AN IMPROVED OPTICAL NULL INFRARED SPECTROPHOTOMETER

Author Institution: The Dow Chemical Company“A new spectrophotometer is described and some demonstrations of its performance are given. The design embodies several new features directed toward improved quantitative accuracy of chemical analysis, particularly differential analysis. An arrangement for by-passing a small amount of radiation around the sample cell provides a dynamic balance at zero transmission; a means for automatic gain control during differential recording permits the beam-balancing servo to remain active through absorption bands of the reference material as deep as 95% absorbed; a new type of radiation attenuator is used which permits simpler optical design and provides exact linearity between pen movement and percent transmitted. Other features include improved scan-speed control, expansion of pen-travel in ordinate direction, and use of a non-linear $\lambda^{1/2}$ wave-length scale. Differential performance is demonstrated using the difficult example of an impurity of propylbenzene in ethylbenzene.