In short
Area percent is a peak's share of the total area in a chromatogram. It is not the same as a compound's share by weight, because detectors respond differently to different compounds, and because anything that didn't elute is absent from the total. It is a useful description of a chromatogram, and it can be a rough guide. It's a poor basis for a restriction calculation unless you know how it relates to true concentration.
What you will learn
- Explain why area percent differs from weight percent
- Apply a relative response factor
- Say what a report can support for regulatory use
Before this lesson: Retention Indices and Identification
Three reasons the numbers differ
| Reason | Effect |
|---|---|
| Detector response | FID responds broadly in proportion to carbon content, and oxygenated compounds give less per gram. MS response varies far more. |
| Hidden components | Water, waxes, resins and nonvolatiles do not appear, so the rest is inflated. |
| Integration | Overlapping or tailing peaks are split by judgment, and small peaks on a noisy baseline are uncertain. |
Correcting with a response factor
With a calibration, you can correct for response. For this practice, let mass be proportional to area divided by the relative response factor (RRF).
| Compound | Area % | RRF | Area ÷ RRF | Corrected % |
|---|---|---|---|---|
| A | 60 | 1.00 | 60.0 | 47.4 |
| B | 40 | 0.60 | 66.7 | 52.6 |
| Total | 100 | 126.7 | 100.0 |
A appears dominant by area, but B is slightly larger by weight. With a real sample, you'd also need to handle what didn't elute, usually through an internal or external standard, which reports true concentration instead of a normalized share.
Levels of quantification
- Area percent, normalized. Descriptive, and common on supplier reports.
- Corrected with response factors. Better, if the factors suit your compounds and instrument.
- Internal standard calibration. A known compound is added, and each analyte is calibrated against it. This gives true concentration, within stated uncertainty.
- A validated method. Tested for accuracy, precision and limits, often through a formal ring trial, such as EN 16274 for a set of allergens.
Consequences for perfumers
A real example of how this matters is in allergen and restriction work. If you use an area percent figure from a certificate of analysis to work out how much citral or eugenol a natural contributes, you may be off by a meaningful amount. The right approach is to ask your supplier for a composition statement intended for this purpose, and to record its method. If you need your own figures, commission a laboratory, and tell them exactly what you need. Say which compounds matter, at what level, and in what matrix.
Limits matter too. At the EU leave-on threshold of 0.001%, you are asking about 10 mg per kg, and a method has to be capable of seeing that in your matrix. Ask for the limit of quantification, and consider what's meant when a report says "not detected". It means below what the method could see, which isn't the same as absent.
Questions for any quantitative figure
- Normalized or calibrated?
- Which detector was used for quantity?
- Were standards run for this compound?
- What's the limit of quantification, and the uncertainty?
- Which sample or lot, and on what date?
Practical exercise: Correct a report with response factors
This exercise uses invented data and needs no instrument.
| Compound | Area % | RRF |
|---|---|---|
| C | 50 | 1.00 |
| D | 30 | 0.75 |
| E | 20 | 0.50 |
- Calculate the corrected percentages. (Answers: about 38.5%, 30.8% and 30.8%.)
- Suppose the sample also holds 20% nonvolatile material by weight. What is C's true share? (About 30.8%.)
- Take a real certificate of analysis from a supplier. Write down whether it states area or weight, and the detector, and write a question for the supplier.
You have done this well if your arithmetic is right and you can state, for your real certificate, what you can't conclude from it.
Common mistakes
- Calling area percent "content". Label it for what it is.
- Reading "not detected" as "absent". Ask for the limit.
- Applying someone else's response factors. They are specific to the conditions and compounds.
Key terms
- Relative response factor
- How strongly a detector responds to a compound compared with a reference compound at the same amount.
Knowledge check
Written and reviewed by the RUŌOD Lab team. This is general education, not legal, regulatory, medical or safety advice. Example dilutions and exercises are for learning, are not validated products, and have not been safety-assessed. Completing a lesson confers no qualification. How we check these lessons.