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How to read a peptide COA: a field-by-field guide

A field-by-field guide to a peptide certificate of analysis: matching it to your batch, what each test means, and how to confirm the report with the laboratory that issued it.

Research-use notice: this article is for legitimate laboratory and educational research only. It does not provide guidance for human use, administration, dosing or treatment.

A peptide certificate of analysis (COA) is a laboratory’s record of named tests on one identified sample, so you read it batch first and numbers second. Check that the batch number, the peptide and the issuing laboratory match the material in front of you, then read each test with its method, its limit and its measured value. High-performance liquid chromatography (HPLC) purity, mass spectrometry (MS) identity and peptide content answer three different questions, and none of them is a safety test. A COA supports only the tests it lists, for the sample that was tested.

What is a certificate of analysis?

A COA summarises the analytical tests carried out on a sample from one batch. It may come from the manufacturer’s own quality-control laboratory or from an independent laboratory that tested a sample sent to it. Both are called COAs, and both are read the same way.

The clearest published description of what a COA should contain is in ICH Q7, the International Council for Harmonisation (ICH) guide to good manufacturing practice for active pharmaceutical ingredients. It was written for the ingredients of medicines, not research materials, but it makes a useful yardstick. It says authentic COAs should be issued for each batch on request. Each should give the material’s name, batch number and date of release, an expiry or retest date where one applies, and every test performed with its acceptance limits and numerical values. It should be dated and signed by authorised quality staff.

How to read a peptide COA, field by field

Work from the top of the document down. The table lists the fields you will usually see and the check that goes with each.

Field What it shows What to check
Laboratory and report number Who issued the report, with a unique reference The laboratory is named, you can find its contact details independently, and the reference appears on every page
Peptide name and description The material tested, often with its sequence, molecular formula or expected molecular mass The sequence or expected mass matches the peptide you meant, not just the name
Batch or lot number The code that links the sample to one production batch The same code is printed on your vial label
Dates When the sample was received, tested and reported; sometimes a retest date The dates fit the batch you hold, and the report is not for an earlier batch
Test, method and specification Each test, the method used and its acceptance limit Every value has a stated method; a bare number is hard to interpret
Measured value and units For example % area, mass in daltons (Da) or endotoxin units (EU) The units suit the test, and each value is compared with its limit
Attachments Chromatogram, mass spectrum and integration table The data carry the same report number and sample name as the summary page
Authorisation A signature or electronic approval A named person and a date are present

The batch number is the field that matters most. The peptide name tells you what the material should be. The batch number tells you which material was tested.

What does each test on a peptide COA mean?

Each test answers one question. The table summarises them, and the notes below explain the ones that cause most confusion.

Test What it tells you What it does not tell you
HPLC purity (% area) The main peak’s share of the detected peak area, under that method Identity, how much of the powder is peptide, or sterility
Identity (MS) Whether the measured mass matches the expected peptide Whether a same-mass variant is present, or how much peptide there is
Peptide content (assay) How much of the powder’s mass is peptide Whether it is the right sequence, unless paired with an identity test
Water content How much of the mass is water Purity or identity
Counterion content Which salt form is present, for example trifluoroacetate (TFA) or acetate, and how much mass it accounts for Peptide purity
Bacterial endotoxins The endotoxin level in the tested sample, in EU Sterility, or contamination introduced after sampling
Sterility Whether micro-organisms were found in the units tested That every unit in the batch is sterile

HPLC purity

HPLC separates the dissolved sample into its components, which leave the column at different times and appear as peaks. The chromatography text harmonised across the European, Japanese and US pharmacopoeias describes the calculation behind a percentage figure: the area of one peak as a percentage of the total area of all peaks, leaving out peaks from solvents, reagents, the mobile phase or the sample matrix, and those at or below the reporting threshold. The purity figure is therefore a share of detector signal, not of weight. Our article on what 99% purity actually means works through the arithmetic.

Identity by mass spectrometry

MS measures the mass of the molecule, which is compared with the mass expected from the sequence. The European Medicines Agency (EMA) guideline on synthetic peptides, in effect since 1 June 2026, says variants of MS can be used to determine a peptide’s molecular mass and to confirm its amino acid sequence. For medicinal peptides it recommends at least two orthogonal (independent) identification methods. ICH Q2(R2), the guideline on validating analytical procedures, says an identification test should be applied to closely related materials to confirm that they do not give a positive result. A matching mass is strong evidence, but on its own it does not prove every residue. Our comparison of HPLC and mass spectrometry explains the difference.

Peptide content

Content, sometimes called assay or net peptide content, answers a different question from purity: how much of the powder is the peptide. The EMA guideline names a chromatographic assay against a reference standard and lists alternatives including amino acid analysis, nitrogen analysis and quantitative nuclear magnetic resonance (qNMR). It expects chromatographic assay limits to be expressed on a counterion-free, anhydrous basis. A 2016 consensus paper in Clinical Chemistry, on peptides used as standards in MS assays, set out recommendations for quantifying peptides and highlighted the need for carefully characterised peptide standards so that amino acid analysis methods agree more closely.

