Sedimentation

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SedimentationSedimentation

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PrinciplePrinciple

2 2m r - r(m ) vf

centrifugal force - buoyancy = frictional force

angular velocityr: distance from the center of spinningm: mass of the molecule partial specific volume: density of solutionv: velocity of sedimentationf: frictional coefficient

2

v m(1- ) r

sf

Sedimentation coeff. = velocity per unit centrifugal field

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Analytical CentrifugeAnalytical Centrifuge

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Boundary sedimentationBoundary sedimentation

equivalent2

d ln(r )1sdt

2

v m(1- ) r

sf

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Application: Application: HomogeneityHomogeneity

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Application: Molecular weight determination Application: Molecular weight determination

•Sedimentation Equilibrium– Sedimentation at low speeds so that the sedimentation is counterbalanced by

diffusion

1 2E E- 1 kT

2

c ec

1

2

21 2 r

m (1- )rdrr

E E

222

1ln)-(1

2RT Marc

c

a

r

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Application: Shape determinationApplication: Shape determination

6o o of R

Stokes

6.7 S 6.2 S 5.4 S

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Application: PurificationApplication: Purification

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Zonal - Density CentrifugationZonal - Density Centrifugation

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Practical tipsPractical tips

• K-factor:

• Pelleting time (t):

• But if you spin at speed less than the maximum speed (wmax) k is smaller:

• g force

sk t

k k2

maxadj

2

000,11.12r g

135 max min max min

2 2

2.53x10 ln(r / ) ln(r / ) 10( /1000) 3600

r rkRPM