= SEC pore access
{c}
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.
\Image[/size-exclusion-chromatography-pore-paths.png]
{title=<SEC pore access>: excluded species leave before species that explore pore liquid}
{id=image-size-exclusion-chromatography-pore-paths}
{description=Original schematic of a porous packing and representative excursions. Colored routes distinguish exclusion, partial access, and extensive access; they are not measured trajectories or a calculation of travel times. The circles represent effective <hydrodynamic sizes>, not actual <molecular geometry>. There is no <adsorption> in this illustration.}
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>.
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