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Absorption Spectrophotometry /26 : I 



cules, as for instance the heme proteins, which are difficult to separate 

 by chemical techniques. However, each of the heme proteins has a 

 characteristic absorption spectrum which makes possible not only its 

 identification but also a measure of the amount of the compound present, 

 both in the test tube and in the living cell. 



Absorption spectroscopy has been important both for compounds with 

 characteristic spectra and for many others which form characteristically 

 colored compounds on reacting with another substance. In measuring 

 blood sugar level, for example, a reaction is produced which leads to a 

 colored product. The amount of this characteristic product is deter- 

 mined spectrophotometrically. 



Nor is absorption spectroscopy limited to visibly colored pigments. 

 Although the eye responds to electromagnetic energy only if its frequency 

 is within a particular octave, there exist detectors which can measure 

 electromagnetic energy from very low frequencies to very high fre- 

 quencies. The different parts of the electromagnetic spectrum are pre- 

 sented in Table I. The various ranges are purely arbitrary; there are 



no sharp dividing lines. Each range demands the use of a different 

 type of emitter and detector. However, between any two adjacent 

 regions, there is an overlap where both techniques may be employed. 

 This fact emphasizes the essential continuity of the electromagnetic 

 spectrum ; all the types of radiation described propagate as disturbances 

 in the electrical and magnetic fields. In a plane wave, these disturb- 

 ances are at right angles to each other as well as to the direction of 

 propagation. 



