Pushing the Limits of Capacitively Coupled Contactless Conductivity Detection for Capillary Electrophoresis.

Capillary electrophoresis with capacitively coupled contactless conductivity detection (CE-CD) has proven to be an efficient technique for the separation and detection of charged inorganic, organic, and biochemical analytes. It offers several advantages, including cost-effectiveness, nanoliter injection volume, short analysis time, good separation efficiency, suitability for miniaturization, and portability. However, the routine determination of common inorganic cations (NH, K, Na, Ca, Mg, and Li) and inorganic anions (F, Cl, Br, NO, NO, PO, and SO) in water quality monitoring typically exhibits limits of detection of about 0.3-1 μM without preconcentration. This sensitivity often proves insufficient for the applications of CE-CD in trace analysis situations. Here, we explore methods to push the detection limits of CE-CD through a comprehensive consideration of signal and noise sources. In particular, we (i) studied the model of CD and its guiding roles in CD and CE-CD, (ii) optimized the bandwidth and noise performance of the current-to-voltage (-) converter, and (iii) reduced the noise level due to the strong background signal of the background electrolyte by adaptive differential detection. We characterized the system with Li; the 3-fold signal-to-noise (S/N) detection limit for Li was determined at 20 nM, with a linear range spanning from 60 nM to 1.6 mM. Moreover, the optimized CE-CD method was applied to the analysis of common mixed inorganic cations (K, Na, Ca, Mg, and Li), anions (F, Cl, Br, NO, NO, PO, and SO), toxic halides (BrO) and heavy metal ions (Pb, Cd, Cr, Co, Ni, Zn, and Cu) at trace concentrations of 200 nM. All electropherograms showed good S/N ratios, thus proving its applicability and accuracy. Our results have shown that the developed CE-CD method is feasible for trace ion analysis in water quality control.