Chromatography and Instrumental Analysis
Chromatography is a technique used to separate the substances in a mixture and to help identify them, based on how strongly each substance is attracted to a mobile phase, the solvent moving through the paper, compared with a stationary phase, the paper itself. Paper chromatography results are interpreted by calculating the Rf value of a spot, while instrumental methods such as gas chromatography linked to mass spectrometry (GC-MS) give faster, more sensitive analysis for Higher tier.
Method
- Set up the experiment correctly: draw a pencil baseline near the bottom of the chromatography paper (pencil is used because it does not dissolve in the solvent or react, unlike ink), add small spots of each sample on the baseline, and stand the paper in a shallow layer of solvent that is below the level of the baseline, so the samples do not dissolve straight into the solvent.
- Explain the separation: as the solvent (the mobile phase) travels up the paper by capillary action, each substance in a spot distributes between the solvent and the paper (the stationary phase); a substance that is more soluble in the solvent spends more time in the mobile phase and travels further up the paper, while a substance more strongly attracted to the paper travels less far.
- Calculate the Rf value for a spot using Rf = distance travelled by the substance divided by distance travelled by the solvent, both measured from the baseline; every Rf value is between 0 and 1, since a spot can never travel further than the solvent front.
- Use the number of spots to judge purity: a pure substance produces exactly one spot on the chromatogram, however many different solvents are tried, while a mixture usually separates into two or more spots.
- Identify unknown substances either by comparing their Rf value with published reference values for the same solvent, or, more reliably, by running known reference substances on the same plate alongside the unknown and comparing spot positions and Rf values directly under identical conditions.
- For Higher tier, explain the advantages of instrumental methods of analysis over manual tests: they tend to be much more sensitive (able to detect very small amounts of a substance), more accurate, and faster, though they need calibrating against samples of known concentration.
- For Higher tier, describe gas chromatography linked to mass spectrometry (GC-MS): gas chromatography separates a mixture into its individual components, each leaving the column at its own characteristic retention time; the mass spectrum recorded for each component shows a molecular ion peak whose mass to charge ratio gives the relative molecular mass of that compound, helping to identify it.
Worked example
A student runs a paper chromatogram of an ink mixture. The solvent front has travelled 9.0 cm from the baseline. The ink separates into two spots, A and B. Spot A has travelled 3.6 cm and spot B has travelled 7.2 cm. Calculate the Rf value of each spot, and state what the two spots show about the ink.
- Write the Rf formula: Rf = distance travelled by the spot / distance travelled by the solvent.
- Calculate Rf for spot A: 3.6 / 9.0 = 0.4.
- Calculate Rf for spot B: 7.2 / 9.0 = 0.8.
- Interpret the two spots: since the ink separates into two spots with different Rf values, the ink is a mixture of at least two different coloured substances, not a pure single dye.
- Final answer: Rf(A) = 0.4 and Rf(B) = 0.8, and the two spots show the ink is a mixture of two components.
Practice questions
Type your answer and press Check to be marked straight away, or reveal the answer and mark yourself.
Q1Write the formula for calculating an Rf value.Show answer
Answer: Rf = distance travelled by the substance (spot) / distance travelled by the solvent, both measured from the baseline.
Q2In a chromatography experiment, a dye spot travels 4.5 cm and the solvent front travels 6.0 cm. Calculate its Rf value.Show answer
Answer: 0.75 (4.5 / 6.0).
Q3Explain why the baseline is drawn in pencil rather than ink.Show answer
Answer: Pencil marks are insoluble in the solvent and do not react with it, so the baseline stays fixed; ink would dissolve into the solvent and run with the samples, making the results unreliable.
Q4Explain why the solvent level in the container must be below the pencil baseline.Show answer
Answer: If the solvent covered the baseline, the sample spots would dissolve directly into the bulk solvent rather than being carried up the paper, so no separation would take place.
Q5A sample produces only one spot when run in three different solvents. What does this suggest about the sample?Show answer
Answer: It suggests the sample is a pure substance rather than a mixture, since a mixture would be expected to separate into more than one spot in at least one solvent.
Q6State one advantage of running known reference substances alongside an unknown sample on the same chromatogram, rather than only comparing to published Rf values.Show answer
Answer: The reference and the unknown experience exactly the same paper, solvent, temperature and distance travelled by the solvent, so their spot positions can be compared directly and reliably, whereas published Rf values can vary between different experimental conditions.
Q7State two advantages of instrumental methods of analysis, such as GC-MS, compared with manual chemical tests.Show answer
Answer: They are generally more sensitive, able to detect very small quantities, and faster; they can also be more accurate than manual tests, though they must be calibrated against known standards.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A student wants to find out which of four food dyes, labelled P, Q, R and S, are present in a green sweet coating. She runs a paper chromatogram with a spot of the sweet coating extract and a spot of each of the four reference dyes, using the same solvent for all five spots. The solvent front travels 10.0 cm. The sweet coating extract produces two spots, at 3.0 cm and 8.0 cm from the baseline. Reference dye P travels 3.0 cm, Q travels 5.0 cm, R travels 8.0 cm and S travels 9.0 cm. Explain how the student can identify which dyes are in the sweet coating, and state which dyes she should conclude are present.
Show mark scheme
Tick each line you got. Your score builds from the marks on the scheme.
Nothing ticked yet - 6 available
Explain why gas chromatography linked to mass spectrometry (GC-MS) can identify individual components in a mixture more reliably than paper chromatography alone.
Show mark scheme
Tick each line you got. Your score builds from the marks on the scheme.
Nothing ticked yet - 3 available
See real GCSE Science past-paper questions, with official mark schemes →
Free printable worksheet
Want more practice on paper? Download the chromatography and instrumental analysis worksheet pack - 14 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 27 of GCSE Chemistry Workbook, the whole course as one free printable PDF.
Next topics
Not quite what you needed?
Tell us what is missing on chromatography and instrumental analysis, or which topic to write up next. Every request is read, and we reply to every one.
Build a full practice pack.
This topic is one of hundreds in the library - pick the ones a student needs and generate a printable PDF in minutes.