7
A. DESCRIPTION OF INSTRUMENT
A.1 GENERAL DESCRIPTION
A.1.a Purpose and Applications
The Model 302 TDA Differential Refractometer is the basic detector component of a
system for polymer analysis by gel permeation chromatography (GPC), also called size
exclusion chromatography (SEC). SEC involves separation through the hydrodynamic volume
of the molecular distribution. The refractometer can also be used to determine the
concentration of solutions after calibration.
SEC SEPARATION
In SEC, columns are packed with porous packing material. Molecules which are smaller
than the pores are trapped for short periods of time within the pores with which they come in
contact. A common term for this is that “the molecules are totally included.” Molecules larger
than the size of the pores are not trapped at all, and will elute earliest. These molecules are
said to be “totally excluded.” In these two cases, the column has little or no separation as a
function of molecular weight and the values fall on a non-linear section of the calibration curve
(log Hydrodynamic Volume (V
h
) vs. Retention Volume). Within a given column, there is a
distribution of pore sizes which allows most molecules to stay resident in many of the pores
while being excluded from many others. Molecules in this size range, generally will fall on a
fairly linear calibration range of the calibration curve.
MEASURING THE INTRINSIC VISCOSITY DISTRIBUTION
The Model 302 TDA is specifically designed for polymer analysis by SEC
3
or FIPA.
In an SEC
3
system, the Deflection-type refractometers are the most common type used in SEC
separation; they operate by measuring the deflection of a light source caused by a difference in
the refractive index of a sample-solvent mixture and pure solvent. Because different polymer
types have different differential refractive index responses with concentration (dn/dc), each
polymer type must have a unique RI response factor. Therefore, the area of the RI is
proportional to the product of the concentration, injection volume and inverse flowrate of the
sample only across a single sample type. The RI area is exactly proportional to the quantity of
the mass injected of the sample and its dn/dc value.
Thus, the refractometer provides a signal which is generally proportional to concentration of
sample as it elutes from the column.
[A.1-1]
Y
k
dn
dc
C
i
ri
i
=
where for species
i
,
k
ri
= Refractometer response constant
dn
dc
= Polymer refractive index increment
C
i
= Concentration
If the usual assumption is made that all of the sample injected into the column elutes from the
column, the sum can be taken over all data points in the concentration chromatogram. Then
Equation [A.1-1] can be normalized, yielding a cancellation of constants.