Chromatography basics

CIE A-Level Chemistry· Unit 16: Introduction to analytical chemistry· 10 min read

1. Core Principles of Chromatography★☆☆☆☆⏱ 3 min

All chromatography techniques separate mixtures of soluble substances based on differences in how components distribute between two immiscible phases: a stationary phase that does not move, and a mobile phase that moves through or over the stationary phase.

📘 Definition

Chromatography Separation Principle

Separation occurs because different components have different affinities (attraction) for the stationary and mobile phases. Components with stronger affinity for the mobile phase move faster, separating from components more attracted to the stationary phase.

Example:

A mixture of amino acids separates based on differences in their polarity and attraction to each phase.

📐 Worked Example

A student runs paper chromatography to separate two food dyes. Dye A moves 3 cm up the paper, while dye B moves only 1 cm. Explain this observation.

1
First identify the two phases in paper chromatography:
2
Stationary phase = water adsorbed onto cellulose paper; Mobile phase = solvent moving up the paper
3
Dye A moves further, so it has a stronger affinity for the mobile solvent phase than dye B.
4
Dye B is more strongly attracted to the stationary water phase, so it moves more slowly up the paper.

2. Calculating Retention Factor ($R_f$)★★☆☆☆⏱ 4 min

The retention factor ( $R_{f}$ ) is a quantitative value that describes how far a component moves relative to the solvent front. For a given component, $R_{f}$ is constant under fixed conditions (same solvent, stationary phase, temperature), so it can be used to identify unknown components.

📘 Definition

Retention Factor ($R_f$)

R_f

The ratio of the distance moved by a component from the origin (baseline) to the distance moved by the solvent front from the origin.

R_f = \frac{\text{distance moved by component}}{\text{distance moved by solvent front}}

📐 Worked Example

In a paper chromatography experiment, the solvent front moves 8.0 cm from the baseline. A separated coloured component moves 5.2 cm from the baseline. Calculate the $R_{f}$ value of the component.

1
Write the formula for $R_{f}$ :
2
$R_f = \frac{d_{\text{component}}}{d_{\text{solvent front}}}$
3
Substitute the measured values:
4
$R_f = \frac{5.2 \text{ cm}}{8.0 \text{ cm}} = 0.65$
5
Confirm the result: $R_{f}$ values are always dimensionless (no units) and between 0 and 1, which matches our calculation.

✓ Quick check

Test your understanding:

What is the maximum possible $R_{f}$ value for any component?
- 0
- 0.5
- 1
- Depends on solvent
Reveal answer
1 —
The maximum $R_{f}$ is 1, achieved when a component moves the same distance as the solvent front, meaning it has no affinity for the stationary phase.

3. Interpreting Chromatograms★★☆☆☆⏱ 3 min

Chromatography has two common uses in chemistry: identifying unknown components in a mixture, and testing the purity of a sample. Identification relies on comparing $R_{f}$ values of unknowns to known reference values measured under identical conditions. Purity is tested by counting the number of spots on the chromatogram.

📐 Worked Example

A student runs chromatography of an unknown painkiller alongside three known references: paracetamol ( $R_{f} = 0.42$ ), aspirin ( $R_{f} = 0.61$ ), ibuprofen ( $R_{f} = 0.75$ ). The solvent front moves 10.0 cm, and the unknown spot moves 6.1 cm. Identify the unknown and comment on its purity if only one spot appears.

1
Calculate the $R_{f}$ of the unknown:
2
$R_f = \frac{6.1}{10.0} = 0.61$
3
Compare to reference values: 0.61 matches the $R_{f}$ of aspirin.
4
A pure compound produces only one spot on a chromatogram, so the unknown aspirin sample is pure under these conditions.

4. Common Pitfalls

Wrong move:

Measuring distance from the baseline to the edge of the spot instead of the centre

Why:

The position of a spot is defined by its centre, so measuring the edge gives an incorrect $R_{f}$ value

Correct move:

Always measure from the origin (baseline) to the centre of the separated spot

Wrong move:

Assigning units of centimetres to $R_{f}$ values

Why:

$R_{f}$ is a ratio of two distances, so units cancel out completely

Correct move:

Remember $R_{f}$ is a dimensionless number between 0 and 1

Wrong move:

Comparing $R_{f}$ values measured in different solvents to identify an unknown

Why:

$R_{f}$ depends strongly on the solvent and stationary phase, so values are only comparable under identical conditions

Correct move:

Run reference compounds alongside the unknown on the same chromatogram for accurate identification

Wrong move:

Assuming two spots with the same $R_{f}$ must be the same compound

Why:

Different compounds can have very similar $R_{f}$ values under the same conditions

Correct move:

Confirm identification by repeating the experiment with a different solvent or use another analytical technique

5. Quick Reference Cheatsheet

Term	Key Definition	Important Notes
Stationary Phase	Immobile phase components bind to	Varies by technique: paper = water on cellulose
Mobile Phase	Moving phase that carries components	Always liquid or gas
$R_{f}$ Value	$\frac{d _{co m p o n e n t}}{d _{so l v e n t f r o n t}}$	Dimensionless, 0 < $R_{f}$ < 1, constant for same conditions
Pure Compound	Produces 1 spot on a chromatogram	Can still be impure if two compounds share same $R_{f}$
Identification	Compare $R_{f}$ to reference values	Always run references on the same chromatogram

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 1
MCQ on Rf value properties
2023 · 2
Calculation of Rf for separated dye
2021 · 3
Interpret chromatogram purity

Going deeper

syllabusCIE 9701 Syllabus Page 44Official syllabus learning outcomes

What's Next

Now that you have mastered the core basics of chromatography, you are ready to learn about specific types of chromatography tested in CIE A-Level Chemistry. All specific techniques build directly on the phase separation and $R_{f}$ calculation principles you learned here, with adapted applications for more complex mixtures and quantitative analysis. A solid understanding of these basics is required for both written exam questions and the practical assessment paper, where you will be expected to process and interpret chromatographic separation data.