Size Exclusion Chromatography

Introduction

This technique, also known by the name Gel Permeation Chromatography (GPC) is used to determine the molar mass distribution of polymers in solution.

A polymer sample in solution is injected on top of a column. A flow of liquid (eluent) takes the sample through the column where the molecules are separated according to their size in solution. The largest molecules elute first.

The separation principle is an exclusion of large molecules from pores in the matrix of the particles in the column, letting them pass the column in a shorter time.

The equipment is in principle a liquid chromatograph, often with the option that samples can be run at elevated temperatures. Polyethylene for example can be run at 140°C in trichlorobenzene. The separation is normally isocratic, using only one eluent.

The detector is often a refractive index detector or a light scattering detector, but any LC detector could be used. The advantage of a light scattering detector measuring in a minimum of two angles is that the absolute molar mass can be determined directly.

Since the diameter of a polymer molecule in solution is dependent on the solvent, all other detectors have to be calibrated using polymer standards of differing molar mass with narrow distributions.