118 



COLORIMETRY-SPECTROPHOTOMETRY 



chromator employs a giating instead of a prism. The Gary instrument 

 is also equipped to operate in the near infrared. Because of the possibility 

 of producing stray infrared in the measuring system, the infrared beam 

 is produced by a second tungsten bulb and passes backward through 

 the double monochromator. Any stray energy is thus removed. 



<A — ^~{|"*0 Phototube 



Der yU 



N^— >— [|-H0 Phototube 

 Sample 



■^^- — T— Q — >-^ I Beam splitter 

 n Chopper ^ 



Hydrogen lamp 



Tungsten 

 lamp 



Fig. 9-4. Optical Schematic Diagram of the Perkin-Elmer Model 

 350. Note the double monochromator and the use of two 

 matched phototubes as detectors. (Courtesy Perkin-Elmer Cor- 

 p>oration.) 



In any spectrophotometer it is necessary to standardize the instru- 

 ment at every wavelength with the pure solvent in the light path because 

 the response of the instrument varies with wavelength. In the recording 

 instruments, the comparison of the sample with the reference solvent is 

 accomplished automatically since the entire spectrum is swept across the 

 exit slit and the wavelength changes rapidly. Reference to Figs. 9-4 and 

 9-5 will show two different means of performing this operation. In the 

 Perkin-Elmer instrument the light beam emerging from the monochrom- 

 ator is chopped by a rotating shutter. This shutter provides pulses of 

 light to the phototubes, yielding a pulsating output which can be ampli- 

 fied. The beam of light is then divided so that half the energy passes 

 through the sample and half through the reference tube, each half- 

 beam arriving ultimately at a separate detector. The outputs from the 

 two detectors are compared electronically, and the difference between 

 the two is plotted on a chart. This arrangement requires a reliable beam- 

 splitter and a pair of carefully matched phototubes but allows the photo- 

 tubes to be placed close to the sample and reference containers. In the 



