494 Absorption Spectrophotometry /26 : 3 



physics courses, it is shown that the condition for reinforcement is 

 given by 



b sin 6 = mX 



where b is the spacing between the lines of the grating and m is an 

 integer. 



A slight extension of this reasoning to light rays not perpendicular to 

 the grating G shows that rotating G changes the wavelength of the light 

 passing through S 2 . A similar slight variation shows that a reflection 

 grating may be used instead of a transmission grating. 



For either prism or grating type monochromators, it is necessary to 

 use components which will permit operation at the desired wavelengths. 

 In the ultraviolet, all the lenses, prisms, and plates through which the 

 light passes are usually made of quartz. Special surfaces are necessary 

 for reflectors, usually coated with either aluminum or silver. In the 

 visible region of the spectrum, glass of various types is used. For infra- 

 red spectrophotometers, prisms are made from rock salt. Lenses, 

 transparent for infrared radiation, are difficult or impossible to con- 

 struct, so focusing must be accomplished with suitably curved mirrors. 

 In general, special parts are necessary for any desired wavelength region. 



C. Sample Holders 



For spectrophotometric measurements in the visible and ultraviolet, the 

 samples are usually held in small glass (or quartz) containers called 

 cuvettes. If a direct measure of the millimolar extinction coefficient is 

 desired, the cuvette must have plane parallel faces, at right angles to 

 the light beam and separated by a known distance, most often 1 cm. In 

 spectrophotometers designed for curved cuvettes, it is necessary to cali- 

 brate against a standard of known optical density. 



D. Detectors and Electronic Circuits 



In the simplest colorimeters, the eye was used as a detector, the relative 

 height of two columns of liquid being adjusted until they both appeared 

 at the same brightness. A hand spectroscope and an eye can resolve 

 sharper bands than some spectrophotometers costing several thousand 

 dollars. However, the eye gives very poor quantitative estimates of 

 relative intensities. In sensitive spectrophotometry, some form of 

 detector is used which converts the light intensity into an electrical 

 current or voltage. Various photocells, phototubes, and photo- 

 multipliers are used. 



For measurements which do not change rapidly with time, the optical 

 density can be determined by reading a meter or by balancing a bridge. 

 To observe rapid reactions, some means of graphic or photographic 



