A variant of gradient elution moving boundary electrophoresis with channel current detection is described, which uses relatively long channels (1.5 cm) for separation and detection. The signal for each analyte is determined to have the shape of a parabola ending with a break in slope. The capabilities of the new method are demonstrated for separation and quantitation of ATP and ADP, a measurement relevant for high-throughput screening assays. The results indicate that the new method is capable of reproducibly measuring the ratio of ADP concentration to ATP plus ADP concentration with an analysis time of less than 2 min and with a 1 SD uncertainty of 0.018 (over a range of 0-1).