Integration of Forward-Scatter Interferometry-Based Refractive Index Detector Into Capillary Electrophoresis With Laser-Induced Fluorescence.

A simple and inexpensive method for integrating universal refractive index (RI) detection with existing capillary electrophoresis (CE) platforms utilizing epifluorescence detection is demonstrated. The approach uses the same epi-illumination laser source used for fluorescence detection to create a forward-scattered interference pattern for RI detection. The forward-scattered interferometry (FSI) approach only requires the addition of a bicell photodiode detector and mechanism for aligning it in the fringe pattern to add complementary universal RI detection with existing highly specific epi-fluorescence detection. FSI and fluorescence electropherograms illustrate the utility of measuring both channels simultaneously. Because both signals are generated from the same laser source focused into the same detection zone, the electropherograms are perfectly registered. The sample plug is directly detected with FSI in every separation, providing an intrinsic marker of the electroosmotic flow (EOF) without the need for neutral markers or separate measurements. Moreover, continuous EOF monitoring is also demonstrated using the introduction of thermal markers that are sensed in the FSI signal. Finally, it is shown that the FSI signal responds to both RI and the separation voltage, as has been shown previously for back-scatter interferometry (BSI) detection. This leads to enhanced signal-to-noise at elevated separation voltages, conditions that reduce analysis time and improve peak efficiency.