How to Read a Peptide Certificate of Analysis (COA)

What a Certificate of Analysis actually is

A Certificate of Analysis (COA) is a document produced by an analytical laboratory that records the results of tests run on a specific batch of material. For a research peptide, it answers two core questions: is this the molecule it claims to be? (identity), and how much of the sample is that molecule versus impurities? (purity). A good COA also reports supporting measurements — water content, counter-ion content, and physical appearance — that put the purity number in context.

The single most important thing to understand is that a COA is batch-specific. Peptides are made in discrete synthesis runs, and purity can vary from lot to lot. A certificate is only meaningful if its lot or batch number matches the vial in your hand. A generic PDF with no lot number, or one reused across every product, tells you nothing about the material you actually received.

The header: product, lot, date, and appearance

Before any instrument data, a COA carries identifying header fields. Read these first, because if they do not match your vial the rest of the document is irrelevant.

• Product name and sequence — the peptide's name and, ideally, its amino-acid sequence and molecular formula, so you can independently look up the theoretical mass.
• Lot / batch number — the unique identifier for this synthesis run. It must match the label on your vial.
• Date of analysis / release date — when the testing was performed. Stale dates on a hygroscopic powder can matter.
• Quantity and appearance — e.g. "white to off-white lyophilized powder." Appearance mismatches (discoloration, oily residue) are an early warning sign.
• Storage conditions — recommended handling for the lyophilized material, which links to your storage practices.

A legitimate certificate also names the testing laboratory (in-house QC or a third party) and the analyst or approver. Third-party testing is generally stronger evidence than a supplier grading its own work, though reputable manufacturers do both.

Identity: reading the mass spectrometry result

The identity section answers "is this the right molecule?" It is almost always confirmed by mass spectrometry (MS), which measures the peptide's molecular weight. The COA lists a theoretical (calculated) mass derived from the sequence and an observed (found) mass measured by the instrument. If the observed mass matches the theoretical mass within the instrument's tolerance, the identity is confirmed.

One quirk trips up first-time readers: MS often reports charge states and adduct ions, so you may see values like [M+H]⁺ (one mass unit above the neutral mass) or [M+2H]²⁺ (roughly half the mass, because it carries two charges). A well-built COA labels these so the numbers make sense. For a deeper look at how MS differs from HPLC, see HPLC vs mass spectrometry.

Purity: reading the HPLC chromatogram

The purity section answers "how much of the sample is the target peptide?" It is measured by high-performance liquid chromatography (HPLC), which separates the sample's components as they pass through a column and records them as a series of peaks on a chromatogram. Each peak is a compound; the area under each peak is proportional to how much of that compound is present.

Purity is reported as the percent area of the main peak — for example, "98.7% by HPLC." That figure is the target peptide's peak area divided by the total area of all peaks. The remaining percentage is impurities: truncated sequences, deletion products, or byproducts of synthesis. A strong COA shows the actual chromatogram image with the main peak labeled and its retention time noted, not just a typed number you cannot verify.

As a rough guide, research peptides are commonly reported at ≥ 95–99% purity. Higher is better, but the number is only trustworthy if the method is credible and the chromatogram is shown. A single suspiciously clean peak with no baseline detail, or a purity figure with no accompanying trace, deserves skepticism.

Water, acetate, and why net peptide mass is lower

Beyond identity and purity, a thorough COA reports content measures that explain the gap between the labeled vial weight and the actual amount of peptide. Peptides are hygroscopic and are usually isolated as salts, so a vial labeled "10 mg" rarely contains 10 mg of pure peptide.

Water content is usually measured by Karl Fischer titration; a few percent residual moisture is normal for a lyophilized cake. Counter-ion content — often acetate (or trifluoroacetate/TFA from purification) — reflects the salt form the peptide was isolated as. Together with water, this is why the net peptide content can be meaningfully below the labeled weight, which matters when you calculate concentrations for research.

Spotting a weak or fake COA

Not every document labeled "COA" is a real analytical report. Some are marketing graphics with numbers typed on top of a stock chromatogram image. Use this checklist to judge credibility.

• No lot number, or the same certificate reused for every product and every batch.
• A purity percentage with no chromatogram — or a chromatogram with no axes, no retention time, and no baseline detail.
• No mass spec data, so identity is never actually confirmed.
• Lot, product name, or date that do not match the vial you received.
• No testing laboratory, method, or instrument named, making the results unverifiable.
• Impossibly perfect numbers (e.g. "100.0% purity") with no supporting trace.

Reading a COA is a skill worth building because it lets you compare suppliers on evidence rather than marketing. Pair this guide with purity vs identity to understand why both measurements are needed, HPLC vs mass spectrometry to interpret the core instruments, and are peptide suppliers legit? for the broader verification picture.