A-level Chemistry: 3.3.16 Chromatography

Where molecules can move

Always liquid or gas

Where molecules can’t move

Solid, or a liquid on solid support

Substance’s solubility in mobile phase and its retention by stationary phase

• Thin-Layer Chromatography

• Column Chromatography

• Gas Chromatography

thin layer of silica (silicon dioxide) or alumina (aluminium oxide) fixed to glass or metal plate

Wear gloves to avoid contamination by substances on your hand

1. Draw line in pencil near bottom of TLC plate (the baseline) & put small drop of each mixture on line

2. Allow spots to dry

3. Place plate in beaker with small volume of solvent (mobile phase)
   

   • Solvent level must below baseline

4. As solvent moves up plate, it carrier substances in mixture with it
   

   • Some chemicals will be carried faster than other and and travel further up plate

5. Remove plate from beaker when solvent has nearly reached top of plate

6. Before it evaporates, use a pencil to make how far solvent travelled up plate (solvent front)

7. Place plate in fume cupboard to dry

So solvent doesn’t dissolve samples away

Fume cupboard will prevent any toxic or flammable fumes from escaping into room

Chromatograph

• UV Light

• Iodine

1. Many TLC plates have special fluorescent dye added to silica or alumina layer that glows when UV light shines on it

2. Can put plate under UV lamp and draw around dark patches to show where spots of chemical are

1. 
   Expose chromatogram to iodine vapour
   

   • i.e. leave plate in sealed jar with some iodine crystals

2. Iodine vapour is a location agent - sticks to chemicals on plate and they’ll show up as brown/purple spots

Count no. of spots formed on plate

By calculating a Rf value

Look up Rf values in table of standard Rf values to identify what the substance is

If composition of TLC plate, solvent or temperature changes even slightly = get different Rf values

∴ if you suspect mixture contains e.g. chlorophyll best to put spot of chlorophyll on baseline of same plate as mixture and run them both at same time

Used for purifying an organic product

(Done to separate product from unreacted chemicals and by-products)

1. Involves packing glass columns with slurry of absorbent material e.g. aluminium oxide, coated with water

2. Mixture to be separated is added to top of column and allowed to drain down into slurry

3. Solvent is then run slowly and continually through column

Water-coated particles of Al2O3

Solvent

1. As mixture is washed through column = its components separate out according to how soluble they are in mobile phase and how strongly they are adsorbed onto stationary phase (retention)

2. More soluble each component is in mobile phase, quicker it'll pass through column

Used to separate mixture of volatile liquids so you can identify them

Solid or solid coated by viscous liquid e.g. oil packed into long tube

Tube is coiled to save space and built into an oven

Unreactive carrier gas e.g. nitrogen

The time taken for a component to travel between the injection point and the detector

Each separate substance will have unique retention time = so use retention time to identify components of mixture

(Have to run known sample under same conditions for comparison)

How much time the component spends moving along with the carrier gas, and how much time it spends stuck to the viscous liquid

Relative amount of each component that's present in mixture

• To find level of alcohol in blood or urine

  • Very accurate

• To find proportions of various esters in oils used in paints

  • Allows picture restorers know exactly what paint was originally used

1. Sample is separated using gas chromatography, but instead of going to detector, separated components are fed into mass spectrometer

2. Spectrometer produces mass spectrum for each component = used to identify each one and show what original sample consisted of

Components separated out by chromatography can be positively identified which can be impossible from chromatogram alone

Computers can be used to match up mass spectrum for each component of mixture against database

Each component has 2 Rf values both of which must match with a known component