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SOLID PHASE MICROEXTRACTION: HOW CHROMATOGRAPHY WORKS

المؤلف:  Max M. Houck، Jay A. Siegel

المصدر:  Fundamentals of Forensic Science

الجزء والصفحة:  p130-131

2026-07-05

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SOLID PHASE MICROEXTRACTION: HOW CHROMATOGRAPHY WORKS

Almost all chromatographic methods work on the same principle. They differ in how the experiment is set up and some of the analytical details. The components of the analyte will show some differences in at least one property that is exploited by the chromatography experiment. That will be the basis for the separation. The original experiment, cited above, where plant pigments are separated in a column of silica, illustrates the principles. In all forms of chromatography, there are two phases present: a stationary phase and a mobile phase. The stationary phase is a finely divided solid material or a vis cous liquid that is contained within a long column. The mobile phase is a liquid (or liquid solution) or a gas under pressure. The mobile phase moves through the stationary phase carrying the analyte mixture with it. Depending upon their affinity for the stationary phase, the components of the analyte move quickly or slowly through the column, separating from other components along the way. In the plant pigment separation cited above, the column of silica is an example of a stationary phase. The plant pigment mixture is dissolved in a solvent that is then poured through the stationary phase. The solvent is the mobile phase. If the stationary phase had no affinity for the analyte components, then they would all travel together right along with the mobile phase and emerge at the bottom together. The stationary phase is designed to attract or, in some cases, repel, certain members of the analyte, each one in a different way. This means that the progress of each component of the analyte will be affected by the stationary phase, hopefully in a different way from all of the others. When the mobile phase has completed its journey through the column, the various pigments have been held up by the stationary phase and form colored bands at various points in the column. To recover one or more of the pigments, they would have to be eluted off the column using a suitable solvent. Each pigment has a distinct visible color and can thus be detected visually. In modern applications of chromatography, most analytes have no color and there are such small quantities being separated, that no color would be visible anyway. Therefore each type of chromatography has its own types of detectors that use various properties of the analytes to signal their presence. In chromatography, the relationship between the stationary phase and mobile phase is often described as normal or reverse phase. In normal phase chromatography, the mobile phase is less polar than the stationary phase. This is always the situation with GC, where the mobile phase is a nonpolar inert gas. In reverse phase chromatography, the opposite is true. The mobile phase is more polar than the stationary phase. In LC and thin-layer chromatography (TLC), either normal or reverse phase can be used.