Size-exclusion chromatography (SEC), also known as gel permeation chromatography (GPC) for organic polymers or gel filtration chromatography (GFC) for aqueous systems, separates molecules based on their hydrodynamic volume rather than chemical interaction with the stationary phase. A sample introduced into the column is carried by the mobile phase through a bed of porous beads. Molecules larger than the largest pores (above the exclusion limit) are unable to enter the pores and elute first. Molecules small enough to penetrate the entire pore network (below the permeation limit) elute last. Between these limits, molecules elute in order of decreasing size.
The stationary phase consists of cross-linked polymer beads with controlled pore size distributions. Common materials include polystyrene-divinylbenzene for organic SEC, dextran (Sephadex), agarose (Sepharose), and polyacrylamide for aqueous SEC. The pore size is selected based on the molecular weight range of the analyte; columns with different pore sizes can be connected in series to extend the separation range. Particle size influences both resolution and backpressure, with smaller particles (3-5 µm) providing higher efficiency.
Molecular weight calibration is performed using narrow-distribution polymer standards of known molecular weight, typically polystyrene, poly(methyl methacrylate), or poly(ethylene glycol), depending on the sample type. The calibration curve plots log(Mw) versus elution volume. From the chromatogram, average molecular weights are calculated: number-average (Mn), weight-average (Mw), and peak molecular weight (Mp). The polydispersity index (PDI = Mw/Mn) indicates the breadth of the molecular weight distribution, with PDI = 1 for monodisperse samples.
Applications of SEC center on polymer characterization, including determination of molecular weight distribution, monitoring of polymerization kinetics, and analysis of copolymer composition. SEC is also employed in biochemistry for protein purification and desalting (buffer exchange), in the food industry for analyzing polysaccharides and proteins, and in petrochemical analysis for characterizing heavy fractions. Aqueous SEC (GFC) is widely used for analyzing water-soluble polymers, proteins, and nucleic acids under non-denaturing conditions.
Limitations of SEC must be considered during method development. Non-size-exclusion effects such as electrostatic interactions (ion exclusion or ion inclusion on charged stationary phases) and hydrophobic adsorption can distort elution profiles. Sample concentration must be kept low (typically < 2 mg/mL) to avoid viscous fingering and overloading effects. Column choice is critical: the pore size must match the sample’s molecular weight range, and flow rates must be kept moderate (0.5-1.0 mL/min) to minimize shear degradation of high-molecular-weight polymers.
