Universal detection in capillary electrophoresis with a micro-interferometric backscatter detector

Abstract

An optically simple, inexpensive, micro-volume refractive index detector was applied to capillary electrophoresis (CE), allowing universal solute detection at the sub-picogram level. The micro-interferometric backscatter detector (MIBD) employs direct, side illumination of an unmodified capillary by an He–Ne laser, producing a 360° fan of scattered light that contains a set of high contrast interference fringes. These light and dark spots are viewed on a flat plane in the direct backscatter configuration. A slit–photodetector assembly accomplishes signal interrogation of the time-dependent fringe shifts, produced or imparted by refractive index (RI) changes. Using an unfocused laser beam to prove the unmodified separation capillary produces a detector volume of 4.7 × 10^–9 L. The separation and quantification of a mixture of organic dyes and simple sugars demonstrate the system’s utility. Submicromolar concentration detection limits of 0.46, 1.1 and 0.72 µM for Bromothymol Blue, Thymol Blue and Bromocresol Green, respectively, are achievable with CE-MIBD in the simplest configuration. The 3ς RI concentration detection limits are 2.5 times superior to those obtained by UV/VIS detection performed under the same conditions. Several carbohydrates (maltose, lactose and D-ribose) are separable and detectable at the ppm level, using no active thermal stabilization. Further demonstrating the utility of MIBD for universal detection with CE.