A chip-scale universal detector for electrophoresis based on backscattering interferometry

Abstract

An on-chip detector based on backscatter interferometry has been developed to perform sub-nanoliter volume refractive index measurements. The detection system consists of a simple, folded optical train based on the interaction of a laser beam and an etched channel in a silica (glass) plate. This etched channel is composed of two radii joined by a flat portion which define a curved surface in the shape of a half cylinder in a silica (glass) plate. The backscattered light from the channel takes on the form of a high contrast interference pattern that contains information related to the bulk properties of the fluid located within the probe volume. Positional changes of the interference pattern extrema (fringes) allow for the determination of refractive index changes at the 10⁻⁶ level in a detection volume of 188 × 10⁻¹² L. Under capillary electrophoresis (CE) conditions, the injected mass detection limits for small molecules with little native absorption ranges from 530 fmol (0.18 ng) for sucrose to 720 fmol (0.43 nanograms) for raffinose. Fluorescein was also used to evaluate the technique for universal CE and under further optimized conditions can be quantified at the 150 μM level. Separation performance for the solutes tested ranged from about 2300 to 15500 plates or 61000 to 400000 N m⁻¹. The results presented here indicate there is potential for using the simple optical train of backscattering interferometry for on-chip universal solute analysis.