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If we need to find out how much of the sample has a molecular weight below
200.000 g/mol, just follow the arrows. The result is that about 45% of the
sample is below 200.000. How much is above 500.000 g/mol?
Such analyses are very important in biomedical uses because strict
regulations may apply to the molecular weight distribution. Typically, only a
certain % may be below a specified molecular weight.
In the same way as for M
w
and M
n
, R
G,n
and R
G,w
can be calculated (after
extrapolating the data to cover the whole chromatogram). Note conventional
light scattering only gives R
G,z
.
6.4.2. R
G
-‐M analysis
from
SEC-‐MALLS
Next, we can directly analyse the R
G
-M relationship, simply by plotting R
G,i
as
a function of M
i
(double logarithmic plot):
Except for data at very high (M > 10
6
) and very low M (M < 10
5
) the data
follow a straight line. Thus, log R
G,I
is a linear function of M
i
, as expected:
log
R
G
,
i
=
α
log
M
i
+
B
(constant)
R
G
,
i
=
BM
α
By using SEC-MALLS we can – in a single experiment
47
- study the M
dependence of R
G
, and find the exponent (
α
) (after regression analysis) which
provides information about the shape of the polymer. In this case we find
α
=
0.57. What is the shape? Let us repeat the basics for idealized shapes:
47
This is only in theory. In practise we must always optimize the experiment by choosing
solvent, column (separation range), injected mass, and so on.
1.0
10.0
100.0
1000.0
1.0x10
4
1.0x10
5
1.0x10
6
1.0x10
7
R.M.S. Radius (nm)
Molar Mass (g /mol)
RM S Radius vs. Molar M ass
1802__10
Figure 51