Improving the compatibility of contact conductivity detection with microchip electrophoresis using a bubble cell

A new approach for improving the compatibility between contact conductivity detection and microchip electrophoresis was developed. Contact conductivity has traditionally been limited by the interaction of the separation voltage with the detection electrodes because the applied field creates a voltage difference between the electrodes, leading to unwanted electrochemical reactions. To minimize the voltage drop between the conductivity electrodes and therefore improve compatibility, a novel bubble cell detection zone was designed. The bubble cell permitted higher separation field strengths (600 V/cm) and reduced background noise by minimizing unwanted electrochemical reactions. The impact of the bubble cell on separation efficiency was measured by imaging fluorescein during electrophoresis. A bubble cell four times as wide as the separation channel led to a decrease of only 3% in separation efficiency at the point of detection. Increasing the bubble cell width caused larger decreases in separation efficiency, and a 4-fold expansion provided the best compromise between loss of separation efficiency and maintaining higher field strengths. A commercial chromatography conductivity detector (Dionex CD20) was used to evaluate the performance of contact conductivity detection with the bubble cell. Mass detection limits (S/N = 3) were as low as 89 +/- 9 amol, providing concentration detection limits as low as 71 +/- 7 nM with gated injection. The linear range was measured to be greater than 2 orders of magnitude, from 1.3 to 600 microM for sulfamate. The bubble cell improves the compatibility and applicability of contact conductivity detection in microchip electrophoresis, and similar designs may have broader application in electrochemical detection as the expanded detection zone provides increased electrode surface area and reduced analyte velocity in addition to the reduction of separation field effects.