Interpret peak areas as mixture composition
Use chromatogram peak areas to estimate component percentages when the detector response is comparable or calibrated.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 37. Use the supplied spectra, shift table and chromatography conditions. Interpret peaks as evidence and state when a short trace cannot uniquely identify a substance. Written practice is self-checked
Before you begin
- Recognise functional groups, molecular structures, polarity, intermolecular interactions and the distinction between equivalent and different chemical environments.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How is a component's chromatographic area percentage calculated?
What you will learn
- Relate resolved peaks to mixture components
- Calculate percentage composition from peak areas
- State the detector-response assumption
Use area, not just peak height
A gas/liquid chromatogram can show separate peaks for components in a mixture. When the detector response is proportional to the amount and response factors are equal or supplied, each peak's integrated area represents its relative amount.
Percentage composition = peak area ÷ total area of all relevant peaks × 100. Use the area values provided, not peak height alone, because peaks can have different widths.
Keep the interpretation bounded
If the detector response factors differ, raw area percentages may not equal actual mole or mass percentages; use calibration or response-factor information when provided. If the question gives only peak areas and expects direct composition, state the proportional-response assumption.
An unresolved overlap makes an area harder to assign to one component. Work from separated peaks or use the deconvolution/calibration assumptions stated in the question.
Pause and explain
Two peaks have areas 25 and 75 units with equal responses. What percentage is represented by the first peak?
Worked example
A two-component chromatogram has areas 30 and 70 units with equal detector response. Find the percentage composition.
Show the worked solution
- Total area is 30 + 70.
- Divide each peak area by the total.
- Multiply each fraction by 100.
Answer Component 1: 30/100 × 100 = 30%. Component 2: 70/100 × 100 = 70%.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Three equal-response peaks have areas 12, 18 and 30. Calculate each percentage.
Give me a hint
Add all areas before calculating each share.
Compare my reasoning
- Total area = 60.
- Compute 12/60, 18/60 and 30/60.
- Multiply each by 100.
20%, 30% and 50%, respectively.
Look for these in your work
- Total correct
- Shares calculated
- Percentages sum to 100%
A peak has area 45 units and the total area is 180 units. Find its estimated composition.
Give me a hint
Use the peak's fraction of the whole chromatogram area.
Compare my reasoning
- 45/180 = 0.25.
- Convert the fraction to a percentage.
- Include the response assumption.
25% of the mixture by detector response, assuming equal or appropriately calibrated response factors.
Look for these in your work
- Fraction correct
- Percentage correct
- Assumption stated
Why may a raw 40% peak-area share not equal exactly 40 mol% if two compounds have different detector response factors?
Give me a hint
The detector signal per amount can differ between compounds.
Compare my reasoning
- Area measures detector response.
- Unequal response factors scale the areas differently.
- Calibration or response factors are needed for corrected composition.
The peak area is proportional to detector response, not necessarily equal molar amount when response factors differ; calibration or supplied response factors are needed.
Look for these in your work
- Signal distinguished from amount
- Response factor considered
- Correction described
Common mistakes
- Using peak height when area is supplied
- Dividing by only one component's area
- Reporting percentages that do not sum to 100%
- Ignoring unequal detector responses when they are stated
What can you explain now?
How is a component's chromatographic area percentage calculated?
Compare with the explanation
Its peak area divided by total peak area × 100
This gives its share of the summed response under the stated assumptions.
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Make it stick
How do peak areas estimate composition, and when does the estimate need a detector-response correction?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.