How to read a peptide Certificate of Analysis

A field-by-field guide to peptide COAs: identity, HPLC purity, peptide content, counterions and the red flags that mean a report shouldn’t be trusted.

A Certificate of Analysis (COA) is the document that connects a vial to the analytical work done on the material inside it. Read well, it tells you what was tested, how, by whom and on which batch. Read carelessly, it can give false confidence: a COA is only as useful as its link to the lot in your hands.

This guide walks through a typical research peptide COA section by section.

1. The header: who, what and which batch

• Product name and sequence. The name alone isn’t enough. A useful COA states the amino-acid sequence, any terminal modifications (for example C-terminal amide or N-terminal acetylation) and, where relevant, salt form.
• Lot or batch number. The most important field on the page. It must match the lot printed on your vial label exactly. A report for a different lot tells you nothing reliable about your vial. See Understanding peptide lot numbers and traceability.
• Molecular formula and theoretical molecular weight. These let you check the identity result further down.
• Dates. Manufacture date, test date and any retest or expiry date. A test date years before the sale, or after a stated expiry, deserves a question.
• Testing laboratory. Is the report from the manufacturer’s in-house QC lab or an independent third party? Both can be legitimate, but the report should say which, and an independent lab should be identifiable.

2. Identity: is it the right molecule?

Identity is usually confirmed by mass spectrometry (MS), often electrospray (ESI-MS) or LC-MS. The report should show an observed mass alongside the theoretical mass. For most small peptides the two should agree closely; a meaningful gap, or a missing observed value, means identity has not been demonstrated.

Purity testing alone does not confirm identity. A sample can show a single clean HPLC peak and still be the wrong peptide.

3. Purity: usually HPLC

Purity is most often reported as an HPLC area percentage, for example “98.6% (HPLC, 220 nm)”. Look for:

• the method (typically reversed-phase HPLC);
• the detection wavelength (214 or 220 nm is common for peptide bonds);
• an attached chromatogram, not just a number.

HPLC purity is useful but limited. It describes the share of UV-absorbing material that elutes as the main peak under that specific method. It does not tell you how much peptide is in the vial. Our companion article covers this in detail: What HPLC purity does—and doesn’t—tell you.

4. Peptide content, water and counterions

A lyophilized peptide powder is rarely 100% peptide by weight. It also contains water and counterions left from synthesis and purification, commonly trifluoroacetate (TFA) or acetate. Reports may include:

• Net peptide content (often by amino-acid analysis or nitrogen analysis): the fraction of the powder’s mass that is peptide.
• Water content (often Karl Fischer titration).
• Counterion content (for example TFA by ion chromatography).

These matter for anyone preparing solutions to a known concentration in the lab: a vial labeled 5 mg of material may contain meaningfully less than 5 mg of the peptide itself. If net content isn’t reported, the COA does not support precise concentration calculations.

5. Other tests you may see

• Appearance (for example “white lyophilized powder”) and solubility.
• Endotoxin (LAL test), relevant for some cell-based laboratory work.
• Residual solvents and heavy metals, less common on research-grade reports.

Absence of a test isn’t automatically a problem; it means the COA makes no claim about that property.

6. Red flags

• No lot number, or a lot number that doesn’t match the vial.
• A purity number with no method, wavelength or chromatogram.
• No mass spectrometry or other identity test.
• Identical results (for example “99.9%”) across many different lots and products.
• A testing lab that can’t be identified or contacted.
• Signs of editing: mismatched fonts, misaligned fields, dates out of sequence.
• A “COA” that is really a generic product specification sheet, with targets instead of results.

7. A quick checklist

1. Lot number on the report matches the vial exactly.
2. Sequence, modifications and theoretical mass are stated.
3. Identity confirmed by MS with an observed mass.
4. Purity reported with method, wavelength and chromatogram.
5. Net peptide content reported if you need accurate concentrations.
6. Testing lab and test date identified.

How Answer North handles COAs

Every report in our COA library is listed with its lot and method, and each product page has a lot lookup: enter the lot printed on your vial and we show only a document for that exact lot. We do not apply a report from one lot to another. Where a product shows “COA pending”, no lot-matched report has been released yet.

For Laboratory Research Use Only — Not for Human or Animal Consumption. Educational content only; not instructions for use outside a laboratory.