Ensuring the stability of spectrophotometric measurement results during a mixture composition analysis

Spectrophotometric methods for studying the composition of mixtures are considered. The need is shown to improve the stability and accuracy of spectrophotometric measurement results in the analysis of substances with small absorption coefficients or a low concentration of the determinand. The article describes a spectrophotometric system with a temperature-controlled cell, which provides a means to study gaseous and liquid mixtures. The results are presented of a study of the absorption spectra of liquid xylene isomers and their mixtures with the use of this system. The author considers ways to improve the stability of spectrophotometric measurement results, as well as hardware and data processing methods required for this purpose. It is shown that the temperature control of both vapor and liquid samples constitutes one of the necessary conditions for ensuring the stability of measurement results. A procedure for conducting experiments and processing the results of spectrophotometric measurements is presented. A differential method was developed for reducing the effect of spectrophotometer signal instabilities on the accuracy in determining the mixture composition. This method consists in using a wavelength derivative of the absorption spectrum rather than the absorption spectrum itself as input data for multiple linear regression. Through a composition analysis of mixtures of two xylene isomers, this method was shown to decrease the standard deviation in determining the composition of mixtures by about 1.6 times. It is noted that additional filtering (smoothing) of the absorption spectrum derivative calculated from the measurement results may be required to reduce the errors in determining the composition of mixtures. The developed method designed to decrease the effect of spectrophotometer signal instabilities on mixture composition analysis is of greatest interest for the analysis of substances with small absorption coefficients or in the case of small concentrations of analyzed substances; it can be applied in environmental monitoring, for example, in the determination of hydrocarbon contaminants in the atmosphere.