Numerical-Simulation-Based Buffer Design for Microchip Electrophoresis with Capacitively Coupled Contactless Conductivity Detection

Abstract

We present a numerical simulation method for designing a buffer system in microchip electrophoresis (MCE) equipped with capacitively-coupled-contactless-conductivity detection (C4D). One of the key design considerations for MCE-C4D is background electrolyte (BGE). This is because a C4D typically exhibits low sensitivity, and optimizing BGE conditions (e.g., base and acid species, pH, and ionic strength) can improve its sensitivity. However, BGE has been traditionally designed through experience or trial and error, which is time- and reagent-consuming. In this study, we employ Simul 5, an open-source electrophoresis simulation software, for rational BGE design. Four BGEs including trimethylamine (TEA)/acetic acid (AcOH, pH 10.6), MES/His (pH 6.1), MES/TRIS (pH 8.1), and TRIS/HCl (pH 7.4), previously used in electrophoresis-C4D of amino acids and protein, were selected for evaluation of our numerical method. Glutamic acid (Glu) was selected as a model analyte for initial simulation verification. Our numerical simulation revealed that the best achievable detection sensitivity was 1.046 × 10^–5 S/(m µM) in the TRIS/HCl buffer because anionic Glu species with a low mobility (27 × 10^–9 m²/Vs) replaced Cl⁻ co-ion of a high mobility (79.1 × 10^–9 m²/Vs) in the analyte zone, leading to a significant negative conductivity peak. TEA/AcOH, MES/His, and MES/TRIS buffers exhibited progressively lower sensitivity. After the initial evaluation, trypsin inhibitor (TI), a more complex proteinous analyte was tested in the MES/His and MES/TRIS BGEs. The best detection sensitivity was 1.032 × 10^–4 S/(m µM) in the MES/TRIS buffer because counter-ionic species TRIS⁺ of a high mobility (29.5 × 10^–9 m²/Vs) was replaced by the ionic TI, characterized by a large charge (− 11.5) and a low mobility (8.08 × 10^–9 m²/Vs), resulting in a strong negative peak. Based on a comprehensive analysis of the impacts of compositional changes in each ionic species of the analyte zone on conductivity-peak height, we propose a BGE design guideline for enhanced sensitivity. Subsequent MCE-C4D confirmation experiments demonstrated excellent qualitative agreement with the simulation results for the Glu and TI analytes. We anticipate that our numerical analysis method will find wide application in designing BGEs for portable MCE-C4D systems by enhancing sensitivity.