Determination of amino acids by capillary electrophoresis with differential resonant contactless conductivity detector

The impedance of a capacitively coupled contactless conductivity detector (C4D) in capillary electrophoresis (CE) was measured by an impedance analysis method. The influence of solution conductivity and capillary dimension on impedance parameters was investigated. Under the experimental conditions used, 86–99.9% of the total impedance of a C4D is composed by its imaginary part from the capillary wall capacitor. With increasing inner diameter of capillary and solution conductivity in detection zone, the wall capacitance increases, which results in the increase in the response signal of C4D. But the wall capacitance is only 0.5–12% of the predicted value according to a cylinder capacitor model. As the change in solution resistance is detected in a resonant C4D (RC4D), the sensitivity of contactless conductivity detection in CE is improved. The application of an end-to-end differential RC4D (DRC4D) system in CE was demonstrated in the determination of 10 amino acids. The running buffer consisted of 2 M acetic acid and 0.1% hydroxyethylcellulose (pH 2.1). The limit of detection for amino acids is in the range of 0.1–0.4 μM. Under our experimental conditions, the sensitivity of DRC4D enhances by a factor of 15–29 as compared with C4D.