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Chromatography separates components because they distribute differently between a stationary phase and a mobile phase. In paper or thin-layer chromatography, a substance strongly attracted to the stationary phase moves more slowly than one favouring the solvent. Place the pencil baseline above the starting solvent level, use small concentrated spots and mark the solvent front promptly. Ink introduces extra coloured components, while submerging the sample spots dissolves them into the solvent reservoir rather than separating them up the plate.
Video by Eliot Rintoul. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Explain stationary and mobile phases
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What is the mobile phase in gas chromatography?
The carrier gas. It carries components through the stationary phase.
After watching 2: Why draw a chromatography baseline in pencil?
Graphite does not dissolve and separate like ink. Ink can introduce extra spots.
What you'll be able to do
Chromatography separates components because they distribute differently between a stationary phase and a mobile phase. In paper or thin-layer chromatography, a substance strongly attracted to the stationary phase moves more slowly than one favouring the solvent. Place the pencil baseline above the starting solvent level, use small concentrated spots and mark the solvent front promptly. Ink introduces extra coloured components, while submerging the sample spots dissolves them into the solvent reservoir rather than separating them up the plate.
An Rf value is the distance travelled by the centre of a component spot divided by the distance travelled by the solvent front, measured from the same baseline. It is a ratio without units and lies between zero and one in an ordinary run. Compare unknowns and references under the same solvent, stationary-phase and temperature conditions. Equal Rf values support a possible identity but do not prove it, because different compounds can co-elute. More than one solvent system provides stronger discriminating evidence.
Gas chromatography uses a carrier gas and a stationary phase in a column. Components have different retention times, affected by volatility and interactions with the stationary phase. Compare retention times under controlled instrumental conditions; peak areas may support quantitative composition after appropriate calibration. Coupling chromatography with mass spectrometry gives a spectrum for a separated component and strengthens identification. A single peak is evidence of purity under that method, not proof that no undetected or co-eluting impurity exists.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: An unknown and a reference have the same retention time. Is identity proven?
No; combine independent evidence.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.