In short
A GC-MS report usually has a chromatogram and a peak table listing retention time, area and a proposed identity. Read it as a set of candidates. Area percent is only a relative signal, a library match is a suggestion, and absence of a peak proves little. The most useful skill is knowing which lines to trust and which to question.
What you will learn
- Read a chromatogram and peak table
- Explain the limits of area percent and library match
- Mark which identifications deserve doubt
Before this lesson: GC-MS Fundamentals: Study Guide
An invented report to read
This report was made up for teaching. It looks like a lavender-type oil, but none of the figures comes from a real analysis, so do not use them as a reference for lavender.
| Peak | Retention time (min) | Area % | Proposed identity | Library match |
|---|---|---|---|---|
| 1 | 7.1 | 0.8 | alpha-Pinene | 94 |
| 2 | 9.4 | 1.2 | Limonene | 91 |
| 3 | 9.5 | 3.1 | 1,8-Cineole | 96 |
| 4 | 11.8 | 34.0 | Linalool | 97 |
| 5 | 13.0 | 0.9 | Camphor | 88 |
| 6 | 16.2 | 41.5 | Linalyl acetate | 95 |
| 7 | 19.6 | 2.4 | Unidentified | 62 |
| 83.9 | Total reported |
Line by line
- Total. It reports 83.9%. Where is the rest? Perhaps many small peaks that were not listed, or a cut-off. Ask.
- Peaks 2 and 3. They are just 0.1 minute apart. If these overlap, each area is uncertain, and a spectrum may be mixed. Coelution of limonene and 1,8-cineole is a known issue on some columns, so this is exactly the kind of point to query.
- Peak 4 and 6. Plausible, and large enough to be measured well, but area is not weight unless the method was calibrated. Linalool and linalyl acetate may not respond identically.
- Peak 5. Match 88. Lower, so check against a retention index before believing it.
- Peak 7. Unidentified. It may be real, and it may be important: some of the most characteristic trace materials are minor peaks.
Understanding the main columns
| Column | Means | Does not mean |
|---|---|---|
| Retention time | When it left this column in this run | A portable number. Use retention indices when comparing with the literature. |
| Area % | Share of the detector's total signal | Weight percent, unless calibrated |
| Proposed identity | A candidate | A confirmed identification, unless checked with a standard or index |
| Match | Spectral similarity | Probability of being right |
What the report cannot show
- Odor impact. A trace peak may dominate the smell, and a 40% peak might be a mild material.
- Enantiomers, unless a chiral column was used.
- Non-volatile components, such as waxes or some resins.
- Very low-level materials, such as the sulfur compounds that matter to some oils.
- Origin. Synthetic and natural linalool can look identical on a standard run. Showing origin requires other methods.
- Safety. Peak lists do not assess a formula, and they may miss restricted constituents present at low levels.
The analytical track goes deeper into retention indices, coelution and how identifications are confirmed. NIST's retention data is one public reference.
Practical exercise: Annotate a sample report
Use the invented report above.
- For each line, mark it as trust, doubt or ask, with a reason.
- Which two lines would you ask the analyst about first?
- Write two statements you could fairly make from this report, and two that you could not.
- If you have access to a real supplier GC report, repeat the exercise, and note how much information it gives you about its method.
You have done this well if you questioned the overlap of peaks 2 and 3 and the 83.9% total, and did not treat any area figure as a weight without a caveat.
Common mistakes
- Treating area percent as composition. Calibration is what makes that possible.
- Treating a match score as a probability. It is not.
- Treating a missing peak as absence. It may simply not be detectable this way.
- Building a formula from one GC list and calling it a copy. The fragrance matching guide explains why it is not.
Key terms
- Chromatogram
- A plot of detector signal against time, in which each peak represents one or more components leaving the column.
- Retention time
- The time a component takes to pass through the column. It depends on the method and instrument.
- Area percent
- A peak's area as a share of the total area. It is a relative signal and not automatically a weight percentage.
- Library match
- A similarity score between a measured mass spectrum and a reference spectrum. A candidate identification, not a proof.
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.