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

An on-chip detector based on backscatter interferometry has been developed to perform sub-nanoliter volume refractive index measurements. The detectio n system consists of a simple, folded optical train based on the interactio n of a laser beam and an etched channel in a silica (glass) plate. This etc hed 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 contr ast interference pattern that contains information related to the bulk prop erties of the fluid located within the probe volume. Positional changes of the interference pattern extrema (fringes) allow for the determination of r efractive index changes at the 10(-6) level in a detection volume of 188 x 10(-12) L. Under capillary electrophoresis (CE) conditions, the injected ma ss detection limits for small molecules with little native absorption range s from 530 fmol (0.18 ng) for sucrose to 720 fmol (0.43 nanograms) for raff inose. Fluorescein was also used to evaluate the technique for universal CE and under further optimized conditions can be quantified at the 150 mu M l evel. Separation performance for the solutes tested ranged from about 2300 to 15500 plates or 61000 to 400000 N m(-1). The results presented here indi cate there is potential for using the simple optical train of backscatterin g interferometry for on-chip universal solute analysis.