11
precision of measurement suffers greatly below
R
g
of 20 nm (MW
≈
250K for flexible coils) and
becomes impossible below 10 nm (MW
≈
80K).
The radius of gyration can be computed from the intrinsic viscosity and molecular weight
via the Ptitsyn-Eizner modification of the Flory-Fox equation.
[A.1-5]
[ ]
(
)
(
)
η
ε
ε
M
R
g
=
−
+
Θ
0
2
3
1 2 63
2 86
6
.
.
where
ε
=
−
2
1
3
a
and
with
a
= Mark-Houwink exponent.
Although this viscometry approach is certainly non-rigorous and would likely be
inaccurate for polymer molecules deviating significantly from the flexible coil model, it has the
necessary virtue of excellent precision over all ranges of molecular size and weight. Precision
and range of applicability are most important because it turns out that any subsequent
calculations of secondary structural effects,
e.g.
, branching, are only suitable for relative
measurements. This is because those theoretical calculations involve the same type of
assumptions that affect Equation [A.1-5].
A.1.b General Layout
LIGHT SCATTERING PLUMBING AND FLOW PATH
Refer to the Detector Plumbing Diagram in
Figure 3
for the flow through the system.
Flow enters the light scattering cell from the in-line RALLS filter and then exits to the
refractometer detector of the 302TDA. A laser beam is focused on the end of the cell and
scattered light is measured at 90 degrees and at 7 degrees.
REFRACTOMETER PLUMBING AND FLOW PATH
Refer to the Detector Plumbing Diagram
Figure 3
for a schematic of the flow through the
refractometer. Note that the diameter of the tubing is not the same throughout the
refractometer. The tubing from the inlet through to the sample side of the cell is 0.01”id; all
other tubing is 0.04”id, from the bottom of the reference cell and out to the RI Purge solenoid.
Although the pressure drop also depends on length, the strong dependence on bore radius or
diameter means that the wide bore tubes shown have small resistance compared to the narrow
bore tubes. Only with this relationship clearly in mind can one understand the flow patterns
described below.
The reference cell of the refractometer is filled with mobile phase solvent by opening the
RI purge valve. Solvent flow is then diverted from the Viscometer to the Reference side of the
RI flow cell. Solvent then passes through a 20 micron filter, the positive Inlet Pressure
transducer cavity and Cross 3. It then flows through the negative Inlet Pressure transducer
cavity, Cross 2 and finally exits to waste via the Outlet bulkhead port fitting.
VAPOR SENSOR DETECTOR
Refer
to
Figure 9
for the location of the vapor sensor in the Model 302. The sensor is
mounted in the rear of the TDA module. The 4-lead sensor plugs into a socket for easy
replacement. Note that the socket has a notch cut for the sensor key alignment. Hence, the