100 E. G. PICKELS 



2. Sampling Techniques 



It is often necessary or advisable to establish a relationship be- 

 tween the material forming an optically recorded sedimentation 

 boundary and some biological or chemical activity. To relate the 

 distribution of some specific property with the boundary, Tiselius, 

 Pedersen, and Svedberg (77) have provided the optical cell of the 

 ultracentrifuge with a porous partition near its center. After a 

 boundary is observed to have passed through this filter, the machine 

 is stopped and the fluid above and below the partition collected for 

 analysis. The angle centrifuge has also been employed with advan- 

 tage for the measurement of approximate sedimentation rates based 

 on sampling (76,78). With proper technique (79) fluid columns 

 can be sampled with fair accuracy at any number of levels. Ad- 

 vantages of the method are that comparatively large samples may be 

 had for analysis and that it affords a knowledge of the concentration 

 of a specific property throughout the fluid column. Relatively sharp 

 sedimentation boundaries can be obtained and approximate homo- 

 geneity as well as average rate determined. Dilute preparations 

 that would otherwise be remixed by convection can be stabilized with 

 a graded concentration of sucrose or some other nonsedimenting ma- 

 terial {76). 



Centrifuges making use of the ''inverted capillary" technique {80) 

 have also been used with good success in the study of several viruses. 

 The material to be investigated is placed in small capillaries and the 

 sedimenting particles are allowed to migrate into a larger reservoir 

 filled with the same material. After centrifugation, the degree of 

 sedimentation is estimated by comparing the average concentrations 

 of activity in the capillaries and the reservoir. Of interest also are 

 the opaque, air-driven ultracentrifuges of the "spinning top" variety, 

 possible applications of which are exemplified by the study of Ruben 

 and associates {81) with radioactively tagged molecules. 



References 



GENERAL 



1 . Svedberg, T., and K. 0. Pedersen, The Ultracentrifuge. Clarendon Press 

 Oxford! 1940 (most complete reference; all aspects of analytical ul- 

 tracentrifugation, with extensive bibliography). 



2 Pickels E. G., "High-Speed Centrifugation," in J. Alexander, Colloid 

 Chemistry, Vol. V, Reinhold, New York, 1944 (analytical and pre- 

 parative centrifuges; design, theory, application; bibliography). 



