two sample plugs from one end of the capillary, and the small size of the contactless conductivity detector allowed the concurrent use of two cells at different points along the capillary for added flexibility. Equally considered was the effect of the pH-value of the background electrolyte as this strongly influences the electroosmotic flow, which also has bearing on the protocol to be used. For dual single-end injection reproducibilities in separation time of about 1–3% were obtained and the peak areas could be reproduced to within about 3–5% (both standard deviations). The detection limits for inorganic ions were all about 1 μM.