Electronic baseline-suppression for liquid conductivity detection in a capillary electrophoresis microchip

Liquid conductivity detection for application in chip-based capillary electrophoresis (CE) has received attention because the sensor electrodes can be integrated in the chip and there is no loss of sensitivity when downscaling the detector. A well known disadvantage of conductivity detection is the high level of the baseline (due to the conductivity of the carrier electrolyte) that has a high influence on the measured values and deteriorates the detection limit. By using a lockin amplifier, making use of its two differential inputs and of its phase-locked sinewave output, we can generate a signal of controlled amplitude and phase, related to the level and phase of the baseline signal. By subtracting this signal from the AC output signal of the detector the baseline is significantly suppressed. Ideally, only the changes of the background signal are then measured. When applying the baseline-suppression method to real measurements, the level of the background signal was decreased 50 times. The detection limit (which is related to the signal-to-noise ratio) was found to be 10 times lower. Reproducible separations with concentrations from 1 mM down to 10 μM of potassium, sodium, and lithium ions were obtained. For our separation and detection system, the concentration of 10 μM could not be detected without the baseline-suppression technique described here.