Ch 1 · Thin-layer chromatography · AS & A Level Chemistry 9701
Topic 1 of 4 in Analytical techniques — AS & A Level Chemistry 9701 — 48 lessons.
Introduction
Thin-layer chromatography (TLC) separates and identifies components of a mixture using their differing tendencies to stay on a solid stationary phase or move with a liquid mobile phase, giving a quick way to compare an unknown sample against known reference compounds.
Key ideas — Key terms in TLC
The stationary phase is a thin layer of an adsorbent solid, usually silica gel or alumina, fixed onto a glass, plastic or aluminium plate. The mobile phase is a liquid solvent (or solvent mixture) that moves up the plate by capillary action, carrying dissolved components of the sample with it. The baseline is the pencil line drawn near the bottom of the plate on which the sample is spotted, kept above the level of solvent in the tank so the sample itself does not dissolve directly into the solvent reservoir. The solvent front is the furthest point the mobile phase reaches up the plate before it is removed and marked, at the moment development is stopped.
🧑🏫 Teacher's explanation
Imagine a crowded hallway (the stationary phase) filled with people standing still. Now imagine a crowd of people (the mobile phase) rushing through the hallway. If you are a person who loves talking to the people in the hallway, you will move very slowly because you keep stopping to chat. If you are in a hurry and don't care about the people in the hallway, you will zip through very quickly.
📖 Definitions
🔍 Worked example — Easy
A component in a mixture travels 4.0 cm from the baseline. The solvent front travels 10.0 cm from the baseline. Calculate the R_f value.
🔍 Worked example — Standard
A sample contains two dyes, A and B. Dye A travels 2.5 cm and Dye B travels 7.5 cm. The solvent front moves 10.0 cm. Calculate the R_f values for both dyes.
🔍 Worked example — Exam-style
In a TLC experiment, Substance X has an R_f of 0.85 and Substance Y has an R_f of 0.20. Explain the difference in their behavior regarding the stationary and mobile phases.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Calculate the R_f value for Compound A. [1] (b) Explain why Compound B has a higher R_f value than Compound A in terms of its interactions with the stationary and mobile phases. [2] [3 marks]
Key ideas — Calculating and interpreting Rf values
The retardation factor, R_f, is calculated for each separated spot as: R_f = distance travelled by the spot (baseline to spot centre)/distance travelled by the solvent front (baseline to solvent front) R_f is always between 0 (the component does not move at all) and 1 (the component travels with the solvent front), and under fixed conditions — the same stationary phase, mobile phase and temperature — a given compound always gives the same R_f value, allowing an unknown spot to be identified by comparing its R_f with that of a known reference run on the same plate.
🧑🏫 Teacher's explanation
Imagine a crowded shopping mall where a group of people is walking through a corridor. The 'mobile phase' is the moving crowd, and the 'stationary phase' is the floor or the walls. If a person is very social and keeps stopping to talk to people (interacting with the stationary phase), they move slowly. If they are in a rush and want to move with the crowd (high solubility in the mobile phase), they move quickly.
📖 Definitions
🔍 Worked example — Easy
A spot travels 3.0 cm from the baseline, and the solvent front travels 12.0 cm from the baseline. Calculate the R_f value.
🔍 Worked example — Standard
In a TLC experiment, a component has an R_f of 0.65. If the solvent front moved 8.0 cm from the baseline, how far from the baseline did the center of the spot travel?
🔍 Worked example — Exam-style
A student runs a TLC plate with two known standards (A and B) and one unknown (X). The R_f values are: Standard A = 0.22, Standard B = 0.85, Unknown X = 0.85. Identify X and explain why its R_f is higher than Standard A.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Calculate the R_f values for the three pigments. [3] (b) Explain why the pigment with the highest R_f value is the most soluble in the mobile phase. [2] [5 marks]
Key ideas — Why Rf values differ between compounds
A compound's R_f value reflects the balance between two competing tendencies: adsorption onto the polar stationary phase, through hydrogen bonding or dipole interactions with the silica surface, and solubility in the mobile phase. A strongly polar compound that hydrogen-bonds well with silica is held back and has a low R_f; a less polar compound that dissolves more readily in the, typically less polar, mobile phase moves further and has a higher R_f. Changing the polarity of the mobile phase changes every compound's R_f, so R_f values from different experiments can be compared only if the same solvent and stationary phase were used.
