COLORIMETRY-SPECTROPHOTOMETRY 1 1 7 



Any spectrophotometer must be a properly balanced collection of com- 

 ponents. Obviously there is no point in coupling a very sensitive detector 

 to a crude optical system. As a rule, the better the instrument, the nar- 

 rower the spectral width of the band of monochromatic light that may 

 be used. Imagine a spectrophotometer which produces a spectrum whose 

 energy distribution is shown by the curve in Fig. 9-3. The actual response 

 of the photocell will depend upon the fraction of the area under the 

 curve which passes through the exit slit. For example, the shaded portion 

 between Xi and X4 would produce a certain electrical response. If the 

 slit is made narrower, a band extending from ^2 to X3 will pass through, 

 and the electrical output is reduced by approximately one-half. As the 

 slit becomes narrower and narrower, the amount of energy falling on the 

 photocell becomes smaller and smaller until eventually there is insuf- 

 ficient energy to produce a usable electrical output. The width of the 

 usable spectral band can be reduced by increasing the responsiveness of 

 the detecting system. The minimum usable spectral band width varies 

 from one part of the spectrum to another, and there is considerable varia- 

 tion among the different commercial instruments. Usually the lower the 

 price, the wider the band widths must be. The effect of spectral band 

 width upon actual measurement is illustrated in Fig. 9-9 on page 124. 



Recording spectrophotometers 



Perhaps the ultimate in convenience is attained in the recording 

 spectrophotometers. These instruments contain a motor-driven mecha- 

 nism that will scan the entire spectrum, that is, the entire spectrum is 

 swept across the exit slit. At the same time the amount of light falling on 

 the photocell is recorded by a moving pen on paper. Several instruments 

 are available on the market, and each uses slightly different principles 

 in its operation. Figures 9-4 and 9-5 compare two of the available instru- 

 ments. Each uses a double monochromator, or, the beam of monochro- 

 matic light produced by one monochromator is passed through a second 

 monochromator to bring about a further reduction in stray light. The 

 Perkin-Elmer Model 350 operates in the visible and ultraviolet and em- 

 ploys two similar quartz prisms. Radiant energy from either the tungsten 

 lamp or the hydrogen lamp enters the monochromator where it is dis- 

 persed, and the narrow beam emerges through the exit slit. In the Gary 

 Model 14 radiant energy from either the tungsten source or the hydrogen 

 lamp follows a somewhat similar path, except that the second mono- 



