29.1 Chromatography Basics

A useful set of techniques for separating mixtures is called chromatography. There are many different chromatography methods.  Each has its own purposes and limitations.

All forms of chromatography work on the same general principle.  There is always a stationary phase which does not move and a mobile phase which does. The various components in the mixture being chromatographed separate from each other because they are more strongly held by one phase or the other. Those which have the greatest affinity for the mobile phase move along the fastest.

As the mobile phase moves along the stationary phase, the sample partially dissolves and travels along the stationary phase. Different substances will travel different distances along the stationary phase. The distance that a substance will travel depends on how strongly it adheres to the stationary phase (a process called adsorption) versus how much time it spends dissolved in the mobile phase.  The more a substance adsorbs, the less it dissolves and the less it moves along the plate. The less a substance adsorbs, the more it dissolves and the farther it travels. The process is continued until a good separation is created. In this manner, a mixture of substances may be separated.

The stationary phase acts as a constraint on many of the components in a mixture, slowing them down to move slower than the mobile phase. The movement of the components in the mobile phase is controlled by the significance of their interactions with the mobile and/or stationary phases. Because of the differences in factors such as the solubility of certain components in the mobile phase and the strength of their affinities for the stationary phase, some components will move faster than others, thus facilitating the separation of the components within that mixture.

Perhaps the simplest of these techniques to describe is paper chromatography (Figure 29.1a.), which is described in the next section.

The development of chromatography is one of the major revolutions in technique in the history of chemistry, comparable to that which followed the development of an accurate balance. Separations which were previously considered impossible are now easily achieved, sometimes with quite simple apparatus. This technique is particular essential to the science of biochemistry, in which complex mixtures are almost always encountered. In the field of environmental chemistry, chromatography has helped us separate and detect very low concentrations of contaminants like DDT or PCB (polychlorinated biphenyls). The major drawback to chromatography is that it does not lend itself to large-scale operation. As a result, it remains largely a laboratory, rather than an industrial, technique for separating mixtures.

Watch Synthesis and Column Chromatography: Crash Course Organic Chemistry #25 on YouTube (12 mins)  *Chromatography starts about 2 min mark

Attribution & References

Except where otherwise noted, this page is adapted by Samantha Sullivan Sauer from:

• “10.23: Chromatography” In ChemPRIME (Moore et al.) by Ed Vitz, John W. Moore, Justin Shorb, Xavier Prat-Resina, Tim Wendorff, & Adam Hahn, licensed under CC BY-NC-SA 4.0 and
• Paragraph that starts with “The stationary phase” is from “Chromatography” In Instrumentation and Analysis by Sean Gottlieb (UCD), Jessica Hosfelt (UCD) and Chemistry LibreTexts, licensed under CC BY-NC-SA 4.0.