Two enantiomers are normally so similar to each other physically that we can't separate them.
Most of the time, we don't get so lucky. The differing interactions between the two enantiomers and the chiral elements of the stationary phase result in the two compounds moving through the column at different rates.
That changes everything. One salt precipitates or forms a solid from the solution, but the other stays dissolved. This one is a diastereomer of the salt in the previous picture. The stationary phase could be simple alumina or silica like very finely powdered sand or it could be a complicated synthetic material protected by a patent.
Chiral compounds, of course, are often compared to hands. The crystals that formed had multifaceted habits that were visibly mirror images of each other. That means we now have a mixture of diastereomers. Two enantiomers are like a right hand and a left hand, mirror images of each other that look exactly the same but are instead completely opposite.
That's because two enantiomers may have identical physical properties, but they may have very different biological properties. The mobile phase could be a gas, allowed to flow through the column from a compressed tank, or a liquid, pushed through with a pump. In case you aren't familiar with chromatography, we should take a look at it in general terms first. The fact that the interaction of chiral compounds with other chiral compounds is influenced by their stereochemistry is often compared to the fit of your two hands to the same glove.
Chromatography describes the separation of compounds through their different physical interactions between two different phases as the compounds pass through a long column or tube.
Compounds that are held more firmly by the stationary phase will move more slowly. The classic example of this method of obtaining one isomer of a compound involves the formation of a diastereomeric salt. Another material, called the mobile phase, flows through the column. Start reading the comics page. Two protons are transferred from the phenylsuccinic acid to the proline.
A tug-of-war ensues between the mobile phase and the stationary phase. One of these is identical among all the molecules in the sample; that's the one from the pure chiral resolving agent. Not surprisingly, one hand fits the glove better than the other one. The phenylsuccinic acid gave up two positives, so it is a dianion. The compound that fits together with the chiral stationary phase will slow down and take longer to come through the column.
The third compound should contain both chiral centres: In the cell, they may encounter other chiral materials, including proteins and sugars.
The column is packed with one material, called the stationary phase, whose job it is to stick to the compounds and keep them from moving through the tube.