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A mass spectrum is a bar graph: the horizontal axis is mass-to-charge ratio (m/z) and the vertical axis is relative abundance. To read one, find the base peak, find the molecular ion (or its soft-ionization equivalent), then use the isotope pattern and the lower-mass fragments to work out what the molecule is. This guide gives that workflow as numbered steps, shows how isotope patterns reveal chlorine and bromine, and lists the reading mistakes that cause most wrong assignments. It is written for students, technicians and researchers who have a spectrum in front of them and need to interpret it.
What you need
- The spectrum itself, with both axes labelled (m/z and relative abundance, or absolute intensity).
- The ionization method and ion mode used. ACD/Labs notes that the ionization method is crucial to interpreting a spectrum, and that positive or negative ion mode must be set correctly for the analysis.
- Any sample context: expected compound class, solvent, and whether the spectrum came from a direct probe or from a chromatograph (see LC-MS explained).
- A periodic table with isotope abundances, and access to a reference library such as the NIST Chemistry WebBook, which lists mass spectra credited to the NIST Mass Spectrometry Data Center.
What the graph shows
How do you read a mass spectrometry graph?
Each line (peak) is one ion. Its position on the horizontal axis is its m/z; its height is its relative abundance. According to the LibreTexts organic chemistry text (Morsch et al.), the lines are labelled by m/z, the tallest line is the base peak, and the base peak is usually given an arbitrary height of 100, with every other peak scaled relative to it. The base peak is the most common fragment ion, which is not the same thing as the heaviest ion in the spectrum.
Steps: how to interpret a mass spectrum
- Check the ionization type and ion mode. Hard ionization imparts excess energy and produces fragmentation; soft ionization produces little fragmentation (ACD/Labs). That decides what the spectrum can tell you: a hard-ionization spectrum is rich in fragments, a soft-ionization spectrum is dominated by the ion of the intact molecule. See electrospray ionization source parameters for one common soft source.
- Read the axes. Confirm the m/z range and whether intensities are relative (scaled to the base peak = 100) or absolute counts.
- Find the base peak. It is the tallest line. Note its m/z, but do not treat it as the molecular mass unless other evidence supports that.
- Find the molecular ion. In a hard-ionization spectrum the molecular ion (M+, also called the parent ion) is formed by knocking one electron off the molecule, and it is typically the highest-m/z line of the main cluster. LibreTexts gives pentane as the example: its heaviest line is at m/z 72. In soft-ionization spectra the intact molecule usually appears with an added or removed charge carrier (for example a protonated molecule), so the signal sits at a value offset from the neutral mass.
- Examine the isotope cluster around that ion (next section). It can tell you about carbon count and the presence of chlorine or bromine.
- Examine the lower-mass fragments. Gaps between the molecular ion and fragment peaks correspond to pieces lost from the molecule; rules for what those losses mean are in the mass spectrometry fragmentation rules guide.
- Compare with a library or database. Search a reference collection such as the NIST WebBook and judge the match; see interpreting NIST library match factors and the organic spectroscopy database overview.
Isotope patterns: M+1 and M+2
What do the small peaks beside the molecular ion mean?
Natural isotopes produce small companion peaks. The M+1 peak sits one m/z unit above the molecular ion and comes from carbon-13, which makes up 1.11% of carbon atoms (Chemguide, read 2026-11-07). As a rough guide the M+1 peak is about 1% of the M+ height per carbon atom, so a two-carbon compound gives about 2% and a three-carbon compound about 3%. Chemguide warns that above three carbon atoms the approximation should not be used.
| Pattern at the molecular ion | What it indicates | Source |
|---|---|---|
| Two peaks 2 m/z apart, heights about 3:1 | One chlorine atom | Chemguide, M+2 page |
| Three peaks (M, M+2, M+4), heights about 9:6:1 | Two chlorine atoms | Chemguide, M+2 page |
| Two peaks 2 m/z apart, nearly equal height | One bromine atom (79Br and 81Br, about 1:1) | Chemguide, M+2 page |
Reading spectra from different sources
An electron-ionization (hard) spectrum shows many fragment lines and a molecular ion that may be weak. A soft-ionization spectrum shows few fragments, so the pattern to read is the intact-molecule signal and its isotope cluster. If the spectrum comes from an LC-MS or GC-MS run, the instrument records a spectrum at each point in the chromatogram, so you also choose which retention-time slice you are reading; see GC-MS vs LC-MS for how the two couplings differ.
What can go wrong
- Mistaking the base peak for the molecular ion. The base peak is the tallest line, not the heaviest; they coincide only sometimes.
- Ignoring ionization method and ion mode. The same compound looks very different under hard and soft ionization, and a spectrum recorded in the wrong polarity will not match expectations (ACD/Labs).
- Reading relative intensity as absolute. Heights are scaled to the base peak unless the axis says otherwise.
- Over-using the M+1 rule. Counting carbons from the M+1 ratio breaks down above about three carbons (Chemguide).
- Treating a library hit as proof. A high match is a candidate, not a confirmation; check it against the isotope pattern and the sample context.
When to get professional help
If the instrument is poorly calibrated, the masses look shifted, or a result will support a regulated decision, ask the facility’s mass-spectrometry specialist to review the data. For mass-axis problems see mass spectrometer tuning and mass calibration, and for quantitation work see internal standards in quantitative mass spectrometry.
FAQ
How to interpret mass spectrometry?
Identify the ionization method, locate the base peak and the molecular ion, read the isotope cluster, interpret the fragment losses, and compare with a reference library. The numbered steps above give the order.
How to read a mass spectrometry graph?
Read m/z on the horizontal axis and relative abundance on the vertical axis; the tallest peak is the base peak, set to 100, and all other peaks are scaled to it.
What is the molecular ion peak?
It is the ion formed when the molecule loses one electron, written M+; it gives the molecular mass in a hard-ionization spectrum (LibreTexts). For a first look at the instrument itself, see what is a mass spectrometer.
Sources
- LibreTexts Chemistry, 12.2 Interpreting Mass Spectra (read 2026-11-07).
- ACD/Labs, A Beginner’s Guide to Mass Spectrometry (read 2026-11-07).
- Chemguide, The M+1 peak and The M+2 peak (read 2026-11-07).
- NIST Chemistry WebBook, webbook.nist.gov, Standard Reference Database 69 (read 2026-11-07).
