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Hydrodynamic size describes the effective dimensions of a dissolved molecule moving with its surrounding solvent. It depends on molecular geometry, internal shape, and solvent conditions, rather than just molar mass. SEC pore access probes size-dependent accessibility of the packing.

Polystyrene is a polymer used as a calibration-standard family in organic-solvent size-exclusion chromatography. Its hydrodynamic size in a specified solvent supplies a conditional SEC molar mass calibration for other dissolved chains.

SEC molar mass calibration measures standards under the same column, solvent, and temperature conditions as the unknown. Over a suitable fractionation interval, a common empirical approximation iswhere $M_{\mathrm{ref}}$ specifies the molar mass unit and $b$ has inverse-volume units. Interpolation is conditional on the standards' size–molar mass relationship. SEC measures access associated with hydrodynamic size, so a branched polymer, an extended chain, and a compact protein need not share that relationship.

In this model the $80,000$, $40,000$, and $20,000\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ standards have hydrodynamic radii $2\sqrt{2}$, $2$, and $\sqrt{2}\mathrm{nm}$ and elute at $7$, $9$, and $11\mathrm{mL}$. An unknown at $8\mathrm{mL}$ would be assigned $M_{\mathrm{app}}=40,000\sqrt{2}\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ by this calibration. But a $40,000\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ species that expands from $2$ to $2\sqrt{2}\mathrm{nm}$ elutes at $7\mathrm{mL}$ and is assigned $80,000\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ without gaining any mass. The numbers demonstrate ambiguity, rather than predict a real unfolding transition.

Changes in conformational isomerism, protein folding, protein denaturation, and protein aggregation can therefore shift peaks. Association usually increases size; compaction can reduce size at unchanged molar mass. Adsorption creates a different ambiguity: delayed elution need not indicate a smaller species. A different solvent can change both polymer coil dimensions and interactions with the packing. The SEC protein conformation experiment tests these alternatives with additional detectors.

In ideal SEC pore access, geometric exclusion changes the fraction of pore liquid a solute can occupy. Diffusion permits entry into and exit from accessible pores; the through-flow chiefly transports the sample between packing particles. A hydrodynamic radius is a useful size descriptor, but actual pore entry also depends on shape and pore geometry. A flexible polymer coil is not a rigid ball with a perfectly sharp entry diameter.

The extra delay is best understood as extra accessible storage, rather than a prescribed longer streamline. Pore visits are reversible and random. An entirely excluded species samples the SEC void volume; a sufficiently small nonbinding species samples the SEC total accessible liquid volume. Intermediate species lie between these limits. Larger hydrodynamic size generally means earlier elution within the column's SEC fractionation range.

Size-exclusion chromatography separates dissolved molecules by their hydrodynamic size in the chosen solvent. A larger species that cannot enter the packing's pores travels with the liquid between particles and elutes earlier. Smaller species explore more internal liquid and elute later. The column is not a sieve that permanently traps the large species: exclusion makes them leave first. “Gel filtration” is common for aqueous separations; “gel permeation chromatography” is common for organic-solvent analysis of polymers. SEC molar mass calibration translates size-dependent elution into a standard-relative estimate of molar mass, rather than a universal absolute measurement.

This makes SEC useful for separating proteins from small dissolved species, comparing protein aggregation, and characterizing dissolved polymer chains. The residual-group monitoring of polyurethane formation experiment already illustrates polymer calibration: its reported molar masses are relative to polystyrene standards. Size-exclusion characterization of chain polymers explains why an elution trace alone cannot count chains or establish their growth mechanism.