🧑🏫 Teacher's explanation
Imagine a crowd of people walking through a shopping mall. The floor (the stationary phase) is covered in sticky patches of honey, and the air (the mobile phase) is a strong wind blowing everyone forward. Some people are wearing heavy boots that stick intensely to the honey; they move very slowly. Other people are wearing smooth-soled sneakers and are very light; the wind carries them quickly through the mall.
📖 Definitions
🔍 Worked example — Easy
A pigment in a leaf extract travels 3.0 cm from the baseline. The solvent front travels 12.0 cm from the baseline. Calculate the R_f value.
🔍 Worked example — Standard
Compound A has an R_f of 0.85 and Compound B has an R_f of 0.30 using the same solvent and stationary phase. Explain which compound is more polar, assuming the stationary phase is polar silica gel.
🔍 Worked example — Exam-style
A student performs TLC on a mixture. Component X travels 4.5 cm and Component Y travels 1.5 cm. The solvent front is at 5.0 cm. Calculate the R_f values for both and determine which is more soluble in the mobile phase.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Explain the difference in R_f values in terms of the interactions between the compounds and the stationary phase. [2] (b) Which compound is more polar? [1] [3 marks]
Key ideas — Comparing TLC with paper chromatography
In TLC the stationary phase is a solid adsorbent layer, silica or alumina, coated onto a rigid or semi-rigid plate; in paper chromatography, the stationary phase is water held within the cellulose fibres of the paper itself, and separation depends mainly on partition between this water layer and the mobile phase rather than surface adsorption. TLC plates are generally more robust, give sharper spots and faster separations than paper, and the choice of adsorbent can be varied to improve a particular separation, whereas paper chromatography offers less control over the stationary phase.
🧑🏫 Teacher's explanation
Imagine a crowd of people walking through a shopping mall. Some people are very interested in the shop windows (the stationary phase) and stop frequently to look, while others are in a rush to get to the exit (the mobile phase). The people who stop most often will be much further back from the exit than the people who keep walking.
📖 Definitions
🔍 Worked example — Easy
A pigment in a leaf extract travels 4.0 cm from the baseline. The solvent front travels 10.0 cm from the baseline. Calculate the R_f value.
🔍 Worked example — Standard
In a TLC experiment, Substance A has an R_f of 0.85 and Substance B has an R_f of 0.30. Explain which substance has a higher affinity for the stationary phase.
🔍 Worked example — Exam-style
A student performs TLC on a mixture. The solvent front moves 5.0 cm. Spot X moves 3.5 cm and Spot Y moves 1.5 cm. Calculate the difference in R_f values between the two spots.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Calculate the R_f value for the Blue dye. [1] (b) Explain why the Red dye has a lower R_f value than the Blue dye in terms of its interaction with the stationary phase. [2] [3 marks]
Worked example
Question. A TLC plate is developed in a tank, and the solvent front travels 8.0 cm from the baseline. Three spots are seen: spot A has travelled 6.4 cm, spot B has travelled 2.0 cm, and spot C has travelled 7.2 cm. (a) Calculate the R_f value of each spot. [3] (b) A reference compound run under the same conditions has R_f = 0.90. Identify which spot, if any, matches this reference. [1] (c) Suggest, in terms of polarity, why spot B has the lowest R_f value. [2]
🔍 Worked example — Easy
A student performs TLC. The solvent front moves 10.0 cm from the baseline. Spot X moves 5.5 cm. Calculate the R_f value of spot X.
🔍 Worked example — Standard
In a TLC experiment, the solvent front traveled 12.0 cm. Three spots were observed with distances of 2.4 cm, 6.0 cm, and 9.6 cm. Calculate the R_f values for all three spots.
🔍 Worked example — Exam-style
A TLC plate is developed. The solvent front travels 8.0 cm from the baseline. Three spots are seen: spot A has travelled 6.4 cm, spot B has travelled 2.0 cm, and spot C has travelled 7.2 cm. (a) Calculate the R_f value of each spot. (b) A reference compound has R_f = 0.90. Which spot matches? (c) Explain why spot B has the lowest R_f value in terms of polarity, assuming the stationary phase is polar silica and the mobile phase is non-polar.
⚠️ Common errors
Common mistakes
Practice
Answers