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Optimum Reference Wavelength Selection in Multi-Wavelength Spectrophotometry of Turbid Media

In-dual wavelength spectrophotometry a sample is observed at two different wavelengths, and the ratio of the photosignals is used in the analysis. Dual wavelength photometry may be used, for example, in studies of the kinetics of turbid chemical systems in which both transmittance and scatter are time-dependent and in liquid chromatographic analysis in which a change in eluting solvent often generates inhomogeneities in the flowing liquid. In both examples, the primary wavelength is selected for optimal change in signal with a change in concentration of the reacting or flowing solution. Ideally, the reference wavelength is selected for cancellation of the signal change associated with the change in turbidity. In fact, this condition cannot be achieved because of the spectral dependency of scattering, and a compromise between scatter compensation and analytical signal degradation must be made. Mathematical treatments of the dual wavelength method have been given by Chance, Cowles, and Rikmenspoel, but the question of optimum reference wavelength selection has not been discussed.