How Mass Spectrometry Confirms a Peptide’s Identity

A purity figure tells you how clean a sample is. It does not tell you what the sample is. A vial could hold a perfectly clean preparation of the wrong peptide and still show a single sharp peak on an HPLC trace. Identity is a separate question, and the standard tool for answering it is mass spectrometry.

What a mass spectrometer measures

A mass spectrometer turns molecules into ions, sorts them by their mass-to-charge ratio (m/z), and counts them. For peptides the ionisation step is almost always one of two techniques:

• Electrospray ionisation (ESI). The sample is sprayed from a charged needle and the peptide picks up several protons at once. A peptide of around 4,700 Da does not appear at m/z 4,700; it appears as a family of peaks for the 3+, 4+, 5+ and higher charge states. Software then works backwards from that family to a single molecular mass — a step called deconvolution.
• MALDI. The sample is mixed into a light-absorbing matrix and fired with a laser. Ions come off mostly with a single charge, so the main peak sits close to the molecular mass plus one proton. It is quick and forgiving, and common for a straightforward identity check.

When the mass spectrometer sits directly behind an HPLC column the combination is called LC-MS. The chromatography separates the components first, so each peak on the purity trace can be given a mass of its own.

Observed mass against theoretical mass

Every peptide sequence has a calculated mass. An identity result is a comparison: the mass the instrument observed against the mass the sequence predicts. Two versions of that predicted mass exist, and a report should say which one it uses.

• Monoisotopic mass assumes every atom is its most common isotope. High-resolution instruments can resolve it, and agreement is quoted in parts per million.
• Average mass weights each element by its natural isotope mix. Lower-resolution instruments, and larger peptides whose isotope peaks blur together, report this one, and agreement is usually quoted to within about one dalton.

A result such as “observed 4,731.2 Da, expected 4,731.4 Da” is what a confirmed identity looks like. A report that says only “identity: conforms” with no numbers is asking to be taken on trust.

What the extra peaks usually mean

A mass spectrum is rarely a single line. Most of the smaller signals have ordinary explanations, and several are worth recognising:

• Adducts. The peptide carrying a sodium ion (+22 Da) or a potassium ion (+38 Da) in place of a proton. These are artefacts of the measurement, not impurities in the vial.
• Counter-ion clusters. Peptides purified with trifluoroacetic acid can show a satellite peak at +114 Da. It reflects the salt form, not a different compound.
• Oxidation (+16 Da). Methionine and tryptophan residues pick up an oxygen with age, air and light. A small +16 signal is common; a large one points to storage or handling problems.
• Deamidation (+1 Da). Asparagine and glutamine slowly convert to their acid forms. The shift is small enough that only a high-resolution instrument separates it cleanly.
• Truncations and deletions. Signals lighter than the target by the exact mass of one amino acid residue — the fingerprint of a coupling step that failed during synthesis.

What mass spectrometry cannot tell you

Mass spectrometry is excellent at answering “is this the right molecular weight”. It has real limits beyond that, and a careful reader keeps them in mind.

• It does not prove sequence order. Two peptides built from the same amino acids in a different order have the same mass. Confirming the order needs tandem mass spectrometry (MS/MS), which breaks the peptide apart and reads the fragments.
• It is blind to some changes. Swapping an L-amino acid for its D-form, a common side reaction in synthesis, leaves the mass unchanged.
• It is not a purity measurement. Different molecules ionise with very different efficiency, so the relative height of peaks in a mass spectrum does not reflect how much of each is present. Purity comes from the HPLC trace, measured by UV absorbance — see how to read an HPLC chromatogram.
• It does not measure quantity. How many milligrams are in the vial is a separate assay, calibrated against a reference standard.

How identity fits with the rest of a certificate

The three questions a peptide certificate should answer are separate, and each needs its own measurement:

1. Is it the right compound? Identity, by mass spectrometry.
2. How clean is it? Purity, by HPLC with UV detection, reported as area percent.
3. How much is there? Content in milligrams, by a calibrated quantitative assay.

A report that answers one of these well and stays silent on the others has not been dishonest, but it has left you to assume the rest. When you read a certificate, check which of the three it actually addresses, then check that the lot code on it matches the lot you received. Our guide to reading a certificate of analysis walks through the remaining fields, and every lot we ship links to its report on the lab results page, where it can be verified directly with the laboratory.

Materials supplied by BMS Labs are for laboratory research use only and are not for human or veterinary use.