How to Read a Mass Spectrum for Peptide Identity
A mass spectrum is the chart that sits behind the MS identity line on a peptide Certificate of Analysis (COA). It plots the masses the spectrometer recorded for a separated component, and reading it is how you check that a detected mass matches the expected mass of the named compound. This page explains the four things to look at on the chart — the m/z axis, the molecular ion, the isotope pattern, and the expected-versus-detected comparison — as documentation only. It does not describe any use, effect, or outcome of any compound.
Research use only. Not for human or animal consumption. This is reference material about an analytical chart. It does not describe a dose, an effect, or a benefit of any compound.
The m/z axis: what the horizontal scale means
A mass spectrum has two axes. The horizontal axis is m/z — the mass-to-charge ratio of an ion — and the vertical axis is relative intensity, usually scaled so the tallest peak reads 100%. Each vertical line (a peak) marks an ion the detector recorded at a particular m/z value.
The key point for reading the chart: m/z is mass divided by charge. A peptide ion carrying a single charge appears at a value close to its molecular mass; an ion carrying two charges appears at roughly half that value, three charges at roughly a third, and so on. So the same molecule can produce a small family of peaks at predictable positions. Knowing the charge state of a peak is what lets you convert its m/z back to a molecular mass to compare against the expected molecular weight. For where the MS result fits on the certificate as a whole, see lc ms identity confirmation explained.
Finding the molecular ion
The molecular ion is the peak that corresponds to the intact molecule (as an ion), and it is the anchor for an identity reading. For peptides analysed by electrospray, the spectrum often shows several charge states of that same intact molecule rather than one peak — for example a singly charged ion and a doubly charged ion sitting at their predictable m/z positions.
To read it, you (or the lab software) take the m/z of a molecular-ion peak, apply its charge state, and work back to a single molecular mass. That recovered mass is the detected mass quoted on the COA. Peaks that are not the molecular ion — fragments, adducts, or background — are not what the identity comparison is built on, which is why a clear certificate states which mass it treated as the molecular ion and what charge state it assumed.
Reading the isotope pattern
Real molecules contain a mix of natural isotopes (mostly the small fraction of carbon-13 among carbon-12), so a single component does not give one infinitely thin line. It gives a small cluster of closely spaced peaks — the isotope pattern — separated by about one mass unit divided by the charge.
That spacing is a useful cross-check. The gap between adjacent isotope peaks tells you the charge state directly: a one-unit spacing indicates a singly charged ion, a half-unit spacing a doubly charged ion. The shape and relative heights of the cluster are also broadly characteristic of a molecule of that size, so a pattern that looks consistent with the expected formula supports the reading, while a clearly wrong shape is a flag to check the assignment. The isotope cluster is part of why a chromatogram and spectrum carry more information than a single quoted number.
Expected vs detected mass
The reading only means something when it is compared against a fixed reference. The expected mass is calculated from the compound's molecular formula and does not change between batches; the detected mass is what this run recovered from the molecular ion. Identity confirmation is the match between the two within the instrument's stated tolerance.
| Term | What it is | Where it comes from |
|---|---|---|
| Expected mass | Theoretical mass of the named compound | Calculated from the molecular formula |
| Detected mass | Mass recovered from the molecular ion | This MS / LC-MS run on this batch |
| Match | Detected sits within tolerance of expected | The comparison of the two figures |
Note which mass convention the certificate uses — the monoisotopic mass (the lowest-mass isotope peak) or the average mass — because the expected value differs slightly between them. A spectrum whose recovered mass matches the expected molecular weight is consistent with the named compound; one that lands on a different mass is describing something else. Remember that this is an identity check, separate from a purity figure — see hplc purity vs ms identity.
Reading checklist
- Confirm the axes — m/z horizontal, relative intensity vertical.
- Identify the molecular-ion peak(s) and their charge states.
- Use the isotope spacing to confirm the charge state.
- Recover the detected mass and compare it to the expected molecular weight within tolerance.
- Check the result is tied to a lot / batch number and a report date.
For where the molecular formula, weight and lot appear on the document, see how to read a peptide coa.
Research use only. Not for human or animal consumption. This page is reference documentation about an analytical chart and does not describe any use, effect, benefit, dose, or outcome of any compound.
FAQ
What do the axes on a peptide mass spectrum mean?
The horizontal axis is m/z, the mass-to-charge ratio of each recorded ion, and the vertical axis is relative intensity, usually scaled so the tallest peak reads 100%. Each peak marks an ion the detector recorded at a particular m/z. Because m/z is mass divided by charge, a singly charged ion sits near the molecular mass while multiply charged ions appear at lower m/z values.
What is the molecular ion and why does it matter?
The molecular ion is the peak corresponding to the intact molecule as an ion. It is the anchor for an identity reading: its m/z and charge state are used to recover a single molecular mass, which becomes the detected mass quoted on the COA. Fragments, adducts and background peaks are not the molecular ion and are not what the expected-versus-detected comparison is built on.
How does the isotope pattern help read a spectrum?
Natural isotopes give each component a cluster of closely spaced peaks rather than one line. The spacing between adjacent isotope peaks indicates the charge state directly: a one-unit gap means a singly charged ion, a half-unit gap a doubly charged ion. The cluster's shape is also broadly characteristic of a molecule of that size, so a consistent pattern supports the assignment and an obviously wrong shape is a flag to recheck it.
What is the difference between expected and detected mass?
Expected mass is the theoretical molecular weight calculated from the compound's molecular formula, a fixed value that does not change between batches. Detected mass is what the run recovered from the molecular ion. Identity confirmation is the match between the two within the instrument's stated tolerance. Check whether the certificate uses the monoisotopic or average mass, since the expected value differs slightly between them.
Does reading the spectrum tell you how pure the sample is?
No. The mass spectrum is used to recover a mass and confirm identity, which is a separate question from purity. Purity, typically an area-percent figure from HPLC, measures how much of the detected signal is the main component, while the spectrum establishes what that component is. A complete COA reports both, each tied to its named method, lot or batch number, and report date.
Research use only. Not for human or animal consumption.