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Chromatography is a way to separate components of a mixture (like different proteins) based on how they interact differently with a material. We take a mixture of things in a “mobile phase” (like a mix of proteins in a buffer (pH-stabilized salt water) and get it to travel through some “stationary phase” (like little beads called resin packed into a column). As the things in the mix travel through, they interact differently with the mobile phase (e.g. some proteins bind the beads, others don’t), so they get separated based on which phase they’d rather hang out with (if you’re doing column chromatography, either some things get stuck and don’t come out, or they come out, but with a delay). 

If you came to this page from one of my blog posts, I was probably referencing a specific type of chromatography called protein chromatography, which separates proteins based on how different proteins interact differently with different types of resin (little beads) that we fill cylindrical glass or plastic columns with. There are different types of protein chromatography and a lot of them involve the protein you want sticking to the column under initial conditions (due to charge-opposite charge interactions in ion exchange chromatography, or some specific feature like a genetically-engineered affinity tag (e.g. 6XHis or Strep) in affinity chromatography). If your protein sticks but others don’t, you can wash all those others off and then change the conditions so your protein gets pushed off. An exception is size exclusion chromatography (SEC, aka gel filtration (GF)), in which the proteins don’t stick to the beads, but they travel through tunnels in the beads and smaller proteins can fit through more tunnels so they have to travel further (and thus slower) and thus you can separate proteins based on their size/shape.

Sometimes we do protein chromatography using “gravity flow”, relying on gravity to drip the protein through. Other times, to help make the process easier, we often use a liquid handling machine, like an AKTA FPLC machine. FPLC stands for fast protein liquid chromatography & it uses pumps to push your proteins through the column. More here: http://bit.ly/meettheakta