Chromatography
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Chromatography USES
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Chromatography is used to seperate components in a mixture - used on a small scale for analysis or used on a bigger scale for purification
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Link between stationary phase and mobile phase
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Mobile phase flows past stationary phase carrying the components of the mixture with it
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Several different types of chromatography:
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- Paper chromatography (filter paper SP and water MP) - Thin layer chromatography** - High pressure liquid chromatography (silica SP and solvent MP) - Gas chromatography (liquid or solid SP and gas MP)
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THIN LAYER CHROMATOGRAPHY uses:
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TLC (abbreviated to) is widely used by chemists to assess the progress of reactions and purity of products
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THIN LAYER CHROMATOGRAPHY general info:
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Stationary phase of TLC is a thn layer of solid, e.g. silica gel. Mobile phase of TLC is an organic solvent
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Explanation of events in THIN LAYER CHROMATOGRAPHY
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1) Small spots of mixture to be seperated are added to TLC plate 2) Plate is then dipped in organic solvent WITHOUT immersing the spots of the mixture 3) As solvent moves up plate components of mixture are carried up a certain distance as well (depending on factors) 4) Migration up the plate of the spot is slowed by adsorption of the components to the stationary phase (thin layer of solid)
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The rate at which the spot of product moves up the plate depends on two factors:
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- SOLUBILITY of the component in the mobile phase, the MORE SOLUBLE the component the FASTER it moves up the plate (depends on attractive intermolecular forces) so BIGGER RF value - ADSORPTION of component to stationary phase, the STRONGER the adsorption the SLOWER the migration up the plate so SMALLER RF value
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RF values are..
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RF stands for retention factor values.
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RF values are used to identify the components in the sample:
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Rf values are a ratio of time a component has spent in the stationary phase relative to time spent in mobile phase
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RF equation
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Rf = distance travelled by the component / distance travelled by the solvent (small / big)
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Predicting RF values
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No reliable way to predict Rf values: BUT... compounds with similar functional groups will tend to have similar interactions with the plate & the solvent AND... most common type of TLC has silica gel as stationary phase which is polar so in general polar molecules will tend to adsorb more to it so have lower Rf values than non polar molecules.
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GAS CHROMATOGRAPHY:
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Two main sorts of gas chromatography, gas liquid chromatography (MORE important) & gas solid chromatography
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Gas - solid chromatography: general details
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Solid on a solid support is stationary phase and mobile phase is a gas.
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Gas - liquid chromatography: general details
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Liquid adsorbed onto an inert solid is stationary phase and mobile phase is a gas.
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GAS CHROMATOGRAPHY: what actually happens?
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A small sample is injected into the machine and is VAPOURISED. Sample is carried along in the column by an inert gas carrier where it then passes over stationary phase.
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Time taken for each sample to move through the tube will depend on:
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The BOILING POINT-VOLATILITY of the component and how strongly it INTERACTS with the stationary phase.
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In gas - solid chromatography retention times depend on...
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If the stationary phase is solid then the movement of components through the column is slowed by ADSORPTION to the solid surface. Components are separated according to how strongly they adsorb to stationary phase.
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In gas - liquid chromatography retention times depend on...
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If the stationary phase is liquid then movement of the components in the column is slowed by DISSOLVING in the liquid stationary phase. Components are seperated according to SOLUBILITY in stationary phase
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Measure of time spent in column:
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RETENTION TIME is the time taken for a compound to travel through the column to the detector
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Another effector on retention time:
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Volatility of components of mixture, shorter carbon chains have lower boiling point so are most volatile as have weaker vdw due to less electrons so have shorter retention time. The MORE VOLATILE the SHORTER the retention time.
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Interpretation of a GC chromatogram
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Each compound in the mixture will produce a peak, the area under each peak is proportional to the amount of each compound.
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Gas chromatography is often combined with mass spectrometry, uses are:
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Forensic science Airport security Testing for organic pollutants Analytical technique used to work out components of mixtures
