prof mat org chem_10chromatography01

7/31/2019 Prof Mat Org Chem_10Chromatography01

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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, to separate

mixtures of substances.

Stationary phase & mobile phase.

Compounds with greatest affinity for

mobile phase travel further.


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Chromatography

Separates components in

mixture:

Based on- polarity

- boiling point

- ionic strength- size


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All chromatography needs:

support material stationary phase

solvent (or carrier gas) mobile

phase.


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What happens in practice.

Compounds that have high affinity

for mobile phaseemerge first,

(most volatile).

Chromatogram charts recorderresponse against time.

Each component - separate peak.

Retention timecharacteristic of

the compound under given

conditions.


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Thin Layer Chromatography

Here the mobile phase is a liquid

Flowing past a thin layer of powder on a solid

support.

Substances that are less attracted to the solid or are

more soluble in the liquid move faster.

And so move further up the plate by the time that theprocess has been stopped by taking the plate out of the

liqiud. - larger Rf


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Thin Layer Chromatography - t.l.c.

Series of spots forms

Compare samples in

mixture with knownsubstances.

Measure Rfvalues.

Coloured compounds &

colourless compounds.


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Rf= distance moved by substancedistance moved by solvent front

For substances that are very soluble in theliquid Rf will be close to ....

For substances that are rather insoluble in theliquid Rf will be close to ....

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0


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Separation and identification.


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Gas Liquid Chromatography

Here the mobile phase is an unreactive gas( egNitrogen) flowing through a tube.

And the stationary phase is an involatile liquidheld on particles of a solid support.


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Oven

Detector

Injection

port

Nitrogen

cylinder

Colum

n

Recorder


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Gas - Liquid Chromatography G.l.c.

Sample introduced by syringe.

Column separates components.

(Heated in oven)

Detector monitors compounds

emerging from outlet.

Recorder plots signals as

a chromatogram.


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Uses of G.l.c.

Very sensitive - small quantities of

substances detected, explosives, drugs etc.

Separation ofpure substances for collection.

Can be connected to mass spectrometer for

direct identification of substances.


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Factors 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 added

to the column and conditions kept

constant.

Amount of substance area under peak /

peak height.

Relative proportions can be determined.


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Chromatogram of petrol

Suggest identities of some of the unlabelled peaks.


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Typical Gas Chromatogram


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Typical Liquid Chromatogram


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Partition Chromatography

Used in GC & LC

Molecules will partition into the

stationary phase based upon affinityfor stationary phase & eventually

partition into mobile phase again

Thin layer is coated onto inside of

GC column or on small particles on

LC column


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Adsorption Chromatography

Very similar to partition

chromatography

Adsorption just on surface,

partition into thin layer

Not used as widely as

partition used mainly in TLC

& very small particles in LC


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Ion Exchange Chromatography

Separation of either cationsor anions

Separtion based on relative

strength of ionic bondAnion exchange has cationson surface

Used in LC exclusively


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Molecular Exclusion

ChromatographySeparation based on size

Small molecules get

trapped in pores & takelonger to get out


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Gel Electrophoresis

Separation based on size

and charge

Smaller molecules willmigrate further, less

tangled


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