How to Read a Mass Spectrometry COA

On a peptide certificate of analysis, HPLC tells you how pure the material is, but mass spectrometry tells you what it actually is. The mass spec section confirms identity by measuring the molecular mass of the compound and comparing it to the mass the target peptide should have. This guide explains how to read that section without a chemistry background.

What mass spectrometry measures

A mass spectrometer measures the mass-to-charge ratio of ions. In simple terms, it weighs the molecule. For a peptide, the expected mass can be calculated exactly from its amino acid sequence. If the measured mass matches that calculated value, you have strong evidence the vial contains the intended peptide and not a different or mislabelled compound.

Observed mass versus expected mass

The core of the report is a comparison:

• Expected (theoretical) mass: calculated from the peptide’s sequence.
• Observed (measured) mass: what the instrument detected.

When these agree within a small tolerance, identity is confirmed. Small differences are normal and expected, because instruments report either the average mass or the monoisotopic mass, and because measurement has a margin of error. A match within roughly one mass unit for smaller peptides is typically a good sign; a difference of many units suggests a different molecule, a modification, or a problem.

Average mass and monoisotopic mass

You may see two slightly different theoretical numbers. Average mass uses the average weight of each element across its natural isotopes. Monoisotopic mass uses only the most common isotope of each element. For larger peptides the two can differ by a unit or more, so make sure you are comparing the observed value to the right theoretical value. A good COA states which one it is using.

Charge states and adducts

Peptides often pick up more than one charge in the instrument, so the spectrum can show several peaks for the same molecule at different mass-to-charge values. These are charge states, commonly written as [M+H]+, [M+2H]2+, and so on. The software deconvolutes them back to a single molecular mass. You may also see adducts, where the molecule carries a sodium or potassium ion instead of a proton, shifting the apparent mass. These are normal features of the technique, not impurities.

Reading the spectrum itself

If the COA includes the raw mass spectrum, the horizontal axis is mass-to-charge (m/z) and the vertical axis is relative intensity. The tallest peak, the base peak, is set to 100%. You are looking for a clear, dominant signal that deconvolutes to the expected molecular mass. A messy spectrum with many strong unexplained peaks deserves a closer look.

Why identity plus purity matters

A high HPLC purity number is only reassuring once mass spec has confirmed that the main peak is the right molecule. A vial can be 99% one substance and still be the wrong substance. That is why serious testing pairs the two methods.

A quick checklist

• Find the expected mass and confirm it matches the peptide’s sequence.
• Compare it to the observed mass; expect a close, not exact, match.
• Check whether average or monoisotopic mass is being reported.
• Recognize charge states and adducts as normal, not defects.

Testing at LYFE Science

Every LYFE Science lot is verified by HPLC and mass spectrometry, so both identity and purity are documented, and the COA is available per lot. Orders ship Canada-wide by Canada Post, flat $25 and free over $150, in neutral packaging, with Interac e-Transfer and crypto accepted.

Learn to read the mass spec section once and you will never again have to take a purity claim on faith.

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