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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