Field Enhanced Backscatter Interferometry: Improved universal detection method for Capillary Electrophoresis

Description:
Backscatter Interferometry (BSI) is a capillary scale refractive index sensing technique widely used for monitoring binding interactions of biological molecules as well as detecting analytes in Capillary Electrophoresis (CE). BSI has proven useful as a universal detector, able to detect inorganic ions, amino acids as well as serum proteins in CE separations. While previous reports show that controlling external factors such as temperature can lower detection limits, the influence of the separation field on BSI sensitivity needed further investigation. The electric field has a significant effect on the BSI signal due to the Kerr effect. Through simulations and experiments we demonstrate that high electric fields can enhance the BSI signal. Such large field strengths in conventional CE setups can be challenging due to Joule heating, but our planar CE (PCE) system uses a thin-walled capillary that efficient dissipates heat. We also show that the field enhancement of the BSI signal can be accomplished through adjustments in the background electrolyte (BGE) composition, which maximize the conductivity difference between the analyte and BGE. We demonstrate this theoretically as well as experimentally using a system a BGE containing 4M acetic acid (HAc) and arginine as the analyte. Through simulations, we were able to determine that the addition of HCl to the BGE can reduce the relative conductivity of the arginine zone, leading to larger local electric fields. Experimental evidence matches these predictions and shows that the addition of 0.01 M of HCl to 4M HAc can produce a 5 fold increase of the signal-to-noise ratio of the analyte.