7J DIALYSIS STUDIES 105 



As regards the measurement of size, the approach we have followed is 

 similar to that in the well-known method of molecular weight determina- 

 tion of Northrop and Anson, ^ who measured the rate of diffusion of a 

 solute through sintered glass. However, as will be seen later, there are 

 marked differences. 



It was realized in the beginning that the rate of diffusion through a mem- 

 brane would be a function of a number of interdependent variables, when 

 the sole driving force is a concentration gradient across the membrane. 

 These are the nature of the membrane, charge effects on the membrane, 

 effect of solvent, pH, ideaUty of the solute, temperature, etc. 



In prehminary studies cellophane appeared to be the most promising 

 membrane for the investigation. A simple cell was constructed from a cello- 

 phane sac in which both the contents of the sac and the solvent around it 

 were stirred. By measuring the rates of escape of solutes of known size 

 through the sac, it was soon found^ that the escape rates did not bear the 

 relationship to size that would be expected from free diffusion alone and 

 as would be expected from the Northrop and Anson cell. A greater selec- 

 tivity was noted which suggested a membrane effect in addition to the dif- 

 ferential provided by diffusion. 



Fig. 1. Schematic drawing of the microdialysis cell. A 7-mm. glass 

 tube fitted at the top with a 2-ml. syringe passes through two larger 

 sections of glass tubing A and B. The dialysis membrane represented 

 by the dashed line is made by tying a knot in a wet piece of cellophane 

 tubing with care to avoid any stretching of the membrane. A is of such 

 an outside diameter (ca. 15 mm.) that it is a little less than the diameter 

 of the extended sac. B is slightly larger in diameter than A and serves to 

 hold the membrane in position. C is a test tube which is large enough to 

 provide about 2 mm. clearance from the membrane. It holds the outside 

 solvent amounting to about 10-20 times the volume of the solution 

 inside the sac. The upper rim of C is collapsed somewhat in order to 

 keep it centered. The lower end of C rests on the table and can be easily 

 centered. During operation the height of the solutions both inside the sac 

 and outside are adjusted to reach only the top of A. A small hole is 

 punched through the membrane just below B in order to avoid pressure 

 being built up during the run. 



w 



In order to enhance this effect it appeared desirable to try less porous 

 membranes so that the solutes would be even more restricted in their pass- 

 age, or alternatively, use larger test solutes with the same membrane. Either 

 course implied a much slower process, although the time involved had 

 already proved a serious handicap. Therefore, an attempt was made to de- 

 sign a cell which would give a faster rate of net transfer across the membrane. 



It is known* that the net rate of transfer under a concentration gradient 



