Size-exclusion chromatography (SEC) is a pervasively used technique to determine the molecular-weight distributions of all kinds of soluble polymers. Delivering distributions, molecular-weight averages and measures of the dispersity of polymers within one straightforward chromatographic experiment is the great strength of SEC.
A mental picture of the SEC mechanism is that macromolecules can only penetrate part of the pore volume. All analyte polymers are expected to elute between the exclusion volume (total mobile-phase volume in the column outside the packing particles) and the total-permeation volume (exclusion volume plus the total pore volume).
The range of molecular weights covered and the selectivity (the extent of separation) are determined by the pore-size distribution of the column-packing material. The choice of solvent is essential to ensure good sample solubility and absence of energetic interactions between the analyte polymer and the surface of the packing.
To convert SEC elution profiles to molecular-weight distributions a calibration is required. Most commonly this is done by creating a calibration curve that displays the (logarithm of the) molecular weight of a narrow standard against the elution time. Ideally, such narrow standards are structurally identical to the analyte polymers. If these are not available relative data may be reported based on calibration with reference standards.
Some specific detectors exist that – in principle – allow calibration without reference standards. These include static light scattering detectors, such as multi-angle light scattering (MALS) detectors, dynamic light-scattering (DLS) detectors and viscometers.
