114 



COLOmMETRY-SPECTROPHOTOMETRY 



ing to resolve a beam of white light into its spectrum. The spectrum is 

 allowed to fall upon an adjustable slit which allows light of only one 

 color to pass through. A simplified system is diagrammed in Fig. 9-1. 

 Theoretically it makes no difference whether the colored solution is 

 placed in front of or behind the prism. In practice it is usually placed as 

 shown in the diagram. For comparison and to show some of the operating 

 details, the optical system of the Beckman Model DU Spectrophotom- 

 eter is shown in Fig. 9-2. It will be seen that the principle is the same. 

 Many models of spectrophotometers are available on the commercial 

 market. They differ chiefly in the quality of the optical system and in 

 the electrical measuring system. The smaller and less expensive instru- 

 ments typically use a direct reading galvanometer of some sort. The 

 deflection of this meter is proportional to the output of the photocell. 

 The more refined instruments use a Wheatstone bridge as a null 

 measuring instrument, as was described in Chapter 4. 



^Collimating 

 mirror 



Condensing mirror- 



Tungsten 

 lamp 



Diagonal mirror- 



Sample Phototube 



Fig. 9-2. Diagram of Beckman Model DU Optical system. Light from the tung- 

 sten lamp is focused by the condensing mirror and directed in a beam to the 

 diagonal slit entrance mirror. The entrance mirror deflects the light through the 

 entrance slit and into the monochromator to the collimating mirror. Light falling 

 on the collimating mirror is rendered parallel and reflected to the quartz prism 

 where it undergoes refraction. The back surface of the prism is aluminized so 

 that light refracted at the first surface is reflected back through the prism, under- 

 going further refraction as it emerges from the prism. The desired wavelength of 

 light is selected by rotating the Wavelength Selector which adjusts the position 

 of the prism. The spectrum is directed back to the collimating mirror which centers 

 the chosen wavelength on the exit slit and sample. Light passing through the 

 sample strikes the phototube, causing a current gain. The current gain is amplified 

 and registered on the null meter. (Courtesy Beckman Instrument Co.) 



The design and construction of spectrophotometers is an extremely 

 complex subject, so we can discuss only the most gross aspects. The 

 useful range, accuracy, precision, and convenience of a spectrophotometer 



