Gel permeation chromatography
[2] The term gel permeation chromatography can be traced back to J.C. Moore of the Dow Chemical Company who investigated the technique in 1964.
When characterizing polymers, it is important to consider their size distribution and dispersity (Đ) as well their molecular weight.
GPC is a type of chromatography in which analytes are separated, based on their size or hydrodynamic volume (radius of gyration).
This differs from other chromatographic techniques, which depend upon chemical or physical interactions between the mobile and stationary phases to separate analytes.
[5] Separation occurs via the use of porous gel beads packed inside a column (see stationary phase (chemistry)).
The principle of separation relies on the differential exclusion or inclusion of the macromolecules by the porous gel stationary phase.
The entire process takes place without any interaction of the analytes with the surface of the stationary phase.
Conversely, larger analytes relative to the pores sizes spend little if any time inside the column, hence they elute sooner.
Other desirable properties of the gel forming agent are the absence of ionizing groups and, in a given solvent, low affinity for the substances to be separated.
The delivery of a constant flow free of fluctuations is especially important to the precision of the GPC analysis, as the flow-rate is used for the calibration of the molecular weight, or diameter.
Examples of GPC chromatograms of polystyrene samples with their molecular weights and dispersities are shown on the left.
By determining the retention volumes (or times) of monodisperse polymer standards (e.g. solutions of monodispersed polystyrene in THF), a calibration curve can be obtained by plotting the logarithm of the molecular weight versus the retention time or volume.
First of all, it has a well-defined separation time due to the fact that there is a final elution volume for all unretained analytes.
Also, as a technique GPC requires around at least a 10% difference in molecular weight for a reasonable resolution of peaks to occur.
[5] In regards to polymers, the molecular masses of most of the chains will be too close for the GPC separation to show anything more than broad peaks.
Another disadvantage of GPC for polymers is that filtrations must be performed before using the instrument to prevent dust and other particulates from ruining the columns and interfering with the detectors.
Field-flow fractionation (FFF) can be considered as an alternative to GPC, especially when particles or high molar mass polymers cause clogging of the column, shear degradation is an issue or agglomeration takes place but cannot be made visible.
FFF is separation in an open flow channel without having a static phase involved so no interactions occur.
