The Reproducibility and Reliability of UV-vis Higher-Order Derivative Spectroscopy for Quantitative Analysis of Spectral Changes

The reproducibility of higher-order derivative spectra of aqueous solutions of N-methyl-4-(p-formylstyryl) pyridinium salt (FSbQ) as a model compound were examined by the conversion of UV-vis absorption spectra taken on six spectrophotometers into derivatives with aid of software for data analysis as a common tool for differentiation and smoothing. The spectral shapes of crude fourth and eighth derivatives were not only quite noisy, but also considerably dependent on the spectrophotometers. Savizky-Golay smoothing under the same and appropriate conditions, however, provided satisfactorily identical derivative-spectra, confirming the reproducibility of derivative spectroscopy. In order to reveal the reliability of the quantitative analysis on the basis of Beer's law, absorbances (A) of aqueous solutions of FSbQ were plotted against dnA/d-n (D-n) values at subpeaks of the fourth as well as eighth derivatives. Excellent linearity between A and D-n was disclosed when A is smaller than ca. 1, leading to the confirmation that D-n values are usable for kinetic studies on the photoisomerization of FSbQ as an example.