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How to read a mass spectrometry result on a COA: m/z, charge states and mass error

Published 4 Nov 2026 · By The QLD Analytical Team

Informational guide for research samples — not medical advice. QLD Analytical arranges independent testing; we do not sell or endorse any product.

Most certificates of analysis for research peptides include a mass spectrometry result to confirm identity. It may appear as a single mass, a list of m/z values or a simple “conforms” statement. This guide explains what those numbers mean, how to check them against the expected mass, and what a matching mass does and does not prove.

What a mass spec identity test answers

Mass spectrometry (MS) asks a narrow question: does the molecular mass measured in the sample match the mass expected for the compound on the label? For a peptide, the laboratory calculates a theoretical mass from the declared sequence, including any terminal groups or modifications. It then compares that figure with what the instrument observes.

That makes MS the identity half of a standard peptide panel. HPLC tells you how much of the detected material sits in the main peak. MS tells you whether the main component has the right mass. Neither result replaces the other, which is why our guide to reading an HPLC chromatogram pairs well with this one.

Mass, charge and m/z

A mass spectrometer does not weigh molecules directly. It measures ions, and it reports their mass-to-charge ratio, written m/z. To become an ion, a peptide usually picks up one or more protons (H⁺) in positive-ion mode, or loses them in negative-ion mode.

The relationship for a peptide of neutral mass M carrying z extra protons is:

m/z = (M + z × 1.00728) ÷ z

The number 1.00728 is the mass of a proton in daltons (Da). So a singly protonated ion is written [M+H]⁺ and appears at about M + 1.007. A doubly protonated ion, [M+2H]²⁺, appears at roughly half the mass plus one.

Charge states: why one peptide gives several peaks

Electrospray ionisation (ESI), the technique most often coupled to liquid chromatography (LC-MS), tends to put several charges on a peptide. Each charge state produces its own peak. Take a hypothetical peptide with a monoisotopic mass of 1,500.80 Da:

• [M+H]⁺ appears at m/z 1,501.81
• [M+2H]²⁺ appears at m/z 751.41
• [M+3H]³⁺ appears at m/z 501.27

All three peaks come from the same molecule. Someone expecting a single peak near 1,500 can easily misread a spectrum dominated by m/z 751 as “wrong mass”. It isn’t. The software, or the analyst, works back to the neutral mass from the charge states. That calculation is called deconvolution.

The charge state can be read from the isotope pattern. Isotope peaks of a singly charged ion are about 1 m/z unit apart. For a 2+ ion they are about 0.5 apart, and for a 3+ ion about 0.33 apart. MALDI, the other common ionisation method, mostly produces singly charged [M+H]⁺ ions, so its spectra look simpler.

Monoisotopic vs average mass

Carbon, hydrogen, nitrogen, oxygen and sulfur all have heavier natural isotopes, mainly carbon-13. A peptide therefore appears as a cluster of peaks rather than a single line. Two mass figures are in common use:

• Monoisotopic mass: the mass of the molecule made only of the lightest, most abundant isotope of each element. It is the first peak in the cluster and is the usual reference on high-resolution instruments.
• Average mass: the mass weighted by natural isotope abundances. It is what a balance “sees” and is the usual basis for a molecular weight on a label.

The gap between them grows with size. For a peptide around 1,500 Da it is roughly 1 Da. For larger peptides it reaches several daltons, and the monoisotopic peak becomes too small to see clearly. A COA should say which kind of mass it reports. Comparing an observed monoisotopic value with a theoretical average mass, or the reverse, can look like a mismatch when there is none.

Mass error, ppm and what counts as a match

No instrument reads exactly the theoretical value. The difference is usually expressed in parts per million (ppm):

mass error (ppm) = (observed − theoretical) ÷ theoretical × 1,000,000

Using the example above, an observed m/z of 751.409 against a theoretical 751.407 is an error of about 2.7 ppm. High-resolution instruments, such as Orbitrap and time-of-flight analysers, routinely work within a few ppm. Lower-resolution instruments, such as single-quadrupole LC-MS systems, are often reported to the nearest 0.1 to 1 Da.

