Alternate injections coupled with flow-gated capillary electrophoresis for rapid and accurate quantitative analysis of urine samples.

Capillary electrophoresis (CE) is a powerful separation technique with advantages over HPLC in terms of separation efficiency, speed, and cost. However, CE suffers in poor reproducibility in quantitative chemical analysis, which is one of major drawbacks preventing its widespread use in routine analytical laboratories. Here we report a novel strategy to enhance the quantitative capability of flow-gated CE. The platform integrated dual flow branches to respectively supply a sample and its standard additions that were then alternately injected into a single capillary for rapid separations (typically 20-90 s). A micro-fabricated switch was used to enable the alternate injections. It was assumed that the analytical system maintained constant conditions during neighboring injections that served as external self-standards for quantitation. This strategy was expected to reduce uncertainties caused by the fluctuation in capillary conditions and the drift of detection systems. Experimental results demonstrated that the dual-branch flow-gated CE coupled with alternate injections significantly improved the reproducibility with respect to peak height ratios under deliberate variations in injection volumes, separation voltages, optical focusing, and laser power; and thus the interday precision was ensured. To demonstrate its applicability, cyanide and amino acids in human urine were quantified rapidly with the one-point standard addition method after fluorogenic derivatization with naphthalene-2,3-dicarboxaldehyde (NDA), and the measurement accuracy was validated by determining the recovery of standard cyanide added to a urinary matrix. This strategy would be valuable to enable the quantitative capability of flow-gated CE in the measurements of a broad range of analytes, especially those lacking suited internal standards.