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Magnetic Measurements /28 : 5 



5. Limitations and Applications of Magnetic 

 Measurements 



For enzyme studies, two important limitations of magnetic susceptibility 

 measurements are the concentrations needed and the total amount of 

 enzyme needed. The concentration is important for two reasons. 

 First, it is hard to get high concentrations in protein solutions. Secondly, 

 at very high concentrations the proteins may react in a different fashion 

 than at low concentrations. The total enzyme needed is an important 

 limitation because it is difficult to extract large amounts of purified 

 enzymes. 



The resolving time or response time of the apparatus is also important 

 in many enzyme experiments. The higher the concentrations, the 

 faster the reactions become. Many enzyme reactions are over in the time 

 required to make the first measurement with the Gouy balance. At 

 maximum sensitivity, the Rankine balance has a slow response time 

 (about one second), but compensates for this by having a long period of 

 flow at constant velocity. By using moderate velocities, it is possible to 

 have the fluid arrive at the cell only 50 milliseconds after mixing. 



The properties of the three types of apparatus discussed in the last 

 sections are compared in the accompanying table. The large volume 

 needed for the Rankine balance is used to maintain temperature equi- 

 librium. The poor temperature control is a very undesirable feature 

 of the apparatus described in Reference 3 but is in no theoretical way 

 inherent in the equipment. It seems reasonable to suppose that the 



TABLE I 



Properties of Susceptibility Measuring Apparatuses 



Type Gouy Rankine ESR 



balance balance 



* Values depend critically on the line width. 



