lIartin and Ames (6) proposed that, gradients could he designed so that the distances migrated 1)~ particle zones are proportional to the time of ccntrifugation and to the seclimentation coefficients of the particles. No11 (8) generalized on this idea with the term isokinetic gradient< in which particles move at. constant, spcccl. The ndrantagcs of these gradients arc ((1) convenience in estimating scclimcntation coefficients of unknown particles and Cbj a, constancy in zone width. Null's formula for isokinetic gradients maintains :I constant zone widtll, which in the geometry of swinging buckets insures it constant zone volun~c.
The case of sector-shalwd rotors, which inrlutlcs most) zonal rotors, is a quite dlffcrent situation. Hwe a ronstant zone wiclth woultl result in a progressively inrrcssing zone volu~nc~ through tlw familiar ~~lwnomenon of sectorial dilution (cf. 9j. Stecnsgaartl (131 has in fad roniputed isokinetic gradients for the B-SIP rotor, hut it is not wi(lent from his data that radial zone width is constant or that resolution is superior to that previously obtained.
Spragg ef nl. (11 j set, about to optimize resolution in sector-shaped zonal rotors by constructing a graclient in which wctoriwl clilution is exactly counterbalanced by gradient-indud zolw narrowing (lo), so t,li:tt' zone volumes should remain ronstant. Ppragg's isomefric gradients arc also designed to minimize scclimentation t.inics. M%lc the use of these gradients appreciably lower. q tlic limit of lwrticlc weights that may he rcsolrctl in zonal rot,ors, tlicy have three disaclrantagw: (n ) the zone volumes do not, in fact remain constant, but arc subject to sulwtantial anomalous broadening (12'1 , (b 1 resolution is purehasctl at the csl)ensc of limited capacity, and Cc\ t'hc comlwtation of the gradient shapes is espcnsirc, requiring many rrrinufes 011 tllr IRJ2 360 romputcr.