70 



E. G. PICKELS 



weights above several thousand. In the case of paucidisperse prepa- 

 rations that would show multiple boundaries in the absence of dif- 

 fusion, molecular weights generally must be well above 10,000 to ac- 

 complish noticeable differentiation. Also, the smaller the particle 

 size, the greater the uncertainty of differentiating the amount of 

 boundary spreading due to diffusion and the amount due to inhomo- 

 geneity of sedimentation rate, and the greater the necessity of acquir- 

 ing separate diffusion data for making the comparison. Generally 

 speaking, diffusion can hardly be considered a negligible factor in the 

 analytical centrifuge until a size range at least as high as that of the 

 larger viruses (above 50 m/x roughly) is reached. 



I00-- 



< 

 or 



o 



o 



o 



<3 



a: 

 a 



Boundary positions 



i I 1 



Plateau 

 region 



4 



0.4 0.8 1.2 



DISTANCE FROM MENISCUS, cm. 



Fig. 1. Three curves illustrating the concentration 

 distribution of slowly sedimenting material at eighty- 

 minute intervals after boundary has cleared meniscus 

 sufficiently to permit measurements of position. 



As illustrated in Figure 1, the general character of the sedimenta- 

 tion process in the centrifuge and the method of analysis are similar 

 to those described for a gravitational field, except for certain condi- 

 tions imposed by the nonuniformity of the centrifugal field. Since 

 the field is radial, the containing vessel or cell in an analytical centri- 

 fuge is made sector-shaped (SO), having two fiat walls that, if ex- 

 tended, would intersect along the axis of rotation, while the other 

 two side walls are perpendicular to the axis. Thus, particles origi- 

 nally close to any of these walls continue during sedimentation to pur- 

 sue an average course parallel to the wall, so that there is no sedimen- 

 tation against or away from the walls to introduce convective dis- 

 turbances. Also, since in their sedimentation the particles on the 



