Capillary electrophoresis in a liquid-core waveguide with axial Raman spectroscopic detection

Raman spectroscopy is used for detection in capillary electrophoresis (CE) to provide qualitative/quantitative information with an inherently straightforward and easily interfaced instrument such as a Raman microprobe. However, because of the small analyte volume, low analyte concentration and the need for rapid spectral acquisition, high laser power excitation has been necessary to produce adequate Raman signal in CE. This work presents a method for the reduction of required laser power based on liquid core waveguiding.; A liquid core waveguide (LCW) is a liquid filled capillary that supports total internal reflection and behaves like an optical fiber by allowing light to propagate down it. Axial illumination and collection result in signal enhancement based on an increased pathlength as predicted by Beer's law. An LCW can only be formed when the refractive index (RI) of the core liquid is greater than that of the material from which the capillary is fabricated. Very few capillary materials have RIs lower than that of water, and the Teflon AF (TAF) family of polymers does, with RIs ranging from 1.29 to 1.31 (RI water = 1.333).; The low index of TAF is a consequence of high void volume (33%) which also renders the material permeable to small neutral molecules. Permeability causes variable analyte loss through the waveguide walls. Polyelectrolyte multilayers (PEMs) are used to control surface charge and permeability in all TAF capillaries. Finally, in the waveguide mode, Raman detection is performed end-on resulting in acquisition of summed spectra for the entire duration of a separation. Chemometric techniques are used to resolve acquired spectra. This work illustrates the use of waveguiding coupled with chemometric techniques to reduce the laser power requirement for CE-Raman by about an order of magnitude.