So there is no single universal “pass” window. The acceptance range depends on the instrument and the method, and a well-written report states it or reports the observed and theoretical values side by side. A result such as “observed 1,500.8 Da, theoretical 1,500.8 Da, conforms” is more useful than a bare “pass”.

Adducts, oxidation and other extra peaks

Extra peaks in a spectrum do not automatically mean the wrong compound. Several common mass shifts have well-known causes:

• Sodium and potassium adducts. [M+Na]⁺ sits about 22 Da above [M+H]⁺, and [M+K]⁺ about 38 Da above. They come from trace salts and glassware and are usually reported as the same compound.
• Oxidation (+16 Da). Methionine, and to a lesser extent tryptophan, can pick up an oxygen atom. A +16 peak is a real chemical change, not just an ionisation artefact.
• Deamidation (+0.98 Da). Asparagine and glutamine can lose an amide nitrogen and gain an oxygen. The shift is small and can be hidden inside the isotope cluster on a low-resolution instrument.
• Deletion sequences. A peak one residue lighter than the target, for example −57 Da for glycine or −71 Da for alanine, points to a synthesis by-product rather than the full-length peptide.
• Disulfide state. Forming a disulfide bond removes two hydrogens, so the oxidised (bridged) form is about 2 Da lighter than the reduced form.

Counter-ions such as trifluoroacetate (TFA) or acetate are not part of the peptide. They separate from the peptide in solution, so they do not add to the expected peptide mass. That is why the declared salt form matters when the laboratory works out what mass to expect.

What mass spec cannot tell you

A matching mass is strong evidence of identity, but it has limits:

• Isomers have the same mass. Leucine and isoleucine are both 113.08 Da, and an L- or D-form of a residue weighs the same. A scrambled sequence with the same composition also has the same intact mass. Fragmentation (MS/MS) and chromatography can help separate these cases.
• MS is not a purity test. Signal strength depends on how easily each species ionises, so peak heights in a mass spectrum are not a reliable measure of how much of each component is present. That is HPLC’s job.
• MS does not measure content. It says nothing about how many milligrams of peptide are in the vial. That is a separate net content measurement.
• MS does not cover contaminants. Endotoxin, microbial load and heavy metals need their own tests. See our guide to endotoxin (LAL) testing.

Checklist for a mass spec result on a COA

1. Is the compound and sequence the result was compared against stated, including terminal groups and modifications?
2. Does the report give a theoretical mass and an observed mass, not just “pass”?
3. Does it say whether the masses are monoisotopic or average?
4. If m/z values are listed, are the charge states identified?
5. Is the acceptance window or mass error stated?
6. Are significant extra species, such as +16 or deletion peaks, mentioned?
7. Do the test number, sample description and batch match the vial you hold?

Identity testing through QLD Analytical

QLD Analytical is a testing intermediary. Samples are analysed offsite by Apex Labs Pty Ltd. Our standard panel covers mass spectrometry identity, HPLC purity and net content. When you order, enter the compound name and labelled strength exactly as printed on the vial. If the sample is a salt form, such as acetate or trifluoroacetate, note it on the order form so the laboratory can interpret the mass correctly.

Depending on how the laboratory reports the method for that sample type, the certificate may show identity as a numeric mass, a match statement, or both. As with every test we arrange, the result applies only to the sample sent in. Packing and postage details are in our customer guide and FAQ.

Sources

• NIST Physical Measurement Laboratory — Atomic Weights and Isotopic Compositions (isotope masses and natural abundances behind monoisotopic and average mass).
• Unimod protein modifications database — Oxidation, Deamidation and Sodium cation mass shifts.
• Wikipedia — Mass-to-charge ratio (overview of m/z notation).
• QLD Analytical — Mass spectrometry identity testing (salt forms and how identity is reported).