Light at the end of the tunnel: recent analytical applications of liquid-core waveguides Tim Dallas, Purnendu K. Dasgupta Optical chemical analysis systems are the most important tools to analytical chemists in need of sensitive measurement techniques. Liquid-core wave- guides (Fig. 1) have proved to be an important innovation that has led to improvements in detection limits when incorporated into many types of optical analysis systems, especially UV–Vis absorbance, fluorescence and Raman measurements. ª 2004 Published by Elsevier B.V. 1. Introduction It has long been recognized that photolumi- nescence and related (chemiluminescence, electroluminescence, etc.) spectroscopies are excellent ways to detect and to characterize materials. Although specific electron-exci- tation mechanisms differ, the radiation in- teractions that produce electron transitions in atoms and molecules provide an unique optical signature of the sample under study. It has become increasingly important to have field-portable tools for measurement of trace amounts of chemicals, especially those that pose environmental and human health hazards. Analytical chemistry has benefited from innovations in optics, light sources and detectors, resulting in improved sensitivity and speed, while greatly reducing size and cost. Readily available light-emitting diodes (LEDs) [1], across a wide wavelength range, even deep into the ultraviolet, now permit precise tailoring of instrumentation to applications. Many diode lasers have even become inexpensive commodity items. For any photometric analysis technique, it is important to capture and to detect as much of the optical signal as possible, while minimizing background and noise. There are many excellent bench-top spec- troscopy systems that can accomplish these goals. However, cost and size issues can often pre-empt them from a sensing application. One approach, increasingly used to enhance sensitivity without special optics, involves a liquid-core waveguide (LCW). Analogous to an optical fiber, a typical LCW is a tube (cladding) with a refractive index (RI) lower than the RI of the fluid (core) inside it. Light propagates through the fluid core by total internal reflection (TIR) if the RI conditions are satisfied. The LCW serves as the ideal flow cell, as it is both the optical component and the fluid conduit. LCWs have been used for fluorescence [2–22] and Raman [23–33] measurements, as well as long-path-length UV–Vis absorption/transmission [10,20, 34–62] and IR [63,64] spectrometry. Water has an (RI) of 1.33 (Na D-line). Until the late 1980s, there were no coating or tubing materials available with a lower RI. The use of glass or polymer tubing as an LCW meant that either non-aqueous liquids with even higher RI have to be used as the core or one must rely on TIR at the cladding-air interface. Air, with an index of refraction of 1.0, is the ultimate low-RI material. However, to achieve efficient TIR at the cladding-air interface, the surface of the tubing must remain scrupulously clean to prevent light loss through absorption or scattering. Light propagating through the LCW passes through the cladding region, effectively shortening the interaction dis- tance within the core [45,65]. Absorption, fluorescence, and scattering in the clad- ding is possible as well. These problems were eliminated when DuPont introduced a fluoropolymer that had a RI less than 1.33 [66]. The Teflon AF family of fluoro- polymers (2,2-bistrifluoromethyl-4,5-di- fluoro-1,3,-dioxole) have RI in the range *Corresponding author. Tel.: +1-806-742-3064; Fax: +1-806-742-1289; E-mail: sandy.dasgupta@ttu.edu Tim Dallas, Purnendu K. Dasgupta* Texas Tech University, Lubbock, TX, USA 0165-9936/$ - see front matter ª 2004 Published by Elsevier B.V. doi:10.1016/S0165-9936(04)00522-9 385 Trends in Analytical Chemistry, Vol. 23, No. 5, 2004 Trends