OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
(g) separation of biological compounds using thin layer/paper…
Full specification wording
Use of apparatus and techniques: (g) separation of biological compounds using thin layer/paper chromatography or electrophoresis
Chromatography and electrophoresis both separate compounds by movement, but by different mechanisms.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 7 of 14
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Chromatography and electrophoresis both separate compounds by movement, but by different mechanisms. In paper or thin layer chromatography a sample is spotted on the stationary phase and a solvent moves up by capillary action; compounds partition between stationary and mobile phases and travel different distances according to solubility and affinity. The Rf value is the distance travelled by the compound divided by the distance travelled by the solvent front, and it is compared with known standards. In electrophoresis, charged molecules such as amino acids or proteins migrate through a gel under an applied voltage; the direction depends on charge and the rate on charge and size. Buffer maintains pH so charge is controlled. Both methods separate a mixture so components can be identified.
What earns the marks
- Chromatography separates compounds by differential partitioning between a stationary phase and a moving mobile phase.
- The Rf value equals the distance moved by the compound divided by the distance moved by the solvent front, measured from the origin.
- Rf values are compared with standards run under the same conditions to identify a compound.
- Electrophoresis separates charged molecules in a gel under an applied voltage; direction depends on charge and rate on charge and size.
- A buffer controls pH so that the charge on the molecules, and therefore their movement, is predictable.
Common mistakes and how to put them right
Measuring Rf from the wrong baseline.
Put it right by measuring both distances from the origin to the centre of the spot and to the solvent front.
Assuming electrophoresis separates only by size.
Put it right by stating that charge determines direction and both charge and size affect rate.
Ignoring the need for identical conditions when comparing Rf values.
Put it right by running standards alongside the sample under the same solvent and temperature.
Exam tips
- Write the Rf formula as a ratio of two measured distances and state that it has no units.
- For electrophoresis, link the applied voltage to movement of charged molecules and name the buffer's role in fixing pH.
- When identifying a compound, say that its Rf or migration is compared with a standard rather than guessed.
Try the lesson
The start of the Use of apparatus and techniques: (g) separation of biological compounds using thin layer/paper chromatography or electrophoresis lesson, made from checked teaching notes for this specification point.
Have a go first
Which technique separates charged biological molecules such as amino acids or proteins using an electric field?
Listen
Chromatography and electrophoresis both separate compounds by movement, but by different mechanisms. In paper or thin layer chromatography a sample is spotted on the stationary phase and a solvent moves up by capillary action; compounds partition between stationary and mobile phases and travel different distances according to solubility and affinity. The Rf value is the distance travelled by the compound divided by the distance travelled by the solvent front, and it is compared with known standards. In electrophoresis, charged molecules such as amino acids or proteins migrate through a gel under an applied voltage; the direction depends on charge and the rate on charge and size. Buffer maintains pH so charge is controlled. Both methods separate a mixture so components can be identified. A strong answer earns marks for these points. Chromatography separates compounds by differential partitioning between a stationary phase and a moving mobile phase. The Rf value equals the distance moved by the compound divided by the distance moved by the solvent front, measured from the origin. Rf values are compared with standards run under the same conditions to identify a compound. Electrophoresis separates charged molecules in a gel under an applied voltage; direction depends on charge and rate on charge and size. A common slip: Measuring Rf from the wrong baseline. Instead, measuring both distances from the origin to the centre of the spot and to the solvent front. Exam tip: Write the Rf formula as a ratio of two measured distances and state that it has no units.
See it
Put it in order
Put these steps in the order they happen.
- 1. Compound identified
- 2. Compare Rf with standards run under same conditions
- 3. Compounds partition between stationary and mobile phases
- 4. Rf = distance moved by compound / distance moved by solvent front
- 5. Sample spotted on stationary phase at origin
- 6. Solvent moves up by capillary action
- 7. Compounds travel different distances
Key words
Say what each word means before you turn the card.
Partition between stationary and mobile phases
Try answering aloud or in your head before turning the card.
Rf values and comparison with standards
Try answering aloud or in your head before turning the card.
Electrophoretic separation by charge and size
Try answering aloud or in your head before turning the card.
Role of buffer in controlling charge
Try answering aloud or in your head before turning the card.
See every definition
- Partition between stationary and mobile phases
- Compounds in a sample spotted on the stationary phase travel different distances as the solvent moves up by capillary action, according to solubility and affinity.
- Rf values and comparison with standards
- The distance travelled by the compound divided by the distance travelled by the solvent front, compared with known standards to identify a compound.
- Electrophoretic separation by charge and size
- Charged molecules such as amino acids or proteins migrate through a gel under an applied voltage, the direction depending on charge and the rate on charge and size.
- Role of buffer in controlling charge
- Maintains pH so that the charge on the molecules, and therefore their movement, is controlled.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Use of apparatus and techniques: (g) separation of biological compounds using thin layer/paper chromatography or electrophoresis
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge