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

Capillary electrophoresis with capacitively coupled contactless conductivity detection (CE-C 4 D) 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 4 + , K + , Na + , Ca 2+ , Mg 2+ , and Li + ) and inorganic anions (F - , Cl - , Br - , NO 2 - , NO 3 - , PO 4 3- , and SO 4 2- ) 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-C 4 D in trace analysis situations. Here, we explore methods to push the detection limits of CE-C 4 D through a comprehensive consideration of signal and noise sources. In particular, we (i) studied the model of C 4 D and its guiding roles in C 4 D and CE-C 4 D, (ii) optimized the bandwidth and noise performance of the current-to-voltage ( I - V ) 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-C 4 D method was applied to the analysis of common mixed inorganic cations (K + , Na + , Ca 2+ , Mg 2+ , and Li + ), anions (F - , Cl - , Br - , NO 2 - , NO 3 - , PO 4 3- , and SO 4 2- ), toxic halides (BrO 3 - ) and heavy metal ions (Pb 2+ , Cd 2+ , Cr 3+ , Co 2+ , Ni 2+ , Zn 2+ , and Cu 2+ ) 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-C 4 D method is feasible for trace ion analysis in water quality control.