chromatography is a versatile technique for separating mixtures
strategy: flow the mixture over a material that retains some components more than others, so different components flow over the material at different speeds
a simple analogy for chromatography
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mixed swarm of bees and wasps swept by the wind over a flower bed...
bees visit flowers; wasps stay airborne
wasps leave the bed first; swarm separated on the basis of affinity for flowers
in chromatography, a mobile phase sweeps the sample over a stationary phase (as the wind sweeps the swarm over the flower bed)
Advantages of chromatography
can separate very complex mixtures
drugs, plastics, flavorings, foods, pesticides, tissue extracts, fuels, air samples, water samples, ...
very small sample sizes
separated components can be collected individually
analyses can be highly accurate and precise
Sheet chromatography
paper chromatography (PC)
stationary phase is liquid soaked into a sheet or strip of paper
mobile phase is a liquid solvent
some components spend more time in the stationary phase than others
components appear as separate spots spread out on the paper after drying or "developing"
thin layer chromatography (TLC)
stationary phase is a thin layer of adsorbent (Al2O3 or SiO2, usually) coating a sheet of plastic or glass
some components bond to the adsorbent strongly; others, more weakly
as with paper chromatography, components appear as spots on the sheet
Column chromatography
gas chromatography (GC)
sample mixture is injected into a long tube (the column)
mobile phase is an inert gas that sweeps the sample down the tube
stationary phase lining the tube selectively adsorbs or dissolves components
the stationary phase is a solid or very syrupy liquid
silicone polymers (like Silly Putty!) are often used as stationary phases in gas chromatography
a detector responds to separated components as they leave the tube
What is chromatography used for?
finding concentrations
gas chromatogram of gasoline
ion chromatogram of orange juice
each peak corresponds to a separate component in the mixture
area of each peak is proportional to concentration
chemical fingerprinting
species identification
"killer" bees can be distinguished from native bees by comparing gas chromatograms of cuticle extracts
