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All forms of chromatography are differential migration processes where sample components are selectively retained by a stationary phase. The stationary phase may be an active solid or it may be an immobile liquid.
Liquid chromatography (LC) is a separation technique. The sample partitions between two phases:
- The stationary phase of large surface area
- The mobile liquid which passes over the stationary phase
Liquid chromatography can be subdivided as follows:
- Liquid/liquid chromatography (LLC): Liquid/liquid chromatography is partition chromatography. The sample is retained by partitioning between the mobile liquid and the stationary liquid. A subgroup of liquid/liquid chromatography is paper chromatography. In paper chromatography, the stationary phase is actually water held on the fibers of cellulose.
- Liquid/solid chromatography (LSC): Liquid/solid chromatography is adsorption chromatography. The adsorbents, silica gel, alumina, molecular sieve or porous glass, are packed in a column and the sample components displaced by a mobile phase. Thin layer chromatography can be considered essentially liquid/solid chromatography.
- Ion exchange chromatography: Ion chromatography refers to any efficient method of separating and determining ions. Modern ion chromatography involves using conventional LC equipment with a variety of polymeric columns for determining the ions in question. The mobile phase in ion exchange chromatography (IEC) is usually an aqueous buffer, the pH and ionic composition of which determines a solutes retention time. The stationary phase in IEC is a crossed-linked polymer resin with covalently attached ionic functional groups.
- Exclusion chromatography: Ion-exclusion chromatography is not a form of ion chromatography because molecular species rather than ions are separated. However, ion-exclusion chromatography is an excellent way to separate molecular substances from larger amounts of ionic material in the sample. Separations are carried out on high-capacity polystyrene-based exchange resins. Ionized analytes are excluded from the polymer.
References
- D. Harvey, “Modern Analytical Chemistry”, McGraw-Hill Companies Inc., (2000)
- V.R. Meyer, “Practical High – Performance Liquid Chromatography”, Wiley, 2010
- M. McMaster, “LC/MS A Practical User’s Guide”, Wiley-Interscience, 2005
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