Mass spectrometry and peptide identity

Identity is the first question about any sample: is this the peptide the label names? A purity figure cannot answer it, because on its own it shows how clean a sample is, not what the main peak is. The usual answer is mass spectrometry, which measures the mass of the molecule and can also confirm the order of its amino acids. A certificate may cite other identity checks too, such as amino acid analysis or peptide mapping.

The wording below is from the European Medicines Agency's guideline for synthetic peptides in medicines, a European standard. We use it to explain the methods; it says nothing about any product.

What mass spectrometry does

The EMA guideline introduces the method this way:

"Mass spectrometry (MS) is a powerful analytical tool for the structure elucidation of peptides."

European Medicines Agency, Guideline on the Development and Manufacture of Synthetic Peptides, section 4.3.1, retrieved 12 October 2026.

It then says what the method is used for:

"to determine the molecular mass of a peptide and to confirm its amino acid sequence."

European Medicines Agency, Guideline on the Development and Manufacture of Synthetic Peptides, section 4.3.1, retrieved 12 October 2026.

Two separate things are in that line. The first is mass: every peptide sequence has a mass that can be worked out in advance from its amino acids, and the instrument measures the mass of what is actually there. The second is sequence: the same set of amino acids in a different order can have the same total mass, so a mass match alone does not prove the order. Confirming the sequence is a further step.

How to read a mass result

On a certificate, a mass result is useful when it gives two numbers side by side: the expected mass, calculated from the sequence, and the observed mass, measured by the instrument. If they agree within the stated tolerance, that supports the identity. If the line only says "conforms" or "consistent with structure", you cannot see what was compared.

ICH Q7, the international guide for active ingredients, sets the general rule for what a certificate should show:

"The Certificate should list each test performed in accordance with compendial or customer requirements, including the acceptance limits, and the numerical results obtained (if test results are numerical)."

ICH Q7, section 11.42, retrieved 12 October 2026.

A mass is a numerical result. Under that rule, the number belongs on the certificate.

Other ways to confirm identity

Mass spectrometry is not the only method. For a medicine's specification, the EMA guideline lists a set of identity approaches:

"Identification by mass, relative retention time (RRT), LC-MS, peptide mapping, bioactivity"

European Medicines Agency, Guideline on the Development and Manufacture of Synthetic Peptides, section 4.4.2, retrieved 12 October 2026.

In plain words:

• Mass: the measured mass of the molecule, as above.
• Relative retention time: where the main peak comes out of a chromatography column compared with a reference standard run the same way.
• LC-MS: chromatography and mass spectrometry run together, so each peak gets its own mass.
• Peptide mapping: cutting the peptide into pieces at known points and checking the pieces.
• Bioactivity: listed in the guideline for medicines; this library does not cover it.

On peptide mapping, the guideline adds when it fits:

"Peptide mapping may be applicable for longer peptides"

European Medicines Agency, Guideline on the Development and Manufacture of Synthetic Peptides, section 4.3.1, retrieved 12 October 2026.

Amino acid analysis and NMR

Two more methods appear in the guideline's section on structure. The first breaks the peptide down and counts its building blocks:

"Amino acid analysis as described in the Ph. Eur. general chapter 2.2.56"

European Medicines Agency, Guideline on the Development and Manufacture of Synthetic Peptides, section 4.3.1, retrieved 12 October 2026.

Amino acid analysis tells you which amino acids are present and in what ratio. It does not tell you their order. The second is nuclear magnetic resonance:

"NMR can be used for determination of the number and types of proton nuclei and of the peptide sequence"

European Medicines Agency, Guideline on the Development and Manufacture of Synthetic Peptides, section 4.3.1, retrieved 12 October 2026.

The guideline places both under the characterisation of structure.

Why one method is rarely enough

Each method sees something different. Mass spectrometry sees mass, and with more work the sequence. Chromatography sees how many substances there are and how much of each. Amino acid analysis sees composition. A certificate that names one identity method and one purity method, each with a number, tells you far more than one that lists a single word for both.

This is also why identity and purity are separate lines. A sample can be the right peptide and impure, or very pure and the wrong peptide. Our page on peptide purity and HPLC covers the purity side, and what is in a synthetic peptide besides the peptide covers the by-products that sit close to the target in mass.

Why identity matters in Canadian law

The Food and Drugs Act forbids describing a drug in a way that misleads about what it is:

"9 (1) No person shall label, package, treat, process, sell or advertise any drug in a manner that is false, misleading or deceptive or is likely to create an erroneous impression regarding its character, value, quantity, composition, merit or safety."

Food and Drugs Act, s. 9(1), retrieved 4 October 2026.

"Character" and "composition" are about what a product is. An identity test is the evidence behind a label's name. It is not evidence that a product may be sold: for that, see DIN, NPN and how to check a product.

To put the identity line in context with every other line, read how to read a peptide certificate of analysis.

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