Beginner's Guide to Size-Exclusion Chromatography

• Why is GPC important?
• How GPC works
• GPC systems

Gel permeation chromatography (GPC) is one of the most powerful and versatile analytical techniques available for understanding and predicting polymer performance. It is the most convenient technique for characterizing the complete molecular weight distribution of a polymer.

Waters commercially pioneered GPC in 1963. Since then, Waters has continued to develop and explore new GPC applications and improve the instrumentation that makes GPC so powerful.

Why is GPC important?

GPC can determine several important parameters. These include number average molecular weight, weight average molecular weight, Z weight average molecular weight, and the most fundamental characteristic of a polymer its molecular weight distribution.

These values are important, since they affect many of the characteristic physical properties of a polymer. Subtle batch-to-batch differences in these measurable values can cause significant differences in the end-use properties of a polymer. Some of these properties include:

Materials characterization

Understanding the makeup of a polymer is particularly important due to the variety of resins available for the same purpose, the high cost of specialty resins or compounds, and the value added to the polymer during manufacturing. For example, the cost of a resin used in a printed circuit board is very low, but the cost of the finished board is very high. Poor quality resin can result in an unacceptable finished circuit board.

Where a polymer's end-use application requires precision performance or endurance under harsh conditions, the need for polymer characterization is particularly acute. Because GPC fulfills these needs better than any other single technique, it has become an extremely valuable tool for materials characterization in the polymer industry.

Telling good from bad

Two samples of the same polymer resin can have identical tensile strengths and melt viscosities, and yet differ markedly in their ability to be fabricated into usable, durable products. These differences can be attributed to subtle, yet significant variations in the molecular weight distributions of the two resin samples. Such differences, if undetected, can cause serious product defects.

How GPC works

GPC separates molecules in solution by their "effective size in solution." To prepare a sample for GPC analysis the resin is first dissolved in an appropriate solvent.

Inside the gel permeation chromatograph, the dissolved resin is injected into a continually flowing stream of solvent (mobile phase). The mobile phase flows through millions of highly porous, rigid particles (stationary phase) tightly packed together in a column. The pore sizes of these particles are controlled and available in a range of sizes.

The width of the individual peaks reflects the distribution of the size of molecules for a given resin and its components. The distribution curve is also known as the molecular weight distribution (MWD) curve. Taken together the peaks reflect the MWD of a sample. The broader the MWD, the broader the peaks become and vice versa. The higher the average molecular weight, the further along the molecular weight axis the curve shifts and vice versa.

You can see then how easily the MWD profiles of two resins can be compared. If the MWD profile of an incoming resin doesn't match that of the control resin (i.e. one that is known to process well) closely enough, the incoming resin can be modified or process conditions can be changed to make sure the resin processes properly. If the differences between the control resin and the incoming resin are too severe, the incoming resin can be returned to the supplier as unacceptable.

GPC Systems

In designing instrumentation for GPC, a variety of requirements must be satisfied. Injectors are needed to introduce the polymer solution into the flowing system. Pumps deliver the sample and solvent through the columns and system. Detectors monitor and record the separation. Data acquisition accessories control the test automatically, record the results, and calculate the molecular weight averages. The gel permeation chromatograph contains a number of different components that work together to provide optimum system performance with minimum effort. Schematic of a basic gel permeation chromatograph.

Pumps the polymer in solution through the system.

Different polymers produce solutions of different viscosities. To compare data from one analysis to the next, the pump must deliver the same flow rates independent of viscosity differences. In addition, some detectors are very sensitive to the solvent flow rate precision. Such constant flow must be a critical feature of the instrument.

Introduces the polymer solution into the mobile phase.

The injector must be capable of small volume injections (for molecular weight determinations) and large volume injections (if fraction collecting is desirable). The injector should not disturb the continuous mobile phase flow. It should also be capable of automatic multiple sample injection when the sample volume is large.

Efficiently separates sample components from one another.

High efficiency columns give maximum separating capability and rapid analyses. Every column must provide reproducible information over extended periods for both analytical and fraction collecting purposes.

Monitors the separation and responds to components as they elute from the column.

Detectors must be nondestructive to eluting components if they are to be collected for further analysis.

In addition, the detectors must be sensitive and have a wide linear range in order to respond to both trace amounts and large quantities of material if necessary.

Since all compounds refract light, the differential refractometer (RI) is referred to as a "universal" detector. As a result it is the most widely used detector to monitor molecular weight distribution. The refractive index of polymers is constant above approximately 1000 MW. Therefore, the detector response is directly proportional to concentration.

Beside information about molecular weight averages and distribution obtained with RI, the use of UV absorbance detectors may provide information about composition, while on-line light scattering detectors and viscometers provide information about polymer structure.

Automatically calculates, records, and report numerical values for Mz, Mw, Mv, Mn, and MWD.

Data systems can also provide complete control of GPC systems so that large numbers of samples can be run unattended and raw data can be automatically processed. Today's GPC software offerings need to be able to provide special calculations for multi-detection processing, band broadening correction, special calibration routines and polymer branching determination, just to name a few.