Chemistry Reference
In-Depth Information
different chemical compositions. The poly-
mer fractions that appear between 1.5 and
2.2 mL elute with the initial mobile phase
and, thus, are soluble in EGMBE at 140
8
C.
Taking into account the results of our pre-
vious investigations
[33]
these fractions can
be assumed to be EP copolymers with low
ethylene contents. In addition, these frac-
tions could contain some atactic PP. The
next elution region correlates with the sol-
vent peak (n-decanol) at an elution volume
of 2.3 mL. The polymer fractions that
elute with the sample solvent n-decanol are
assumed to be EP copolymers with medium
ethylene contents. Polymers that are inso-
luble in EGMBE and do not elute with
n-decanol appear later in the chromato-
grams. They elute with the TCB gradient in
the elution volume region of 7-9 mL. These
polymer fractions are supposedly EP copoly-
mers with higher ethylene contents. As has
been shown by the CRYSTAF experiments
(see Figure 1) the samples do not contain
PP or PE homopolymer fractions.
Coupling of HT-gradient HPLC and FTIR
As is shown in Figure 3 the chromatograms
indicate that the EP copolymer samples
are separated with regard to chemical
composition by high-temperature gradient
HPLC. To prove that this is the case and
to get more information on the separation
mechanism, the HPLC system was coupled
to FTIR spectroscopy. The most suitable
way of combining the two techniques is by
using the LC-Transform interface where
the eluate coming from chromatography
is sprayed on a rotating Germanium disc.
Under high temperature spraying condi-
tions the mobile phase evaporates and the
polymer fractions are deposited as solid
layers at different positions of the disc.
Subsequently, the disc is placed in a FTIR
spectrometer and spectra are taken from all
polymer fractions.
One major issue of the spray deposition
procedure is the boiling point of the mobile
phase. In particular, when conducting gra-
dient HPLC separations, problems arise
4
4
EP 2
EP 1
3
3
2
2
1
1
0
0
0
2
4
6
8
10
12
0
2
4
6
8
10
12
Elution Volume (mL)
Elution Volume (mL)
EP 3
4
EP 4
4
3
3
2
2
1
1
0
0
0
2
4
6
8
10
12
0
2
4
6
8
10
12
Elution Volume (mL)
Elution Volume (mL)
Figure 3.
Gradient HPLC chromatograms of samples EP1-EP4, stationary phase: Nucleosil 500, mobile phase: linear
gradient EGMBE-TCB, column temperature: 140
C, detector: ELSD, sample solvent: n-decanol, sample concen-
8
tration
1.3 mg/mL.
