How Does Mass Spectrometry Confirm a Peptide’s Identity?

Paul Bakhtiar, MBA | Published October 15, 2026

Short answer. Mass spectrometry weighs the molecule. The lab measures the mass of the main peak and compares it with the theoretical mass calculated from the peptide’s sequence; a match within the instrument’s tolerance confirms the intended compound is present. HPLC alone cannot do this, because a wrong peptide can be 99% pure: seized products labeled as growth hormone secretagogues have turned out to be closely related analogs.

What this means for your clinic. Look for two numbers on the certificate, the theoretical mass of the sequence and the observed mass from the instrument, and the difference between them. If the certificate carries only a purity percentage, identity has not been tested, and that is a gap no purity figure can close.

What are observed and theoretical mass, and where do charge states come in?

Every peptide has a mass that follows from its sequence. Add up the residue masses and the terminal groups and you have the theoretical mass. The instrument does not read that number directly. It measures the mass-to-charge ratio, written m/z, of ions made from the sample. To make an ion, the source attaches one or more protons to the peptide, and each proton adds about 1.007 to the mass and one unit to the charge.

So one peptide shows up at several m/z values, one for each charge state, and the lab converts each back to the same neutral mass. The table uses an illustrative peptide with a monoisotopic mass of exactly 1,000.00.

Charge state	What the instrument reports (m/z)	Converted back to mass
1+	(1,000.00 + 1.007) / 1 = 1,001.007	1,001.007 – 1.007 = 1,000.00
2+	(1,000.00 + 2 x 1.007) / 2 = 501.007	501.007 x 2 – 2 x 1.007 = 1,000.00
3+	(1,000.00 + 3 x 1.007) / 3 = 334.340	334.340 x 3 – 3 x 1.007 = 1,000.00

A certificate that reports identity by mass spectrometry should show the theoretical mass, the observed mass and the charge state it was read from. It should also say whether both figures are monoisotopic or average, because the two differ and the gap grows with the size of the molecule. This is the identity half of a certificate of analysis, and the clinic’s guide to verifying peptides treats it as a separate test from purity for the reasons below.

What does a small mass difference mean?

Observed and theoretical rarely match to the last decimal, and the instrument’s tolerance is part of the method. What matters is a difference that exceeds the tolerance, because particular differences point to particular things.

About +16: an oxidized residue, most often methionine.

About +1: deamidation of an asparagine or glutamine.

About +22 or +38: a sodium or potassium adduct in place of a proton, an artifact of the measurement rather than of the product.

A shift of minus 57, 71 or 87: a missing glycine, alanine or serine.

A shift of plus 57 or 71: an extra glycine or alanine.

A peptide also gives a cluster of peaks spaced one mass unit apart, because a fraction of its carbon atoms are carbon-13. The spacing of that cluster tells the lab the charge state, and its shape should match the pattern predicted from the formula. Mestria and colleagues (2020) identified peptides in eight police-confiscated samples without any reference standard by combining accurate mass, the isotope pattern and the masses of fragment ions.

What did the studies of seized products find?

Two studies of products seized by customs authorities show why a clean chromatogram is not an identity test.

Krug and colleagues (2018) analyzed black-market products by liquid chromatography with high-resolution mass spectrometry. A 22 kDa protein product turned out to carry 192 amino acids instead of 191, an extra alanine at the N-terminus, with a monoisotopic mass of 22,195 Da against the genuine 22,124 Da: a shift of 71, exactly one alanine residue. Three short peptide products in the same study each carried an extra glycine at the N-terminus, confirmed by high-resolution tandem mass spectrometry and, for two of them, by synthesizing the analog and matching it.

Gajda and colleagues (2018) examined powders seized by Danish customs authorities and compared them with reference standards by the same technique. Every product was an analog of a known peptide with an additional glycine at the N-terminus.

On a UV chromatogram, each of those products would have given one clean, dominant peak. The extra residue shows up as a mass shift and nowhere else. A wrong peptide can be 99% pure, which is the whole reason identity is a separate line on the certificate.

What is the next level of identity confirmation?

A matching mass is strong evidence, but two different sequences can share a mass: swap the order of two residues and the mass does not change. Tandem mass spectrometry (MS/MS) goes further. The instrument breaks the peptide ions into fragments and weighs the pieces, and the ladder of fragment masses reads out the sequence. Krug’s study used it to characterize the analogs it found; Mestria’s used fragment masses to identify peptides without a reference standard.

Amino acid analysis is the other check. It breaks the peptide into its amino acids and confirms that the right ones are present in the right ratios. McCarthy and colleagues (2023) describe identity testing for peptide reference standards using nuclear magnetic resonance, mass spectrometry and chromatography together, with extra techniques where a sequence contains isobaric residues such as leucine and isoleucine, which share a mass.

The regulatory benchmark for identity is 21 CFR 211.84(d)(1): at least one test to verify the identity of each component. Part 211 binds drug manufacturers and is cited here as the benchmark a legitimate supplier meets, not as an obligation on a clinic. A certificate that carries only a purity figure does not show that test was done.

How do you read the EV-01 mass spectrum?

The independent laboratory’s report for lot IGF1A122925 (EV-01) is an HPLC-UV-MS report, so the identity check sits beside the purity trace. Read it in this order.

Find the theoretical mass and note whether it is monoisotopic or average.

Find the observed mass, the charge state it was read from and the m/z value behind it. Convert the m/z back to a neutral mass yourself; it takes seconds.

Compare the difference with the tolerance the report states. If no tolerance is stated, ask the laboratory for it.

Check that the spectrum was taken from the main chromatographic peak, not from the whole sample, so the identity belongs to the 99.80%.

Confirm the sample ID and lot number on the spectrum match the chromatogram and the summary page.

How Helios does it

Under the Helios verification standard, step 2 requires an independent laboratory report for the exact lot as part of the certificate set, carrying the panel the standard specifies, and identity by mass spectrometry is on that panel. Step 4 requires the laboratory to confirm the report number, lot and results, reached through contact details found independently of the supplier, and it produces the confirmation record. The standard also calls for confirmatory testing on a schedule, which produces the confirmatory report, and Helios Consultants LLC applies both steps lot by lot. The full sequence is in how Helios verifies every lot. If you want help setting this up in your clinic, book a consultation.

Sources

Mestria S, et al. Drug Test Anal. 2020. PMID 33245851. https://doi.org/10.1002/dta.2986

Krug O, et al. Growth Horm IGF Res. 2018. PMID 29864719. https://doi.org/10.1016/j.ghir.2018.05.001

Gajda PM, et al. Drug Test Anal. 2018. PMID 30136411. https://doi.org/10.1002/dta.2489

McCarthy D, et al. Pharm Res. 2023. PMID 36949371. https://doi.org/10.1007/s11095-023-03493-1

21 CFR 211.84, Testing and approval or rejection of components. eCFR. https://www.ecfr.gov/current/title-21/chapter-I/subchapter-C/part-211/subpart-E/section-211.84

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