Linearity and effective optical pathlength of liquid waveguide capillary cells.

The validity of using Beer's Law to describe liquid waveguide capillary cells (LWCC) as absorption cells with increased optical pathlength was investigated. Experimental and theoretical results for two types of LWCC are presented. "Type I" LWCCs are constructed with solid TEFLON AF tubing. "Type II" LWCCs consist of quartz tubing with an outer coating of TEFLON AF. UV/Vis absorbance spectra versus chromophore concentration were found to be linear for both LWCC types within the wavelength range and absorbance accuracy of the spectrophotometer used. The ratio between "effective" and "physical" pathlength, EPLR was determined experimentally for both LWCC types. Type I cells had an effective optical pathlength that was statistically indistinguishable from the physical pathlength on a 95% probability basis (EPLR = 1.008 +/- 0.006, n=4). Type II cells had an effective optical pathlength that was slightly shorter than the physical pathlength, dependent on the cell's inner diameter and wall thickness (ID= 400 mu m: EPLR= 0.913 +/- 0.003, ID= 550 mu m: EPLR= 0.940 +/- 0.010, ID= 700 mu m: EPLR= 0.954 +/- 0.009, wall thickness= 50 mu m, n=4). A theoretical model explaining Type I LWCC results is presented. Our results indicate that Beer-Lambert's Law can be applied to both types of LWCCs for UV/Vis absorption spectroscopy.