Chemistry Reference
In-Depth Information
FIGURE 3.9
Typical GPC raw data. The units of the vertical axis depend on the detectors used, while
those on the horizontal axis are elapsed time. In this case the lower curve is that of the
differential refractometer, while the upper curve is the trace produced by a continuous
viscometer (which is described briefly in
Section 3.4.4
). The curve proceeds from left to
right.
To normalize the chromatogram, a baseline is drawn through the recorder
trace, and chromatogram heights are taken for equal small increments of elution
volume. (Accurate operation requires that the baseline be straight through the
whole chromatogram.) An ordinate corresponding to a particular elution volume
is converted to a weight fraction by dividing by the sum of the heights of all the
ordinates under the trace. (Recall the mention of normalization in
Section 2.3
.)
Corrections for instrumental broadening (also called axial dispersion) are also
sometimes applied
[16]
. This phenomenon arises because of eddy diffusion and
molecular diffusion at the leading and trailing edges of the pulse of polymer solu-
tion
[17]
. The result is asymmetrical, Gaussian spreading of the GPC chromato-
gram in which the observed range of elution volumes e
xce
eds that which
corresponds to
th
e real range of solute sizes. The calculated
M
n
is lowered and
the calculated
M
w
is raised to a lesser extent as a consequence. A related phenom-
enon involves a skewing of the GPC trace toward higher elution volumes and
lower molecular weights. This results from the radial distribution of velocities in
fluid flow (
Fig. 3.6a
). Its importance varies with the viscosity of the solution
and depends therefore on the high-molecular-weight tail of the polymer molecu-
lar weight distribution. A number of procedures have been proposed to correct
the raw GPC trace for instrumental broadenin
g
[1
8]
.
Such adjustments can
be neglected for most synthetic polymers w
ith
M
w
=
Ske
wi
ng correc-
tions require independent measurements of
M
n
by osmometry or
M
w
by light
scattering.
M
n
.
~
2
:
