Handbook of Size Exclusion Chromatography and Related ...

The injectionvolume should be comparable to the mobile phase volume of
thecolumn.Mostconveniently,injectioncanbeswitchedfromsolutiontosolvent
upon detection of polymer at the column outlet. This practice guarantees that
transport of the polymer solution through the column is uniform at least for the
frontendofthetransportedsolution.Whenthesolventisinjectedintothecolumn
filled with viscous polymer solution, displacement of the viscous solution by the
nonviscous solvent may not be uniform. Rather, solvent channels may be formed
to facilitate penetration of the nonviscous fluid through the packed bed imbibed
with the viscous fluid. Then, mass transfer between the stationary phase and the
mobile phase will not be efficient. This phenomenon is known as viscous
fingering, and is widely observed at the interface between two fluids vastly
different in viscosity (19). The viscous fingering will affect mostly middle to late
fractions in HOPC.
There is asevere restriction on the flow rate available in HOPC. On the one
hand,extremelyslowflowwillcauseaproblematthefractioncollector,especially
whenthesolventisvolatile.Evaporationofsolventatthetipofthetubingwillclog
the tubing or form acolumn of partially dried polymer hanging from the tip. On
the other hand, ahigh flow rate not only increases the already high backpressure
but also increases the chance of nonuniform transport of the solution through the
column. Furthermore, the mass transfer problem will become more serious.
Typically,aflowrateof0.1or0.2mL=minshouldbeusedforacolumnof3.9mm
interior diameter.
It must be borne in mind that alarge-volume injection of the viscous
solution poses aserious problem of high backpressure, often exceeding several
thousandpsi,whichisthelimitofmostHPLCpumpsandhardware.Injectionofa
solution of a high-MW polymer through the pump head (Fig. 3b) may be
especially troublesome, because the concentrated solution can be viscoelastic,
causing malfunction of the check valves when the pump head employs a
reciprocating plunger.
HOPC is a batch separation process. A typical procedure is summarized as
follows. Prior to injection, the column is washed with the same solvent as the one
used to dissolve the polymer to separate. A solution of the polymer is injected into
the column at a constant flow rate. When the first polymer is detected in the eluent,
the injection of the solution is stopped, and instead pure solvent is injected into the
column to collect the polymer by the fraction collector and wash the column.
Polymer can be recovered from solution by evaporation or by adding a nonsolvent.
6 EXAMPLES OF SEPARATION IN A GOOD SOLVENT
HOPC studies have been carried out nearly exclusively in good solvent conditions
(1–10). This is partly to avoid deposition of injected polymer onto the pore surface
© 2004 by Marcel Dekker, Inc.