Loading...
7405 · RP12Required practical investigationsCore
RP12: Thin-Layer Chromatography
TLC uses a thin stationary phase, commonly silica, on a plate and a moving solvent. Different components move different distances because their interactions with both phases differ. Mark a pencil baseline above the solvent level and apply small concentrated sample and reference spots with a capillary. Let applications dry; repeated small applications can concentrate a sample without making a broad initial spot.
Before you start
- Calculate a ratio and explain competing attractions to stationary and mobile phases.
- Use these notes to prepare for teacher-approved laboratory work. Follow the laboratory risk assessment, eye protection, handling and disposal instructions; a reading or quiz does not record Practical Endorsement evidence.
Watch, pause, try
Video by Royal Society of Chemistry. Use the demonstration alongside this lesson’s specification reference and written explanations.
Viewing focus: Plan a TLC separation with references
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Why must the baseline start above the solvent level?
Show answer and reasoning
To prevent sample dissolving into the reservoir. The sample should be carried upward from a defined starting position.
After watching 2: What distance is the denominator in Rf?
Show answer and reasoning
Solvent-front travel from the baseline. Both distances must share the same origin.
What you'll be able to do
- Plan a TLC separation with references
- Calculate Rf using the correct origin
- Evaluate resolution, identification and apparent purity
1
Control the conditions that create separation
TLC uses a thin stationary phase, commonly silica, on a plate and a moving solvent. Different components move different distances because their interactions with both phases differ. Mark a pencil baseline above the solvent level and apply small concentrated sample and reference spots with a capillary. Let applications dry; repeated small applications can concentrate a sample without making a broad initial spot.
Place the plate in the development chamber without submerging the baseline. A cover reduces solvent evaporation and helps provide more consistent chamber conditions. Avoid disturbing the plate. Follow the approved solvent and ventilation arrangements; many mobile phases are volatile and flammable. Handle a plate by its edges to avoid contaminating the stationary surface.
2
Make a measurement that remains interpretable
Remove the plate before the front reaches the top and mark the solvent front immediately in pencil, before it evaporates. Measure from the baseline to the centre of each spot and from that same baseline to the front. Rf = spot distance/front distance, so it has no unit. Using the plate edge for one distance and the baseline for the other gives an invalid ratio.
Colourless compounds need a suitable visualisation method. Use UV only with the specified shielded viewing equipment and protection; do not look directly into an unshielded source. Some approved visualising reagents alter the sample, so record the developed pattern as instructed. A streak rather than a discrete spot may indicate overloading or an unsuitable solvent system.
3
Distinguish useful evidence from proof
Run unknown and reference on the same plate with the same solvent. Rf depends on the mobile phase, stationary phase and conditions, so a tabulated number from a different system is not a reliable direct comparison. A matching spot supports a possible identity; different compounds can travel together. A second solvent system or independent spectral evidence provides a stronger test.
More than one spot supports a mixture if the spots arise from sample components. One spot does not prove complete purity: components may co-migrate or escape the visualisation method. Improve resolution with a suitable solvent composition, smaller starting spots and controlled development. Do not merely develop longer if the front is already near the plate edge.
4
Worked applications
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Example 1The solvent front moves 6.4 cm from the baseline. A sample has spot centres at 1.6 and 4.8 cm. Calculate Rf values.
1First component: 1.6/6.4 = 0.25.
2Second component: 4.8/6.4 = 0.75.
Answer0.25 and 0.75, without units; two detected components are present.
Example 2An unknown gives one spot at Rf 0.60, matching a reference on the same plate. What further evidence would strengthen the identification?
1A match in one solvent system can result from co-migration.
2Compare in a different suitable solvent system or use independent spectroscopic evidence.
AnswerA second independent comparison strengthens identity; the single spot alone does not establish complete purity.
Common mistakes to avoid
Reporting Rf in cm.
The distance units cancel.
Claiming one matching spot proves identity and purity.
State the limits and seek independent evidence.
Key takeaways
- You can plan a TLC separation with references.
- You can calculate Rf using the correct origin.
- You can evaluate resolution, identification and apparent purity.
Apply it in an exam
Question 1: An unknown gives one spot at Rf 0.60, matching a reference on the same plate. What further evidence would strengthen the identification?
Show answer and reasoning
A second independent comparison strengthens identity; the single spot alone does not establish complete purity.
- A match in one solvent system can result from co-migration.
- Compare in a different suitable solvent system or use independent spectroscopic evidence.
Explore further
- AQA 7405 official specification (opens in a new tab)
Check the assessed scope and practical or mathematical requirements.
- Read or explore this topic (opens in a new tab)
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Return to this tomorrow
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
- What distance is the denominator in Rf?
- What does one observed TLC spot establish?
Test yourself
Answer a few questions, then review anything you missed.
Next step
Browse practice topicsKeep practising
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.