Chromatography (OCR Gateway GCSE Chemistry: Combined Science)

Paper Chromatography

• Chromatography is used to separate substances and provide information to help identify them
• The components have different solubilities in a given solvent (e.g. different coloured inks that have been mixed to make black ink) and different adhesion to the supporting medium - usually paper
• A pencil line is drawn on chromatography paper 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 some of the coloured substances with it
• Different substances have different solubilities so will travel at different rates, causing the substances to spread apart
  • Those substances with higher solubility will travel further than the others

• This is because they spend more time in the mobile phase and are thus carried further up the paper than the less soluble components

The pigments in ink can be analysed using paper chromatography

• All chromatography techniques use two phases called the mobile phase and the stationary phase
• In paper chromatography:
• The mobile phase is the solvent in which the sample molecules can move, which in paper chromatography is liquid e.g. water or ethanol
• The stationary phase in paper chromatography is the actual chromatography paper itself
• Different dissolved substances have different affinities for the mobile and stationary phase which determines the speed they move through them

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) supported on a flat, unreactive surface
• The mobile phase, like paper chromatography, is a solvent

• These values are used to identify the components of mixtures
• The R_f value of a particular compound is always the same but it is dependent, however, on the solvent used
• If the solvent is changed then the value changes
• Calculating the R_f value allows chemists to identify unknown substances because it can be compared with R_f values of known substances under the same conditions
• These values are known as reference values

Calculation

• Retention factor = distance moved by compound ÷ distance moved by solvent
• The R_f value is a ratio and therefore has no units

• Pure substances will produce only one spot on the chromatogram
• If two or more substances are the same, they will produce identical chromatograms
• If the substance is a mixture, it will separate on the paper to show all the different components as separate spots
• An impure substance therefore will produce a chromatogram with more than one spot

Gas Chromatography 

• This is used to separates a mixture of gases 
• The mobile phase is an unreactive carrier gas e.g nitrogen
• The stationary phase is a thin layer of an unreactive liquid  e.g silica or aluminium powder
• Gas chromatography occurs as follows:
  • The mixture sample is injected into the column 
  • The mixture is carried by the carrier gas through the column 
  • Different substances in the mixture will  take different times to travel through the column (how long each one takes is known as the retention time and is due to their attraction to the stationary phase)
  • Substances with more attraction to the stationary phase will take longer to move through the column
  • As each component leaves the column, a peak is plotted again the travel time on a chromatogram generated by a computer 

Interpreting Gas Chromatograms

• A gas chromatogram tells you some key details about the mixture
  • the number of peaks- the number of compounds in the mixture 
  • the height of the peak- how much of each compound is present (the higher the peak, the more compound there is)
  • the position of the peak- retention time of compound (how long it took to move through the column

Answers:

i) D (the larger the relative size of the peak, the greater the quantity of that substance present)

ii) B and C (the peak sizes are equal)

iii) D (the larger the retention time, the greater the interaction of that component with the stationary phase)