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Chemical Ideas 7.6

Chromatography

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The general principle.Use to separate and identify

components of mixtures.

Several different types - paper, thin

layer, gas-liquid. All use the principle of partition -

affinity between two phases, toseparate mixtures of substances.

Stationary phase & mobile phase.

Compounds with greatest affinity for mobile phase travel further.

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All chromatography needs :

ChromatographyType

Stationary phase Mobile phase

Paper Paper Organic oraqueous solvent.

Thin layer Silica gel

supported onplastic film

Organic or

aqueous solvent.

G.L.C. High boilingpoint liquid oninert solidsupport.

Inert gas e.g.nitrogen.

support material stationary phase

solvent (or carrier gas ) mobilephase.

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Thin Layer Chromatography - t.l.c.

Series of spots formsCompare samples inmixture with knownsubstances.Measure R f values.

Coloured compounds& colourlesscompounds.

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Separation and identification.

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Gas - Liquid Chromatography

G.l.c.

Sample introduced by syringe.

Column separates components.(Heated in oven )

Detector monitors compoundsemerging from outlet.

Recorder plots signals asa chromatogram.

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What happens in practice.

Compounds that have high affinityfor mobile phase emerge first,(most volatile).

Chromatogram charts recorderresponse against time.

Each component - separate peak.

Retention time characteristic ofthe compound under givenconditions.

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F actors affecting retention

time:

length of column

packing materialtype of carrier gas

flow rate of carrier gas

temperature of column.

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Interpreting the trace

Calibration known compounds are addedto the column and conditions keptconstant.

Amount of substance area under peak /peak height.

Relative proportions can be determined.

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Uses of

G.l.c.

Very sensitive - small quantities of substances detected, explosives, drugs etc.Separation of pure substances for collection.Can be connected to mass spectrometer for direct identification of substances.