IX. ELECTROPHORESIS 287 



organisms (18) and the effects thereon of various surface-active agents 

 (19) can be ekicidated by such methods. The interaction of proteins 

 with high affinity anions such as that of metaphospiioric acid can be 

 studied and the nature and extent of the interaction with changing 

 pH and anion concentration can be followed by microelectrophoresis 

 (20). 



D. MOVING-BOUNDARY METHOD 



1. Applications and Relative Advantages 



The mobilities of substances that are submicroscopic in particle 

 dimensions and are not dependably adsorbable on larger microscopi- 

 cally visible particles may be studied only by use of the moving-bound- 

 ary technique. Examples are the many lyophobic colloidal sols such 

 as gold sol, arsenic trisulfide sol, etc. The moving-boundarj^ method 

 is unique also in that it can yield information on the mobility charac- 

 teristics of mixtures of substances such as native solutions of proteins 

 (e.g., blood serum). Some of the advantages and realms of best ap- 

 plication of this method may be summarized as follows : 



(1) Of greatest importance is the capacity of the moving-boundary 

 method, used in conjunction with the refractive index methods employed for 

 the detection of the boundaries, to yield information as to (a) the electro- 

 phoretic homogeneity or heterogeneity of the disperse phase in an unknown 

 solution, (b) the number of components in a heterogeneous disperse system 

 that are electrically separable, (c) the degree of homogeneity of each compo- 

 nent, (d) the moMity of each component, and (e) the relative concentration of 

 each component in such a mixture. 



(2) This method is applicable to a wide variety of high or low molecular 

 substances that form solutions in which there are no microscopically visible 

 particles. 



(3) Electrophoresis measurements can be made in solutions of consider- 

 ably higher ionic strength than is generally feasible with the microscope 

 method. The upper limit of ionic strength that can be employed in either 

 method is governed by the wattage dissipation in the region in the cell where 

 measurements of mobility are being made and the efficiency with which the 

 heat so generated is removed, i.e., the efficiency with which convection cur- 

 rents are avoided in that region of the cell. In the U-tube method the cell 

 can readily be thermostated. 



(4) Mobilities of a given substance as a function of pH may be studied 

 throughout the whole aqueous pH range. 



