Design and Fabrication of a Cyclical Electrical Field-Flow Fractionation for Differential Retention of Ions

Ionic solutions, including cations and anions, play critical roles in various applications, from industrial processes to health and environmental monitoring. Although ion chromatography (IC) is commonly used for ion analysis, its limitations have driven interest in alternative techniques. In this work, we explore the potential use of field-flow fractionation (FFF), particularly the electrical FFF (ElFFF) method, for ion separations. This research examines the potential of CyElFFF, an ElFFF variant, in retaining inorganic ions. An optimal channel height between 20 and 30 μm for effective ion retention and separation was identified through analytical analysis, numerical modeling, and experimentation. The retention of Na⁺, K⁺, and Cl^- in a 25 μm channel showed the potential of this method and the fabricated channel. Further analysis revealed that cations are mainly found in the retention peak, and better retained as compared to anions that are still primarily in the retained peak, but also found more commonly in the void, and release peaks, suggesting different behavioral responses to electric fields in these conditions. Significant retention was found when operating the CyElFFF system at higher voltages and lower frequencies, or mode III as it is called. Higher frequencies and lower voltages were not found to generate significant retention, and little to no retention was observed for DC ElFFF. Overall, ions were retained using CyElFFF and similar retention ratio results were observed between the numerical model, analytical model, and experimental results, but discrepancies at lower retention parameters suggest the influence of ion surface interactions and the need to overcome overvoltage or polarization effects at low voltages and to adjust the effective field estimates in these conditions.