Liquid–Solid Adsorption Chromatography
In liquid–solid adsorption chromatography (LSC) the column packing also serves as the stationary phase. In Tswett’s original work the stationary phase was finely di- vided CaCO3, but modern columns employ porous 3–10-μm particles of silica or alumina. Since the stationary phase is polar, the mobile phase is usually a nonpolar or moderately polar solvent. Typical mobile phases include hexane, isooctane, and methylene chloride. The usual order of elution, from shorter to longer retention times, is
olefins < aromatic hydrocarbons < ethers < esters, aldehydes, ketones < alcohols, amines < amides < carboxylic acids
For most samples liquid–solid chromatography does not offer any special advan- tages over liquid–liquid chromatography (LLC). One exception is for the analysis of isomers, where LLC excels. Figure 12.32 shows a typical LSC separation of two am- phetamines on a silica column using an 80:20 mixture of methylene chloride and methanol containing 1% NH4OH as a mobile phase. Nonpolar stationary phases, such as charcoal-based absorbents, also may be used.
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