Page 325 - Multidimensional Chromatography

Industrial and Polymer Applications 315

this being achieved by a forward flow of HPLC eluent and then backflushing after
the elution of the three-ring PAC. Analysis of the backflush fraction revealed the
presence of some low-molecular-mass polar PACs, including nitrogen-containing
PACs as well as higher-molecular-mass PAC species, e.g. containing 4–5 rings.

12.6 SEC–REVERSED PHASE LC APPLICATIONS

Quantitative determination of the polymer additives in an acrylonitrile– butadiene–
styrene (ABS) terpolymer by using microcolumn size exclusion chromatography
with THF as the eluent, coupled on-line to a reversed phase column employing typi-
cal solvent systems such as acetonitrile–water has been demonstrated (16). A pre-
separation was carried out by size exclusion chromatography using a
narrow-pore-size packing (50 Å). This packing completely resolved the smaller-
molecular-size additives from the polymer fraction, as can be seen in Figure 12.10
(a). For the transfer from the SEC system to the reversed phase LC system, a 6-port
Valco valve with a 10 L external loop was used. The transferred fraction was then
analyzed by reversed phase HPLC for determination of the polymer additives
(Figure 12.10(b)). The introduction of 6 l of the THF fraction from the SEC sys-
tem into the reversed phase system allowed good resolution between the compo-
nents, and gave peak shapes which were not broad or distorted. The use of
microcolumn SEC permitted a small THF volume containing the additive fraction
to be transferred. However, if a larger i.d. column had been used, a greater volume
of THF would need to be transferred, thus causing broad and distorted peaks in the
second dimension.
Johnson et al. (17) have coupled an SEC system, operating in the normal phase
mode, using Micropak TSK gels with THF as the eluent, to a gradient LC system
in the reversed phase mode, using MicroPak-MCH (monolayer octadecylsilane
phase) with acetonitrile–water as the eluent, for the analysis of various additives
in rubber stocks. These additives include carbon black, processing oils, antioxi-
dants, vulcanizing accelerators and sulfur. The rubber stocks that were analyzed
were butadiene–acrylonitrile (Chemigum N-615) and styrene–butadiene (Plioflex
1502) copolymers. The compositions of the compound rubber stocks is presented
in Table 12.1.
The system used in this analysis consisted of four columns, with the first three
being size exclusion columns (50 cm 3000H, 50cm 2000H and 80 cm 1000H
Micropak TSK gels), followed by a C18 analytical column (Micropak MCH). The
chromatogram of the Chemigum rubber stock obtained from the three coupled SEC
columns can be seen in Figure 12.11(a). After this stage, a 10 L fraction of each
peak in the SEC chromatogram was transferred to the RPLC system via an injection
valve. In this mode, a gradient was applied to elute the more hydrophobic compo-
nents. Standards were run in order to identify the retention times of the components
in the SEC coupled system, as well as in the SEC–RPLC system. RPLC traces
are shown for two the fractions, i.e. dibutylphthalate and elemental sulfur,