Water and counterion content

A freeze-dried peptide is rarely peptide alone. A 2020 review in Pharmaceuticals explains that synthetic peptides are commonly cleaved from the resin and purified using trifluoroacetic acid, so cationic peptides are obtained as TFA salts, which can later be exchanged to acetate or hydrochloride. The EMA guideline lists counterion identity and content, residual TFA and water content as separate tests. It notes that peptides are often very hygroscopic powders, and expects hygroscopicity to be indicated. This is why a report can show high purity and a peptide content well below 100%. Our article on peptide counterions covers the salt form in more depth.

Endotoxin

Bacterial endotoxin is lipopolysaccharide (LPS), a major component of the outer membrane of most Gram-negative bacteria. It is commonly measured with limulus amoebocyte lysate (LAL) tests, which the US Food and Drug Administration (FDA) has accepted in place of the rabbit pyrogen test for more than 30 years. The FDA’s 2026 question-and-answer guidance notes that endotoxin assays are subject to interference from the physical and chemical properties of the material tested, so the test has to be shown to work for each material. The value matters most for cell work, as the research section below explains.

Sterility

A sterility test looks for viable micro-organisms in the units tested. ICH’s evaluation of the harmonised pharmacopoeial sterility texts (Q4B Annex 8) records that a satisfactory outcome “only indicates that no contaminating micro-organism has been found in the sample examined in the conditions of the test”. A pass describes the units tested, not every vial in the batch, and it says nothing about a vial once it has been opened.

How to match a COA to your batch

  1. Batch number. Read the batch or lot number on the vial label and find the same code on the report.
  2. Peptide. Check the name and, where given, the sequence or expected mass.
  3. Dates. A report for an earlier batch, or an undated example certificate, does not describe your vial.
  4. Attachments. The chromatogram and mass spectrum should carry the same report number and sample name as the summary page.
  5. Records. File a copy of the report with your lab notes, together with the batch number and the date the material arrived.

Thistle shows a batch laboratory report on each product page, and you can look up a batch on our batch report lookup.

How do you verify a COA with the issuing laboratory?

A COA is a copy of a laboratory record, so the laboratory is the authority on whether it is genuine. Data integrity guidance from the Medicines and Healthcare products Regulatory Agency (MHRA) defines the original record as the first capture of the data, such as the electronic raw data file, and a true copy as one verified to contain the same information as the original. The PDF you are shown is a report drawn from those records.

  • Find the laboratory yourself. Use contact details or a verification page from the laboratory’s own website, not from the PDF.
  • Quote the report number. Ask whether the report exists, which sample and batch it covers, and whether the values match.
  • Check reissued certificates. ICH Q7 expects a new certificate issued by or for a repacker, agent or broker to name the laboratory that did the analysis and to refer to the original manufacturer and the original batch certificate, with a copy attached.
  • Ask for the data. If the chromatogram or mass spectrum is missing, ask for it.

Some independent laboratories offer an online verification page, where a report can be checked against the laboratory’s own record. A verified report still has limits, which we cover in what a COA cannot tell you. To assess a supplier as a whole, use our UK peptide supplier checklist.

What the evidence says about COAs

Most of what is written about COAs comes from regulatory guidance rather than from studies. Three pieces of research show why the fields above are worth reading closely:

  • Analytical study. In a 2015 study in the Journal of Pharmaceutical Analysis, researchers retested 98 synthetic peptides ordered for research at a requested purity of at least 95.0%. They reported a large discrepancy between the purity stated on the suppliers’ COAs and their own measurements: 43 of the 98 met the specification.
  • Review. The 2020 counterion review notes that TFA can be toxic or inhibit cell proliferation in some settings, and that it was found to stimulate the growth of glioma cells in another. Salt form can therefore matter in cell assays.
  • In vitro study. A 2014 study in PLOS ONE found that very low LPS concentrations, equivalent to the endotoxin levels the authors detected in some commercially available recombinant proteins, were enough to activate human CD1c+ dendritic cells. The work used a cell line and primary human immune cells, not people.

None of this makes COAs worthless. It means a COA is evidence to be matched, checked and understood, not a stamp of quality.

Peptide COA FAQs

What does COA mean for peptides?

COA stands for certificate of analysis: a laboratory document listing the tests run on a sample from a batch, the methods used and the values found. For a research peptide it usually covers HPLC purity and MS identity, sometimes with content, water, counterion or endotoxin tests.

Is HPLC purity the same as peptide content?

No. Purity is the main peak’s share of detected peak area. Content is how much of the powder’s mass is peptide. A powder can show 99% purity and still contain substantial water and counterion.

What should a peptide COA include?

At minimum: the peptide name, batch number, issuing laboratory, report number, dates, and each test with its method, limit and measured value. Identity and purity are the core tests; content, water, counterion and endotoxin tests add useful information.

How can I tell if a peptide COA is genuine?

You cannot tell from the document alone. Contact the issuing laboratory through details you find yourself, and ask it to confirm the report number, the sample and the values.

Does a COA mean a peptide is safe to use?

No. A COA records analytical tests on a sample and contains no human safety data. Research peptides are not for human use, and we cannot advise on personal use.

Sources

Published by Thistle BioLabs, a UK supplier of research materials. This article summarises external research, not studies conducted by Thistle. Educational information only; no medical advice or recommendation for personal use.

Educational information only. This journal does not recommend a substance for personal use or provide medical advice.