How to Read a Peptide Mass Spectrometry Report

When a COA says “identity confirmed by mass spectrometry”, it is describing a spectrum that is rarely shown in full. This guide explains what the spectrum measures, how to check the numbers yourself, and what the result cannot tell you. It is general analytical background for laboratory researchers.

What mass spectrometry measures

A mass spectrometer measures the mass of ions. For peptides, the usual approach is electrospray ionisation (ESI), which gives the molecule one or more positive charges and reports it by its m/z value: the mass divided by the charge.

The point of the test is identity. If the observed mass matches the expected mass for the stated sequence, that supports the sample being the right molecule. It is a different question from purity, which is addressed in how to read an HPLC chromatogram.

Why one peptide shows several peaks

Peptides can pick up more than one charge. A single compound of mass M produces a series of ions, written [M+2H]2+, [M+3H]3+ and so on. Each appears at a different m/z:

m/z = (M + n × 1.00728) / n

where n is the number of charges and 1.00728 is the mass of a proton. For a peptide with a mass of 1,000.00 Da, the 2+ ion appears near 501.01 and the 3+ ion near 334.34. Larger peptides carry more charges and give a longer series, so a spectrum with several evenly spaced peaks for one compound is normal and expected.

Deconvolution: turning the series back into one mass

Many reports show the raw spectrum alongside a deconvoluted result, in which software combines the charge series into a single neutral mass. That is the number that gets compared with the expected mass.

You can check it yourself. Take two neighbouring peaks, work out their charges, and calculate M from each using the formula above. The two answers should agree with each other and with the reported mass.

Expected versus observed mass

• Monoisotopic versus average mass. The monoisotopic mass uses the most abundant isotope of each element. The average mass weights all isotopes. High-resolution instruments usually report monoisotopic values, and low-resolution ones report average values. The report should say which is being compared.
• Tolerance. How close is close enough depends on the instrument. A low-resolution instrument may agree to within a fraction of a dalton, and a high-resolution instrument to a few parts per million. A report should state the tolerance it used.
• The theoretical value. The expected mass should come from the stated sequence or formula, and the report should show it beside the observed value. For how sequences are written, see how to read a peptide sequence.

Extra peaks and what they can mean

Extra signals are common. Some typical ones are:

• Salt adducts. A sodium adduct sits about 22 Da above the expected mass, and a potassium adduct about 38 Da above.
• Oxidation. An increase of about 16 Da is typical for oxidised residues such as methionine.
• Deamidation. An increase of about 1 Da.
• Truncations or deletions. Peaks below the expected mass, differing by the mass of one or more amino acids, point to shorter forms of the peptide.

An adduct is usually harmless noise. A peak that matches a modification or truncation suggests a related impurity, which should also show up in the HPLC data.

What to look for on a real report

1. The spectrum or the deconvoluted result is shown, not only a statement.
2. The expected and observed masses are both given, with the type of mass and the tolerance.
3. The lot number, date and sample name match your vial. See how to read a peptide COA.
4. The lab’s method is named (for example ESI-MS), so the result can be reproduced.

What a mass match does not tell you

• Sequence. Two peptides with the same amino acids in a different order have the same mass. Confirming sequence requires fragmentation (MS/MS).
• Purity. A clean mass match says nothing about how much other material is present. Mass spectrometry is not a quantitative purity method.
• Content. The amount of peptide in the vial is a separate measurement. See purity versus potency.
• Counter-ions and water. Neither shows up as the peptide.

Where to go next

Terms such as adduct, monoisotopic and charge state are explained in the peptide research glossary. The identity test often paired with mass spectrometry is covered in FTIR identification on a peptide COA. Published reports for our lots are on the COA page.

This page is general analytical background for laboratory researchers. It does not describe uses, doses or effects of any compound in humans or animals. For laboratory research use only.