Sorting by Electric Charge
So far our stationary phases have held molecules back by greasiness. But molecules can also carry an electric charge — positive or negative — and opposite charges attract. So coat the stationary phase with fixed charges, and it will grab oppositely-charged molecules from the flow and let the rest pass. This charge-based separation is ion-exchange chromatography.
Picture a stationary phase studded with negative charges. Positively-charged molecules stick to it and are held back; neutral and negative ones flow on by. To release the trapped positives later, you flush the column with a mobile phase full of competing positive ions — a flood of rivals that elbow the analytes off the charged sites and back into the flow. Ion exchange is the heart of water-softening filters and the standard way to separate charged molecules like amino acids and proteins.
Sorting by Size: A Maze of Tiny Pores
Here is a separation that turns intuition upside down. Pack the column with porous beads riddled with tiny tunnels. Big molecules are too large to fit into the tunnels, so they can only flow around the beads — a short, direct path — and leave early. Small molecules wander deep into the maze of pores, take a long, winding detour, and leave late. Sorting purely by molecular size this way is size-exclusion chromatography.
Because it sorts by size, this mode is the favourite for sizing up big molecules like proteins and plastics — telling apart a giant chain from a medium one from a small fragment. It is also gentle, since nothing has to stick and let go, which suits delicate biological molecules that might be damaged by clinging too hard to a stationary phase.
Sorting by a Lock-and-Key Fit
The most exquisitely selective mode catches just one molecule out of thousands. The trick is to bolt onto the stationary phase a special partner — a molecular "lock" — that the target molecule, and only the target, fits like a key. Pour a complex mixture through, and everything washes straight past except the one molecule that snaps into the lock and is held. This shape-recognising method is affinity chromatography.
Then you change the conditions just enough to loosen the lock's grip, and the one captured molecule washes off — now spectacularly pure, plucked from a crowd. Affinity chromatography is how laboratories purify specific antibodies and many medicines: a separation built not on stickiness or size, but on precise biological recognition.
No Mobile Phase: Racing in an Electric Field
Now drop the idea of a flowing mobile phase entirely. Take a slab of soft jelly-like gel, place charged molecules at one end, and switch on an electric field across it. Because opposite charges attract, charged molecules feel a steady pull and crawl through the gel toward the far electrode. This separation by movement in an electric field is electrophoresis — literally "carried by electricity."
Two things decide how fast a molecule travels: how much charge it carries (more charge, stronger pull) and how easily it threads through the gel's mesh (smaller and more streamlined, faster). The gel itself acts like the size-sorting maze from earlier, slowing big molecules. So molecules sort out into bands by a blend of charge and size — and for many molecules, charge depends on the surrounding acidity, since at a special acidity called the isoelectric point a molecule carries no net charge and simply stops moving.
Electrophoresis Goes Down a Hair-Thin Tube
Slab gels are slow and fiddly. The modern upgrade shrinks the whole race into a single hair-thin glass tube filled with liquid, with the electric field applied across its length and a detector watching near the far end. Molecules race down the narrow channel and are clocked as they pass, drawing a chromatogram-like trace of peaks. This miniaturised, instrument-friendly form is capillary electrophoresis.
The thin tube has a quiet advantage: it sheds heat fast. An electric field warms the liquid, and heat blurs the bands; a hair-thin tube cools so well that you can crank the field up high for fast, razor-sharp separations. Capillary electrophoresis can resolve molecules that differ only slightly in charge, making it a favourite for analysing DNA fragments and proteins with exquisite resolution.