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241
The two regression lines for q = 0 and c = 0 are used as before to find M
w
, A
2
and R
G
, except that the slope for
θ
= 0 is 2A
2
k.
Some data programs (such as Astra) automatically select the constant (k - it
may even be negative!) to effectively spread the data for visual inspection
6.2.8. A note on polyelectrolytes
in
relation
to
light
scattering
Many biopolymers are indeed polyelectrolytes as they have fixed electric
charges. In such cases light scattering must be carried out at sufficiently high
ionic strength, typically 0.01 M or higher. At lower strengths electrostatic
charges between individual molecules may be so strong that the criterium of
independent (uncorrelated) movements is violated. The theory does not apply
in such cases.
6.2.9. Some other practical aspects
Solutions for light scattering analysis need to be free from dust or particles
that scatter strongly. Even if they occur in small amounts (< 1%) they can
sometimes dominate the scattering and produce entirely false results. It is
customary to use appropriate filters, selectively removing contaminants. In
some cases ultracentrifugation may be needed.
A second criterion is that the polymer concentration must be accurately
known, since c enters the equations. This is far from trivial, as many
Zimm plot
y = 3.00E-07x + 5.05E-06
y = 3.85E-06x + 5.05E-06
2.00E-06
7.00E-06
1.20E-05
1.70E-05
2.20E-05
2.70E-05
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0
sin
2
(
!
/2) + kc
Kc/R
!
!
=0
30
60
90
120
150
"c=0"