IX. ELECTROPHORESIS 289 



trotysis products that may form at the electrodes through which the 

 electric field is introduced into the system containing the boundary. 

 Also any disturbances that could be transmitted to the boundary due 

 to gas formation at the electrodes must be avoided. (6) The appara- 

 tus must be so designed that a sharp boundary can be formed initially 

 between the solution containing the component to be followed and a 

 solution that does not contain this component, (c) Disturbances of 

 the boundary due to convection currents in the boundary region 

 arising from wattage dissipation in the fluid column containing the 

 boundary must be obviated, (d) It must be possible to detect ac- 

 curately^ the position of the boundary, visually or otherwise, both 

 initially and after a measured lapse of time during which a constant 

 electric field is maintained throughout the column of fluid containing 

 the boundary, (e) A means must be available for measuring and 

 maintaining constant the imposed electrical field strength under 

 which migration of the boundary occurs. (/) The electrolyte compo- 

 sition, pH, and specific conductivity of the two solutions that meet 

 at the boundary must always be as nearly identical as possible while 

 still maintaining the disappearance in the boundary of the component 

 to be studied. As shall be shown, this requirement is the most dif- 

 ficult to approximate and is the source of the major limitation to the 

 use of this method. 



3. Modern Apparatus and Technique 



The series of moving-boundary apparatus that have been devised 

 actually constitute progressive stages of development in which the ob- 

 jective has been to meet more and more effectively the various re- 

 quirements for quantitative measurements by the method. An ap- 

 paratus devised by Tiselius (24,^5) represents such a great improve- 

 ment over earlier instruments that these are now primarily of historic 

 interest only. Tiselius introduced the use of the refractive index or 

 schlieren method for the detection of the position of the boundary in 

 the U tube and also emphasized the importance of avoiding convec- 

 tion currents in the regions of the boundaries; he recommended 

 methods by which these currents can be minimized. The use of the 

 refractive index method for characterizing the boundary has made 

 possible also the analysis of the contours of the concentration gradients 

 occurring in the boundary region and the analysis, therefore, of the 

 electrophoretic homogeneity or heterogeneity of the material that 

 disappears in the boundary. Tiselius incorporated in his apparatus 



