Mass spectrometry on a peptide COA: how ESI-MS and MALDI-TOF confirm identity

Prepared by: Research Desk. Sources are linked for verification.

Published 23 October 2026

Why a peptide COA needs both HPLC and MS

On a certificate of analysis (COA) for a synthetic peptide, high-performance liquid chromatography (HPLC) and mass spectrometry (MS) answer two different analytical questions:

• HPLC separates molecules by hydrophobicity on a C18 stationary-phase column (RP-HPLC) and reports the relative area of the ultraviolet-absorbing peaks. It tells the reader whether a sample contains one dominant peak or several, but a retention time alone does not prove which molecule is in that peak.
• Mass spectrometry ionises the molecules in the sample and measures their mass-to-charge ratio (m/z). It checks whether the main component has the molecular mass expected from the amino-acid sequence.

Analytical work on falsified or mislabelled research peptides has shown why neither test is sufficient on its own. In a study of seized suspected peptide products, Janvier et al. (2018) combined liquid chromatography–tandem mass spectrometry (LC-MS/MS) with nuclear magnetic resonance Spectroscopy and found products where the paperwork did not match the actual chemical contents. For how HPLC purity differs from weight-based peptide content, see HPLC purity vs net peptide content.

The two ionisation methods used for peptides

Peptides are polar, non-volatile molecules that decompose if heated into a gas. Modern peptide mass spectrometry relies on two "soft" ionisation techniques that transfer intact peptide ions into the gas phase with minimal fragmentation:

Electrospray ionisation (ESI-MS)

In ESI-MS, the dissolved peptide is sprayed through a high-voltage capillary, forming charged droplets that evaporate until gas-phase ions enter the mass analyser (راجعة / review of ESI-MS peptide interactions, PubMed). Because basic side chains (arginine, lysine, histidine) and the N-terminal amine can each pick up a proton in acidic mobile phases, a peptide with three basic sites often appears as a family of peaks—a "charge envelope"—corresponding to [M+2H]2+, [M+3H]3+ and [M+4H]4+.

As general analytical context, each peak in an ESI charge envelope follows the relationship:

• m/z = (M + z × 1.0073) / z, where M is the neutral molecular mass of the peptide, z is the integer charge (number of added protons), and 1.0073 Da is the mass of a proton.

For example, a peptide with a molecular mass of 1,420.0 Da carrying two protons (z = 2) produces a doubly charged ion [M+2H]2+ near m/z = (1420.0 + 2.0) / 2 = 711.0, and a triply charged ion [M+3H]3+ near m/z = (1420.0 + 3.0) / 3 = 474.3. When a COA prints an ESI spectrum whose tallest peak is around 711 while the header lists a molecular weight of 1420, that is not a contradiction: the peak is the doubly protonated [M+2H]2+ ion.

MALDI-TOF mass spectrometry

In MALDI, the peptide sample is mixed with a small organic acid matrix (such as alpha-cyano-4-hydroxycinnamic acid or sinapinic acid) and dried on a metal target plate. Pulsed ultraviolet laser shots desorb and ionise the matrix and peptide together, and a time-of-flight (TOF) tube measures how fast the ions reach the detector (kinetic and ion-yield review in Mass Spectrometry Reviews, 2015, PubMed).

Because proton transfer in the MALDI plume strongly favours singly charged species, a MALDI-TOF spectrum usually displays a single dominant [M+H]+ peak at M + 1 Da, sometimes accompanied by smaller sodium ([M+Na]+, M + 23 Da) or potassium ([M+K]+, M + 39 Da) adduct peaks. MALDI-MS is also widely used with chemical reduction and alkylation to verify disulphide bond pairing in cysteine-containing peptides (review of disulphide determination by MALDI-MS, PubMed).

Average mass versus monoisotopic mass

A subtle source of confusion when checking a COA is the difference between monoisotopic mass and average (chemical) molecular weight:

• Monoisotopic mass is calculated using only the most abundant light isotope of each element (12C, 1H, 14N, 16O, 32S). High-resolution mass spectrometers resolve the individual isotopic peaks of a peptide and report the monoisotopic m/z.
• Average mass weights each element by its natural terrestrial isotope abundance (including 13C at 1.1% and 34S at 4.2%). Low-resolution instruments that merge the isotopic envelope into a single broad peak, and most chemical catalogue headers, report the average molecular weight.

For a short five-residue peptide the two numbers differ by less than 0.3 Da, whereas for a 30- to 40-residue peptide the average mass is typically 1.5 to 3.0 Da higher than the monoisotopic mass. A thorough COA states which mass definition it uses.

What mass spectrometry on a routine COA cannot detect

A single-stage MS spectrum confirms molecular mass, not complete structural purity. As catalogued in a review of peptide-related impurities by D'Hondt et al. (2014), several classes of synthesis errors have the exact same molecular mass (isobaric) or nearly the same mass as the target sequence:

• Diastereomers (epimers). If an L-amino acid racemises to a D-amino acid during coupling, the resulting epimer has the exact same elemental formula and molecular mass (Δm = 0 Da). Single-stage MS cannot distinguish it from the target peptide; chromatographic separation is required.
• Sequence scrambling or isoaspartate rearrangement. Rearrangement of an Asp-Gly or Asn-Gly linkage through a cyclic aspartimide intermediate to an isoaspartyl residue leaves the mass unchanged (Δm = 0 Da) after hydrolysis.
• Deamidation. Conversion of asparagine or glutamine to aspartate or glutamate increases the mass by only +0.984 Da, which overlaps with the natural 13C isotope peak on low-resolution instruments.
• Uncharged or volatile impurities. Water, inorganic salts and counter-ions such as trifluoroacetate are not quantified by a standard positive-ion peptide mass spectrum. Those require separate assays, as explained in our note on TFA, acetate and chloride counterions and our overview of synthetic peptide impurities.

To confirm the order of amino acids rather than just the total mass, laboratories use tandem mass spectrometry (MS/MS), which collides the selected precursor ion with an inert gas to generate sequence-defining b- and y-type fragment ions. Validation principles for analytical identification and purity procedures are set out in the ICH Q2(R2) guideline on the ICH quality guidelines page.

Checklist for reading the MS section of a COA

1. Is the actual mass spectrum shown, with labelled m/z axes, rather than only a typed "Conforms" or single number in a summary table?
2. Are the charge states identified ([M+H]+, [M+2H]2+, [M+3H]3+) and do they back-calculate to the theoretical molecular weight?
3. Are large unassigned peaks present that cannot be explained as charge states or sodium/potassium adducts of the main peptide?
4. Does the batch number on the MS printout match the batch number on the HPLC chromatogram and the vial label?

For the full synthesis-to-testing workflow, see the guide to how research peptides are made and tested.

Supplier documentation

Purita is a store that sells peptides for laboratory research use only. It publishes a certificates page where reports are available for some products; reports are not yet available for every item, so check the page for the specific material.

Purita certificates page

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Related pages

• Peptide design, synthesis and testing, explained
• About DesignPeptide
• How research peptides are made and tested: from synthesis to COA
• HPLC purity vs net peptide content: two numbers that are often confused
• Common impurities in synthetic peptides and how labs detect them
• TFA, acetate or chloride: why a peptide's counterion matters