Size exclusion chromatography, also sold as gel filtration chromatography: choosing a size exclusion chromatography column or a size exclusion column by fractionation range and resin, and why loading volume decides what you get

Setting it up so it resolves

1. Fractionation range must bracket your species. A resin separates only within its stated range. Anything above elutes in the void and anything below elutes together at the end, so aggregate, monomer and fragment all have to fall inside the range if you intend to distinguish them. Choosing the resin from the species you need resolved is the first decision.
2. Load a small fraction of the bed volume. Resolution falls steeply as the loaded volume rises, and this is the dominant error in practice. Size exclusion is a low-capacity technique by design, so a run that loads too much to save time produces a chromatogram that cannot be interpreted.
3. Flow rate and bed height. Slower flow and a taller bed both improve resolution and both cost time. For a polishing step the trade usually favours resolution; for a buffer exchange, speed is fine because no separation is being attempted.
4. Buffer prevents unwanted interactions. The mode is supposed to be purely size based, and it stops being so when the protein interacts with the matrix. Adequate ionic strength usually suppresses this; a protein tailing badly on a size exclusion column is usually telling you the buffer is wrong rather than the resin.
5. Calibrate before you quote a size. Apparent molecular weight from a size exclusion column is relative to the standards used and to the shape of the molecule. An elongated protein reads larger than a globular one of the same mass, so treat the figure as a comparison rather than a measurement unless the column is coupled to light scattering.

Where it fits in a purification route

Size exclusion is a polishing and buffer-exchange step rather than a capture step, because it cannot be loaded heavily and it dilutes. Placing it first wastes its capacity on material an affinity or ion exchange step would have removed in one pass.

As a final step it does two useful jobs at once: it removes aggregate and it delivers the product into the buffer you want. That combination is why it survives as the last step in most protein routes.

Analytical size exclusion

Run analytically, the technique is the standard way to quantify aggregate, and for that purpose the method needs to be treated as a quantitative assay with its own validation and its own records rather than as a check.

Coupling to multi-angle light scattering removes the shape dependence and gives an absolute mass, which is worth the additional instrument wherever the aggregate figure is going to be relied on.

Common questions

What does size exclusion chromatography separate?

Molecules by hydrodynamic size, with larger species eluting first because they are excluded from the pores. Within a resin's fractionation range it is the standard way to separate aggregate from monomer.

How much sample can I load?

A small fraction of the column volume, and resolution falls quickly as that fraction rises. It is a low-capacity technique, which is why it is used for polishing rather than capture.

Is size exclusion chromatography the same as gel filtration?

Effectively yes. Gel filtration is the older name, used most often for aqueous protein separations, while size exclusion is the general term including organic phase work.

Can I use it to measure molecular weight?

Only relative to the standards and the shape of the molecule, since elongated proteins read larger than globular ones of equal mass. For an absolute value, couple the column to multi-angle light scattering.

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