Chromatography (HL IB Chemistry)

What is chromatography?

• Chromatography is a separation technique that enables the separation of mixtures and includes:
  • paper chromatography
  • thin-layer chromatography (TLC)
• These chromatography techniques make use of the principle that components in a mixture when dissolved in a fluid (mobile phase), will flow through another material (stationary phase) at varying rates
• The rate of separation depends upon how the components in the mixture interact with the stationary phase (their retention) and how soluble they are in the mobile phase
• Therefore the rate of separation depends on the intermolecular forces present

For more information on performing chromatography and other separation techniques, see our revision notes on separating mixtures

What is paper chromatography?

• In paper chromatography, the mobile phase is a solvent, and the stationary phase is the chromatography paper

• A pencil line is drawn on chromatography paper, this is the baseline (or origin), and spots of the sample are placed on it 
  • Pencil is used for this as ink would run into the chromatogram along with the samples

• The paper is then lowered into the solvent container, making sure that the pencil line sits above the level of the solvent so the samples don’t wash into the solvent container
• The solvent travels up the paper by capillary action, taking the sample with it

• As the solvent moves up the paper, the components in the mixture are dissolved to different extents depending on their solubility, so will travel with the solvent at different rates
  • The extent of solubility depends on the intermolecular forces present
• The paper contains cellulose fibres which have hydroxyl (OH) groups along their structure
• Substances in the mixture that can form hydrogen bonds with the OH groups will be more attracted to the stationary phase than those which form weaker intermolecular forces
  • This attraction to the stationary phase also affects the rate of separation
• Once the solvent front almost reaches the top of the paper, the paper is removed from the solvent and the solvent front is marked on the paper
• The separated components will appear as distinct spots on the paper

Paper chromatography

A dot of the sample is placed on the baseline and allowed to separate as the mobile phase flows through the stationary phase; the reference compound/s will also move with the solvent and are used to identify the components in the mixture.

What is thin layer chromatography (TLC)?

• TLC works in a similar way to paper chromatography but has a different stationary phase 
• The stationary phase is a thin layer of an inert substance (e.g. silica or alumina) supported on a flat, unreactive surface (e.g. glass)
• The mobile phase, like paper chromatography, is a solvent
• Silica and alumina contain OH groups so can form hydrogen bonds with components in the sample
• The components are adsorbed onto the surface of the stationary phase
• Depending on the strength of interactions with the stationary phase, the separated components will travel particular distances through the plate

What are retardation factors (R_f) values?

• The extent of separation of the component molecules in the investigated sample depends on their solubility in the mobile phase and the extent of adhesion to the stationary phase
• The R_f value is used to quantify the distance a particular component travels relative to the solvent front
• R_f values for compounds are calculated using measurements from the paper chromatogram or TLC plate and can be calculated using the R_f equation:

• These values can be used alongside other analytical data to deduce the composition of mixtures

 Calculation of R_f values

R_f values can be calculated by taking 2 measurements from a chromatogram