An optically simple, inexpensive, microvolume refractive index detector has been applied to capillary electrophoresis, allowing universal solute detection at the subpicogram level. The microinterferometric backscatter detector employs direct side illumination of an unmodified capillary by a He-Ne laser, producing a 360Β°fan of scattered light that contains a set of high-contrast interference fringes. Positional changes of the interference pattern extrema (backscattered light) are directly related to the refractive index (RI), n, of the fluid in the tube, and allow for the determination of β¬n at the 10 Οͺ7 level. These light and dark spots are viewed on a flat plane in the direct backscatter configuration. Signal interrogation is accomplished by a simple slit-photodetector assembly that is capable of monitoring the time-dependent fringe shifts produced or imparted by RI changes. Nanoliter detector volumes (0.27 Ο« 10 Οͺ9 liter) are produced by this unique optical train that requires no modification of the separation capillary. The system's utility is demonstrated by the separation and quantitation of a mixture of cations (K Ο© , Ba 2Ο© , Na Ο© , and Li Ο© ) that are separable and detectable at the parts-per-million level, using no active thermal stabilization.