28 : 4/ Magnetic Measurements 



533 



the spin magnetic moment of one electron (that is, 1/2)3) /x, and to 

 replace the product 2g s S by the spectroscopic splitting factor g. Re- 

 writing Equation 6, it then becomes, in ESR notation 



v = 



g^H 



(7) 



By varying either v or H, one can find a value for g. Technically, it is 

 easier to vary H. The form of the apparatus is shown in Figure 3. 



Oscillator 



Attenuator 



Cavity 

 Resonator 



Detector 



Galvanometer 



Sample 



Figure 3. The principle of magnetic resonance absorption 

 measurements. To tune the apparatus, H is usually varied 

 rather than the oscillator frequency. For a free electron 



whereas for a proton 



v = 2.8//, 



= 2 x 10" 3 //. 



In these formulae, v is the oscillator frequency in mc and H the 

 field strength in gauss. For H = 5 x 10 3 gauss, v el = 14 

 kmc and v pr = 20 mc. After P. Selwood, Magnetochemistry 

 (New York: Interscience Publishers, Inc., 1956). 



For crystalline materials, electron-spin resonance methods give a 

 much higher resolution and more information than does any other 

 method available. However, water, a necessary part of all enzyme 

 reactions, absorbs electromagnetic waves. This problem can be reduced 

 by placing the sample in a capillary tube at an electric node of a wave- 

 guide, but the water still remains the limiting factor. In spite of this 

 limitation, ESR is more sensitive than other magnetic methods for the 

 detection of free radicals. 



