# What does all chromatography have in common?

We're learning about chromatography in class, and I'm confused about all the different types (e.g. those that separate liquid-liquid solutions vs. those that separate liquid-solid solutions vs. those that separate solid-solid solutions, etc.).

Could you provide a summary of the similarities between all forms of chromatography? (just an overview, do not require specifics)

Thanks.

## 2 Answers

Here are some similarities:

• There is always a mobile (e. g. Gas - GC, Liquid - HPLC; GPC) and a stationary phase (liquid; gel - GC, solid - LCs, GPC).
• Compounds in the sample interact different with both phases and are therefore held back stronger or lesser. This results in the accumulation of compounds that interact similar at some point in the system $-$ with some chromatographys, especially affinity chromatography, one might even get pure substances.
• The mobile phase carrys the sample and is commonly less polar than the stationary phase, except for RPCs.
• The structure of a chromatograph is basically like this:

1. transport system: reproductive transport of the mobile phase, adjustable (e. g. piston pump - LCs, compressed gas cylinder - GC)
2. sample application: import of the sample into the flowing mobile phase (e. g. injector, valve)
3. column: stationary phase and its mounting (e. g. capillary column in the column oven)
4. detector: transformation of the chemical or physical properties, which change with substance concentration, into electrical signals (e. g. UV/VIS-detector, thermal conductivity detector)
5. plotting unit: visualisation of the electrical signals (e. g. integrator, computer)

KhanAcademy video

You need to understand that the basis for this chromatography business is separating a mixture using a "stationary phase" and a "mobile phase."

I wouldn't take those terms as intuition, but column chromatography for instance. A fine powder, usually silica(Stationary phase) is put in solution with a solvent(mobile phase), then you add mixture A. The different compounds in mixture A will have different levels of "attraction" to the floating silica powder, and the part of mixture A that is "less attracted" to the stationary phase will flow out of the funnel.

One of the main issues is that sometimes this can take a very long time because the surface area of the silica may be too great decreasing flow out of the separatory funnel substantially - which is why we use high performance liquid chromatography.

It utilizes high pressure machines that offset the large surface area of stationary phase (pressure = force/surface area) thus reducing the amount of time it takes for separation.

• Your answer seems to be limited to liquid chromatography. I agree, however, that the "stationary phase" vs. "mobile phase" thing is really the core of the matter. – Ivan Neretin Sep 24 '15 at 6:23
• The attraction part does not hold for size exclusion chromatography. – WYSIWYG Sep 24 '15 at 12:28