Quantitative analyses of multicomponent mixtures, blends or complex materials by absorbance measurements in the infrared region are subject to uncertainties in accuracy because of variations in spectrophotometer performance between period of calibration and sample measurement. Another variable is path length which is difficult to estimate for cells of less than 0.1 mm thickness or in the case of films of insoluble materials such as certain polymers. A solution to these problems is to employ the absorbance ratio method which has been reported for two and three component systems such as copolymers (1-3). Knowledge of path length is not essential in absorbance ratio methods. A study has been reported of the precision of absorbance ratio methods for mixtures of methyl cyclo-hexane and toluene and for six component mixtures of o, m, p-xylene, toluene, isopropylbenzene, and benzene (4).
Arthur S. Wexler, "Multicomponent Infrared Analysis by the Absorbance Ratio Method with Internal Standard Addition," Appl. Spectrosc. 19, 135-137 (1965)