III. CENTRIFUGATION 69 



ess, there exists for some time, between the diffuse boundary and the 

 transition zone, a region ("plateau" region) in which the concentra- 

 tion remains nearly constant and uniform throughout. This circum- 

 stance permits one to obtain a measurement of the true sedimentation 

 rate in spite of the diffusion, for it is known from the nature of the 

 diffusion process that the position which the boundary would have 

 had in the absence of diffusion is simply the level at which the con- 

 centration is one-half that in the plateau region (/, p. 6). 



Practical measurements that can be completed in a matter of 

 hours with gravitational force alone are in general limited to par- 

 ticles such as red blood cells, etc., in the size range of a few microns 

 or more. The boundary spreading due to diffusion is for such par- 

 ticles negligible in comparison to the boundary movement and hence 

 appreciable inhomogeneity in sedimentation rate is immediately ap- 

 parent from a lack of sharpness in the boundary. Also, if there are 

 several components of discrete particle sizes, their respective bound- 

 aries are relatively sharp and easily differentiated. 



2. Sediiiientatioii in Centrifugal Field of Force 



The sedimentation behavior of materials in the size range consid- 

 erably below a micron, such as virus particles and protein molecules, 

 cannot be studied with gravitational force alone. As a general rule, 

 the smaller the particle size, the slower the sedimentation and the 

 more pronounced the diffusion process. When the diffusion rate is 

 sufficiently high in comparison to the sedimentation rate, a boundary 

 does not become well enough differentiated for analysis. In contain- 

 ing vessels of practical size, no distinct supernatant zone relatively 

 free of particles becomes established before the diffuse boundary be- 

 gins to grade into the transition zone with the consequent loss of any 

 well differentiated plateau region. Furthermore, even if diffusion 

 were not a complicating factor, the time required for a determination 

 of sedimentation rate would be too long for practical purposes in most 

 cases. 



The use of high centrifugal forces instead of gravity permits ap- 

 plication of the method to relatively small particles, although the 

 range is still limited by the same basic considerations. For example, 

 although modern ultracentrifuges are capable of developing cen- 

 trifugal forces in excess of 250,000 times gravity, application of the 

 boundary method Avith any degree of precision is still limited in the 

 case of monodisperse preparations to materials having molecular 



