21
cell window reflections, air bubbles, and particle contaminations. An example of this is where
light scattering data at four different scattering angles is presented: the 90° signal is clearly the
best among all the scattering angles, while the baseline noise is the least.
There is another advantage of choosing the right-angle light scattering detection. In the
current 2-angle or multi-angle LS instrument, a correction factor is needed to convert the
detector angle
θ
DET
to the actual scattering angle
θ
LS
. The difference between these two angles
varies, depending on the scattering angle and the bending of the scattered beam through the
liquid-glass interface. The angular correction factor is needed to compensate for this liquid-
glass interface refraction effect. Unfortunately, this angular correction of
θ
DET
to
θ
LS
is a source
of error in the molecular weight and size calculations. At the 90° angle the scattering beam
exits at a normal angle to the liquid-glass interface. This means that no detector angle
correction is needed for
θ
LS
at the 90° scattering angle.
Since the actual polymer concentration of the samples eluting from the SEC column is
highly dilute, the second term in Equation [A.2-1] is often negligible in the SEC application:
[A.2-8]
( )
θ
θ
P
M
R
KC
w
1
=
In theory, the polymer size parameter can be derived from the
P
(
θ
) function. Again, in
theory, the
P
(
θ
) function and therefore the polymer
R
g
value can be determined by a multi-angle
approach to light scattering detection. However, the molecular size measurement by SEC and
multi-angle LS has may severe practical limitations.
The
P
(
θ
) function approaches unity for molecules having sizes less than 1/20 of the
wavelength of the incident light. When that happens, the scattering intensity alone can no
longer be used as a tool to measure molecular sizes. Under the circumstances, the LS intensity
becomes a function of only the molecular weight. Light scattering loses its angular dependency
and its discriminating power on molecular sizes. As
P
(
θ
) approaches 1.0, LS becomes strictly a
MW measurement, no longer sensitive to molecular size:
[A.2-9]
w
M
R
KC
1
=
θ
or,
[A.2-10]
w
KCM
R
=
θ
Signal-to-noise limitation presents the other problem in the multi-angle light scattering
detection of the SEC effluent. Reliable
P
(
θ
) determination for molecular size measurement is
not yet possible by the SEC-multi-angle LS combination, especially for the determination of the
initial
P
(
θ
) slope at the low scattering angle near
θ
LS
approaches zero. Meaningful
R
g
values are
obtainable only for MW > 200,000 or more.
9,9
SEC-VISCOMETRY-LS
9
W. W. Yau, Chemtracts-Macromolecular Chemistry,
1
, 1 (1990).