In short
A mass spectrum shows which fragments a molecule breaks into and in what proportions: a plot of mass-to-charge ratio (m/z) against abundance. It gives strong clues about structure, but it rarely identifies a fragrance molecule alone, because many isomers fragment alike. A proper identification combines the spectrum with a retention index and, where possible, an authentic standard. This lesson teaches you to read the plot, and to be careful of the number beside it.
What you will learn
- Read a molecular ion, a base peak and common fragments
- Explain why a library match isn't an identification
- Check a spectrum against a reference and a retention index
Before this lesson: How a GC Column Separates a Fragrance
What's happening
In the common electron ionization mode, molecules leaving the column are hit with electrons, usually at 70 eV. A molecule loses an electron, forming a molecular ion. Much of it breaks up into fragments, whose ions are sorted by mass-to-charge. Since the conditions are standardized, spectra can be compared between instruments, which is why libraries work at all.
Reading the plot
- Find the highest meaningful mass. That may be the molecular ion. For a monoterpene hydrocarbon such as limonene, C10H16, it appears at m/z 136.
- Find the base peak. It's the tallest peak, shown as 100%.
- Look at the differences. A loss of 15 suggests a methyl group, a loss of 18 suggests water, as often seen from alcohols, and a loss of 43 might suggest an acetyl or a propyl group.
- Look for known patterns. Many terpenes show ions at m/z 93, 69, 41 and similar. Limonene is known for a strong ion at m/z 68.
- Compare with a reference, ion by ion.
A worked comparison
| Compound | Formula, mass | Notable ions | A point to note |
|---|---|---|---|
| Alpha-pinene | C10H16, 136 | 93 prominent | Very close to beta-pinene and others |
| Limonene | C10H16, 136 | 68 prominent | Enantiomers are indistinguishable here |
| Linalool | C10H18O, 154 | 71, 93; 154 weak | A weak molecular ion is normal |
Notice what the MS can't do. Enantiomers give essentially the same EI spectrum, and many double-bond isomers look alike. A small peak in an isomer's spectrum might be noise, or it could be the only difference.
Library matches
Software scores your spectrum against a library. A high score means high similarity, and nothing more. The risks:
- The library only holds what it holds. A compound not in it can't be matched, so the top hit is merely its nearest neighbor.
- Isomers score alike. Several candidates may be within a few points.
- Coelution mixes spectra. A combined spectrum can match neither compound well, or match a wrong one.
- Trace peaks are noisy. Scores become less reliable for tiny peaks.
Treat the top hit as a hypothesis, check its retention index, and consider whether the compound is plausible in that material. A sound report often marks its identifications by level of confidence, with an authentic standard at the top.
Plausibility
Chemistry and botany help. A library might suggest something never reported in that plant. That may be a discovery, an adulterant, a contaminant or a wrong match, and the last is by far the most common. Check the published literature for that material before you draw a conclusion.
Practical exercise: Compare three published spectra
This exercise uses free reference data and needs no instrument.
- In the NIST WebBook, open the mass spectra of alpha-pinene, beta-pinene and terpinolene.
- For each, write the molecular ion, base peak and three other strong ions.
- Write which features would distinguish them, and how confident you would be.
- Look up each one's retention index on a low-polarity phase, and add it to your table.
- Write a sentence explaining how the retention index changes your confidence.
You have done this well if you noted real similarities rather than forcing differences, and your conclusion uses more than one line of evidence.
Common mistakes
- Taking the top hit as identity. It's only a hypothesis.
- Reading the score as purity. It says nothing about quantity.
- Expecting MS to see chirality. Standard EI spectra do not.
Key terms
- Molecular ion
- The ion formed when a molecule loses one electron. Its mass-to-charge value gives the molecular mass, though it may be weak or absent.
- Base peak
- The tallest peak in a mass spectrum, set to 100% relative abundance. It marks the most abundant fragment, not necessarily the most informative.
- Electron ionization
- The common GC-MS method that bombards molecules with electrons, usually at 70 eV, breaking them into reproducible fragments.
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.