Influence of the mobile phase on the vortex generation in silicon-on-insulator-based column

Taylor–Aris dispersion (also known as the C-term in the van Deemter equation) sets a major restriction on the efficiency of pressure-driven separation techniques such as liquid chromatography (LC). Recently, the application of an active flow into the separation column in the lateral direction (orthogonal to the axial flow) has been introduced as an effective methodology to reduce Taylor–Aris dispersion. The lateral AC electro-osmotic flow (EOF) can be induced in a silicon-on-insulator (SOI) microfluidic channel, allowing for the reduction of Taylor–Aris dispersion up to an order of magnitude [1]. It has been shown that the efficiency of EOF-based lateral flow is dependent on the electrolyte content of the mobile phase and on the aspect ratio (AR) of the separation channel [2]. The mobile phase not only determines the retention of analyte on the stationary phase but also the behavior of the induced electrical double layer in the SOI column and consequently the lateral flow.

Therefore, we examined the impact of the two different electrolytes, potassium nitrate (KNO3) and a mixture of potassium cyanide/sodium dihydrogen phosphate (MP1) on EOF and dispersion reduction potential. The obtained van Deemter curves indicates greater reduction on Taylor–Aris dispersion (4.17-fold vs. 2-fold) in the presence of MP1 even at higher concentrations (1mM vs. 0.1 mM). While the electrical conductivity (Λ) for KNO3 and MP1 at the same ionic strength and temperature shows only a minor difference, variation in the induced impedance inside the SOI-channel (Z) is more pronounced, as ZKNO3 4.9-53.1% is higher than ZMP1 at different frequencies in the range 1-100 kHz, which actively influenced the double-layer capacitance performance.

In summary, the induced EOF-based lateral flow in a SOI microfluidic channel can be tailored using the electrolyte solution, allowing further reduction of Taylor–Aris dispersion and maximizing the separation resolution.