Chemistry Reference
In-Depth Information
scattering as these species elute from the
MTF column. Thus, with knowledge of
both R
G,B
and R
G,L
, one can determine the
branching index, g, across the entire MTF
elution profile. The Zimm-Stockmayer
model can then be used to predict the
number of branch sites per chain across the
MTF elution profile from g.
[13]
This process
can be repeated for each size fraction from
SEC. The distribution information poten-
tially gleaned from the SEC-MTF-light
scattering experiment cannot be obtained
by any other technique at present. For
example, SEC with molecular weight
sensitive detectors provides only an aver-
age number of branch sites at each hydro-
dynamic size interval.
silicas, obtained from Bangs Laboratories
(Fishers, IN), had very narrow size distribu-
tions in contrast to the 0.5
m material. The
nominal sizes of these materials were 0.97
and 0.8
m
m, respectively. The sizes reported
above were estimated from SEM images.
These columns were also packed by Mel
Cabey, Diazem Corporation (Midland, MI)
and the silica was not functionalized prior
to packing.
The dependence of M
C
on packing
particle size is shown in Figure 11. It is
clear that the critical molecular weight falls
off monotonically with decreasing particle
size. However, the correlation does not
appear to be linear, but with only three data
points, it is difficult to estimate the func-
tional form of the relationship with any
certainty. The decrease in M
C
with decreas-
ing particle size is consistent with previous
MTF work performed using monolithic
columns.
[1]
While the exact details of the
MTF separation mechanism are not under-
stood, it is clear that there is a certain
critical solute size that has to be reached
before the MTF mechanism is operative.
m
Particle Size Dependence of M
C
All MTF results presented to this point
were obtained using columns packed with
functionalized, polydisperse silica having
an average particle diameter of 0.5
m. To
learn more about the particle size depen-
dence of MTF, additional columns were
packed with 0.81 and 0.75
m
m silica. These
m
9.00E+05
8.00E+05
7.00E+05
6.00E+05
5.00E+05
4.00E+05
3.00E+05
2.00E+05
1.00E+05
0.00E+00
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Particle size (
ยต
m)
Figure 11.
Dependence of critical molecular weight (M
C
) on packing particle size.
