37.1 Thin-layer chromatography
- Syllabus
- 9701–2028–2029
- Topic
- 37.1
- Level
- A2
Learning objectives
37.1.1And understand the terms• Describe and understand the terms: (a) stationary phase, e.g. aluminium oxide (on a solid support); (b) mobile phase; a polar or non-polar solvent; (c) Rf value; (d) solvent front and baseline37.1.2Rf values• Interpret Rf values37.1.3Differences in Rf values• Explain the differences in Rf values in terms of interaction with the stationary phase and of relative solubility in the mobile phase
A TLC plate separates spots between a fixed solid and a moving solvent
| Term | Meaning in TLC |
|---|---|
| stationary phase | a fixed polar solid such as aluminium oxide on a solid support; solutes adsorb to its surface |
| mobile phase | a polar or non-polar solvent that rises through the stationary phase and carries dissolved solutes |
| baseline | pencil line where sample/reference spots start; placed above the solvent level |
| solvent front | furthest position reached by the mobile phase, marked immediately when the plate is removed |
| Rf | distance travelled by a component from the baseline divided by distance travelled by the solvent front from the baseline |
Rf=distance from baseline to solvent frontdistance from baseline to centre of solute spot
Apply small sample and reference spots to the pencil baseline, allow them to dry, place the plate in a covered container with solvent below the baseline, develop it, then remove the plate and mark the solvent front before evaporation changes its position.
Use pencil because ink could dissolve and separate. If the baseline is submerged, the sample dissolves directly into the solvent reservoir rather than travelling from a defined start.
Calculate Rf, then interpret spots only against the same TLC system
Measure every distance from the same baseline. For a spot that travels 2.4 cm while the solvent front travels 8.0 cm, Rf = 2.4/8.0 = 0.30. Rf has no unit and normally lies from 0 to 1.
| Chromatogram evidence | Supported interpretation |
|---|---|
| sample spot aligns with a reference spot and has the same Rf | sample may contain that reference compound under those identical conditions |
| sample gives several separated spots | sample contains more than one detectable component |
| one expected pure substance gives an extra spot | impurity or an additional component may be present |
| same compound run with a different solvent/stationary phase | Rf may change; do not use the old value as an identity constant |
An Rf match is comparative evidence, not unique proof of identity. Use the same stationary phase, mobile-phase composition, temperature and measurement method, ideally running reference and sample on the same plate.
Do not write cm after an Rf, measure from the plate edge, or identify a compound by comparing values obtained in different chromatographic systems.
Rf reflects competition between stationary-phase attraction and mobile-phase solubility
A solute travels farther when it is relatively more soluble in the mobile phase and interacts more weakly with the stationary phase. Stronger adsorption or intermolecular attraction to the stationary phase holds it back and lowers Rf.
| Relative behaviour | Movement | Rf |
|---|---|---|
| stronger interaction with stationary phase | retained for longer | lower |
| greater relative solubility in mobile phase | carried farther with solvent | higher |
With polar alumina/silica and a relatively non-polar solvent, a solute capable of stronger polar or hydrogen-bonding interactions is often retained more strongly. Changing solvent polarity changes the competition and can change every Rf.
A larger Rf does not mean a larger molecule, higher boiling point or universal lower polarity. It reports the balance of interactions in that specific stationary/mobile phase system.