Attenuation Coefficients of Tubular Conduits for Liquid Phase Absorbance Measurement. Shot Noise-limited Optimum Pathlength.

We trace the history of liquid core waveguides (LCWs, also called liquid core optical fibers) and the role Teflon AF (TAF) has played in their development. We show that, in any shot noise limited situation, the optimum signal-tonoise ratio (S/N) occurs at a path length of 1/alpha(a) {ln[1 + 2(alpha(a)/alpha(b))]}, approximately 2/alpha(b) under most conditions, alpha(a) and alpha(b) being the light attenuation coefficient due to the analyte and the background, respectively. The analysis shows that LCW length should be selected depending on the applicable alpha(b) value. An overly long LCW may exhibit a lower signal-to-noise ratio. Water-filled TAF-clad fused-silica (FS) tubes show the lowest attenuation across the wavelength range. Nevertheless, except at lambda >= 600 nm, the observed alpha(b) values far exceed those reported for pure water: it appears that both impurities in the water and waveguide losses are involved. In examining the attenuation in various water-filled tubes, we find that the transmission of air-surrounded FS tubes is second only to TAF-clad FS tubes and is better than that of TAF tubes or externally mirrored FS tubes. Surprisingly, except for a window centered at similar to 250 nm, light transmission in a water-filled poly(tetrafluoroethylene) (PTFE) tube is worse than in poly(ether ether ketone) (PEEK) tubing. Light transmission in PTFE tubes improves with increasing wall thickness.