Read the phases and distances in thin-layer chromatography
Identify the baseline, solvent front, stationary phase and mobile phase before interpreting a TLC plate.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 37. Use the supplied spectra, shift table and chromatography conditions. Interpret peaks as evidence and state when a short trace cannot uniquely identify a substance. Written practice is self-checked
Before you begin
- Recognise functional groups, molecular structures, polarity, intermolecular interactions and the distinction between equivalent and different chemical environments.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the mobile phase in TLC?
What you will learn
- Define the TLC stationary and mobile phases
- Locate the baseline and solvent front
- Explain why the solvent front must be marked
Name the two phases
In thin-layer chromatography, the stationary phase is a solid adsorbent such as aluminium oxide supported on a plate. The mobile phase is a liquid solvent, which may be polar or non-polar.
A dissolved sample moves with the solvent but also interacts with the stationary surface. The observed separation depends on the balance between those interactions.
Use the plate landmarks
The baseline is the starting line from which the sample and solvent distances are measured. The solvent front is the furthest point reached by the mobile solvent; mark it before the solvent evaporates.
Measure a spot from the baseline to its centre. Measure the solvent front from the same baseline, in the same direction. The two distances form the quantities needed for Rf.
Pause and explain
From where should the spot and solvent-front distances be measured?
Worked example
On a TLC plate, the sample starts on a pencil line and the solvent rises. Name the two phases and the two reference lines.
Show the worked solution
- The solid coating remains fixed.
- The liquid travels up the plate.
- Measure both distances from the starting line.
Answer Stationary phase: aluminium oxide on a solid support. Mobile phase: the solvent. The starting line is the baseline; the furthest solvent mark is the solvent front.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
In a TLC description, identify the stationary phase and mobile phase if aluminium oxide is coated on a plate and propanone rises through it.
Give me a hint
Separate the solid surface from the travelling liquid.
Compare my reasoning
- Aluminium oxide remains fixed.
- Propanone moves up the plate.
- Name each phase.
Stationary phase: aluminium oxide on the support. Mobile phase: propanone.
Look for these in your work
- Fixed phase identified
- Moving phase identified
- Terms used correctly
A solvent front reaches 6.0 cm above the pencil baseline. Where would you measure a spot that travelled 2.4 cm?
Give me a hint
Use the same baseline for both measurements.
Compare my reasoning
- Measure from the pencil line.
- Use the centre of the spot.
- Do not measure from the plate edge.
The spot distance is 2.4 cm from the baseline; the solvent-front distance is 6.0 cm from that same baseline.
Look for these in your work
- Origin consistent
- Spot centre specified
- Distances distinguished
Why should the solvent front be marked as soon as the plate is removed from the solvent?
Give me a hint
The solvent can evaporate, hiding how far it travelled.
Compare my reasoning
- The front is the farthest solvent position.
- Evaporation can make it hard to see later.
- Rf requires that front distance.
Mark the solvent front promptly because the solvent evaporates; otherwise its distance from the baseline may be lost.
Look for these in your work
- Front defined
- Evaporation considered
- Measurement need stated
Common mistakes
- Calling the solvent stationary
- Measuring a spot from the bottom of the plate instead of the baseline
- Forgetting to mark the solvent front
- Using the plate edge as the Rf reference
What can you explain now?
What is the mobile phase in TLC?
Compare with the explanation
A liquid solvent
The solvent moves through the stationary adsorbent.
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Make it stick
Which fixed and moving phases and reference lines define a TLC measurement?
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