Thin Layer Chromatography (AQA A Level Chemistry): Revision Note

Thin Layer Chromatography

• Thin-layer chromatography (TLC) is a technique used to analyse small samples via separation

  • For example, TLC could separate a dye to determine the mixture of dyes in a forensic sample

• There are two phases involved in TLC:

  • Stationary phase

    • This phase is commonly a thin metal sheet coated in alumina (Al₂O₃) or silica (SiO₂)

    • The solute molecules adsorb onto the surface

    • Depending on the strength of interactions with the stationary phase, the separated components will travel particular distances through the plate

    • The more they interact with the stationary phase, the more they will 'stick' to it

  • Mobile phase

    • Flows over the stationary phase

    • It is a polar or non-polar liquid (solvent) that carries components of the compound being investigated

    • Polar solvents - water or alcohol

    • Non-polar solvents - alkanes

    • A component more soluble in the solvent travels further

• Separation depends on the BALANCE between solubility in the mobile phase and affinity for the stationary phase

• If the sample components are coloured, their spots are easily identifiable on the chromatogram

• If the sample components are not coloured, then the spots on the chromatogram can be located and drawn around in pencil

  • To locate the spots, use either:

    • UV light

    • Ninhydrin (harmful skin and eye irritant)

    • Iodine vapour

Conducting a TLC analysis

• Step 1:

  Prepare a beaker with a small quantity of solvent

• Step 2:

  On a TLC plate, draw a horizontal line at the bottom edge (in pencil)

  This is called the baseline

• Step 3:

  Place a spot of pure reference compound on the left of this line, then a spot of the sample to be analysed to the right of the baseline, and allow it to air dry

  The reference compounds will allow the identification of the mixture of compounds in the sample

• Step 4:

  Place the TLC plate inside the beaker with solvent - making sure that the solvent does not cover the spot - and place a lid to cover the beaker

  The solvent will begin to travel up the plate, dissolving the compounds as it does

• Step 5:

  As the solvent reaches the top, remove the plate and draw another pencil line where the solvent has reached, indicating the solvent front

  The sample’s components will have separated and travelled up towards this solvent front

R_f values

• A TLC plate can be used to calculate R_f values for compounds

${\mathrm{R}}_{\mathrm{f}}=\frac{\mathrm{distance}\mathrm{ravelled}\mathrm{by}\mathrm{component}}{\mathrm{distance}\mathrm{travelled}\mathrm{by}\mathrm{solvent}}$

• R_f is dimensionless and is always between 0 and 1

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

Related topics

• Principles of Chromatography

• Interpreting Chromatograms

• Column Chromatography

• Gas Chromatography