Thin-Layer Chromatography (Oxford AQA International A Level Chemistry)

Thin-Layer Chromatography

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

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

• Samples are separated by their solubility in a solvent

The phases in TLC

• Mobile phase (solvent)

  • The mobile phase flows over / through the stationary phase

  • It can be a polar or non-polar liquid

    • Polar solvents / liquids - water or alcohol

    • Non-polar solvents / liquids - alkanes

• Stationary phase

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

  • The components adsorb onto the surface

• The separated components will travel particular distances along the plate

• The movement of components depends on their solubility and retention

  • More soluble components have weaker interactions with the stationary phase, which means that they travel further

  • Less soluble components have stronger interactions with the stationary phase, which means that they do not travel as far

  • Essentially, the more that components interact with the stationary phase, the more they will 'stick' to it

Identifying components

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

• If the sample components are not coloured, then we can locate the spots on the chromatogram and draw around them in pencil

• To locate the spots we can use:

  • UV light

  • Ninhydrin (carcinogenic)

  • Iodine vapour

Conducting a TLC analysis

1. Prepare a beaker with a small quantity of solvent

2. Draw the baseline on a TLC plate, in pencil

   • This is a horizontal line at the bottom edge

3. Place a spot of any required pure reference compounds on the baseline

   • They will allow identification of the components in the sample

4. Place a spot of the sample on the baseline

5. Allow all spots to dry

6. Place the TLC plate inside the beaker with solvent and cover with a lid

   • The solvent must be below the baseline and not cover any spots

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

7. As solvent reaches the top, remove the plate and draw the solvent front, in pencil

   • The components will have separated and travelled up towards this solvent front

8. Interpret the chromatogram

Interpreting thin-layer chromatograms

• The results of thin-layer chromatograph can be analysed qualitatively or quantitatively

Qualitative TLC

• Thin layer chromatography gives an indication about the polarity of components

  • Less polar components travel further up the TLC plate

    • Their R_f values are higher than those closer to the baseline

    • They are more soluble in the mobile phase and get carried forwards with the solvent

  • More polar components do not travel far up the plate

    • They are more attracted to the polar stationary phase

• A typical qualitative analysis of thin-layer chromatograms involves the comparison of sample spots with reference spots on the same chromatogram

• The sample separates into its components, which are seen as individual spots

  • The sample spots will be in a vertical line

• The pure reference compounds each produce a single spot

• The sample spots are compared against the reference spots

  • The same components will appear at the same level horizontally as they will travel the same distance

Quantitative TLC

• Qualitative analysis of thin-layer chromatograms involves the comparison of the R_f values of the sample spots with known values

• To calculate R_f values:

R_f =