Dyer Scientific and Technical Translations
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1. Introduction

2. Paper

3. Thin-layer

4. Column/Liquid

5. Gas

6. High_Pressure

7. Glossary

8. References


Technical translators could happily ignore chromatography, except that it has become very important in chemical analysis, which includes pharmaceutical analysis, etc. Documents about design and construction of chromatographic equipment are relatively rare and detailed, and this paper will not have much about that. On the other hand, references to, and specifications for, chromatographic analyses are very common. I will attempt to help you make sense of those.

Chromatography is really only a method of separating materials, with identification and quantitative measurement of the separated materials done by some other procedure. However, the measurement and often the identification are bundled together with the separation in commercial equipment. As a separation process, chromatography has been used to some extent in production (preparative chromatography) when the products can be sold for enough to pay for the process. As an analytical procedure, it has revolutionized chemical analysis over the past 60 years, and is particularly useful for complex samples that cannot be handled practically in any other way.

The first known work on or with chromatography was done by Mikhail Semenovich Tswett (1872-1919), beginning about 1902. He was primarily concerned with separation of plant pigments (chlorophylls, carotenoids). Tswett's work was almost completely neglected - perhaps five researchers used it up until the early 1930s, and their results were generally ignored. A recent article on Tswett appeared in the magazine LC/GC, May 2003, pages 458-467.

For practical purposes, chromatography began with the British biochemist, A. J. P. Martin, in the early 1940s. Apparently Martin had made some notes with a fountain pen on a paper napkin. When some spilled tea wet a corner of the napkin, it spread across the paper, some of the colored dyes in the ink moved with it and Martin discovered paper chromatography. He also rediscovered column chromatography (apparently independently). The art is still progressing.

The basic concept is simple enough. Suppose that you have a container with two solvents which do not dissolve in each other (they are "immiscible"). Then add a substance that will dissolve to some extent in both of the solvents. Mix thoroughly to establish "equilibrium" and allow the solvents to separate into two "phases". The dissolved substance will be distributed ("partitioned") between the two solvents in proportion to its solubilities in them (the ratio of concentrations is the "partition coefficient"). If it is very much more soluble in one of the two solvents, nearly all of it will be in that solvent. This process is extraction, and you may have done it or seen it done in a "separatory funnel" in a chemistry course. It works well with one or a few "solutes" when the partition coefficients are very large or very small.

If you need to separate two or more solutes with partition coefficients that are not very different, you will need many separatory funnels and many people to shake them. That process was automated by Lyman Craig with the "Craig countercurrent distribution apparatus" (usually called a "Craig") about 1950. It had a long series of 50 to 100 separators mounted on a motor-driven rack which shook all of them vigorously, stopped to let the phases separate, then tilted to pour one of the phases into the next separator (all 50 to 100 of them) for the next extraction step. The effect is that one of the phases moves in steps across the other, with solutions of separated components eventually being poured out of the last separator in the series.

The big step was to make the repeated partitions continuous. Think of one of the two solvents as a thin film of one liquid firmly adsorbed on the surface of a solid "support", with a thin film of the other solvent flowing over it. They are the "stationary phase" and the "mobile phase". Then, at just one spot in the flowing film, add a drop of the solution of substances to be separated. With two thin films, equilibrium between the two phases is established very quickly just by diffusion, and the system acts like a very large number of separatory funnels. Substances that are much more soluble in the stationary phase spend most of their time in that phase are not carried very far or very fast by the mobile phase. Substances more soluble in the mobile phase move faster. Before long they separate. If the mobile phase continues to flow, some of the substances will appear in the solution coming off the end of the stationary phase much sooner than the others. If the flow is stopped before then, the substances will be at different positions along the stationary phase. This is partition chromatography.

Things get more complicated because the mobile phase can be gas or liquid, while the stationary phase, usually liquid, could be a solid in "adsorption chromatography". Solids are not used much because, once the sample components are adsorbed, it is difficult to get them back into solution. However, most of the liquid phases look very much like solids.

Use the menu on the left for a look at some of those systems.

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