288 DAVID R. BRIGGS 



(5) Interactions of one disperse component with another in solution 

 may be detected and studied by this method. 



(6) An advantage this method has to offer the biochemist is tlie possibil- 

 ity of separation in the pure state (or a purer state) of electrophoretically dif- 

 ferent components of a mixture. 



Some disadvantages of the moving-boundary method as compared 

 to the microscopic method are : 



(1) The method requires a mucli longer time for individual measure- 

 ments. 



(2) Larger quantities of disperse substances are usually required. Gen- 

 erally the concentration of disperse phase should be 0.5 to 2.0% and ordinarily 

 about 25 ml. of such a solution is needed to wash out and fill the U tube. 

 This is distinctly a limiting factor in many cases of substances obtainable 

 only with great difficulty. Newer cells and methods will tend to reduce this 

 requirement. (Cells requiring not more than 20 mg. of disperse phase are 

 already in use.) 



(3) Conductivity and pH differences that exist across the boundary be- 

 tween the buffer and the solution containing the disperse phase to be studied 

 lead to boundary uncertainties and anomalies that are not always resolvable. 

 This is particularly true when the buffer solution employed is of low ionic 

 strength (0.05 or less), and may lead to considerable difficulties in the inter- 

 pretation of observations both as to the true mobilities of the components and 

 as to their relative concentrations in the mixture. This constitutes the pri- 

 mary limitation of this method at the present time. 



(4) The moving boundary method apparatus is more expensive than the 

 microelectrophoresis apparatus. 



2. Essential Requirements 



Many variations of apparatus of the U-tube type have been de- 

 vised from time to time {21-23) for the purpose of measuring the elec- 

 trophoretic mobilities of a variety of coUoidally dispersed substances 

 by the moving-boundary method. Since the measurement of the 

 mobility of a given component by this method is obtained by obser- 

 vation of the displacement, under the influence of an electric field, 

 of a boundary between two solutions within the fluid system, only 

 one of which contains the component in question, it is necessary that 

 the following requirements be fulfilled in order that quantitative 

 measurements can be accomplished : (a) The composition, pH, con- 

 ductivity, density, etc. of the fluids in the region of the boundary 

 must be unaffected during the course of the experiment by any elec- 



