How to read a certificate of analysis

A certificate of analysis (COA) is the document that tells you what is actually in a vial. It should accompany every lot of research material and turns a claim of quality into something you can inspect. Knowing how to read one — and what a good one looks like — is a core skill for anyone sourcing research peptides. This is educational reference material, not medical, dosing, or clinical guidance.

What HPLC shows

HPLC — high-performance liquid chromatography — is the workhorse purity test. The sample is pushed through a column that separates its components by how strongly each one interacts with the column material. Different molecules emerge at different times, and a detector records each as a peak on a chromatogram.

The target peptide should appear as one dominant peak. Purity is estimated from the area of that main peak as a percentage of the total area of all peaks. A single tall, clean peak with little else means high purity; a forest of smaller peaks alongside the main one means impurities, truncated sequences, or breakdown products are present. HPLC answers the question, "how much of what's here is the thing I ordered?"

What mass spectrometry shows

HPLC tells you how pure the sample is, but not conclusively what the main peak is. That is the job of mass spectrometry (MS). MS measures the molecular weight of the compound with high precision. Every peptide has a known, calculated mass based on its amino-acid sequence, so if the measured mass matches the expected mass, you have strong confirmation of identity — that the molecule really is the peptide named on the label, not a look-alike.

Used together, the two are complementary: HPLC establishes purity, MS establishes identity. A trustworthy COA shows both, ideally with the actual chromatogram and mass spectrum rather than just numbers typed onto a template.

Reading the purity percentage

The headline figure is usually the HPLC purity, such as "99.4%". Read it as: of everything the analysis detected in the sample, that percentage is the target peptide, and the remainder is other detected species. A ≥99% result is a strong, common benchmark for research-grade peptides. Values in the high 90s are typical of quality material; figures drifting into the low 90s or below suggest meaningful impurity.

Beware of precision without evidence. A number alone is easy to print. What backs it up is the attached chromatogram, the method used, the date, and the lot it applies to.

Net peptide content

Purity and quantity are different questions, and this is where many buyers get confused. Net peptide content (or peptide content) describes how much of the vial's total mass is actually peptide, versus water, counter-ions, and residual salts left over from synthesis. A lyophilized peptide is often only 70–90% peptide by weight even when it is highly pure, because bound water and salts make up the rest.

So a vial can be 99% pure and still be, say, 80% peptide by mass — the two figures are not in conflict. Purity tells you the fraction of the peptide material that is the correct sequence; net peptide content tells you how much peptide mass you're getting for the stated vial weight. For consistent bench work, both matter.

Lot numbers

A COA is only meaningful if it matches your vial. Every certificate should carry a lot (batch) number that also appears on the vial itself. This is what lets you verify that the paperwork describes your material and not a generic sample. A good supplier lets you look up a lot number and retrieve its certificate on demand — which is exactly what a per-lot testing program makes possible.

What "≥99%" means — and the red flags

"≥99%" means the measured HPLC purity met or exceeded 99% for that lot. It is a statement about a specific batch, not a marketing slogan that applies to everything forever — which is why it should be tied to a lot number and a test date.

Watch for these red flags:

• No COA at all, or one available only "on request" that never quite arrives.
• A number with no chromatogram. "99% pure" with nothing to show for it is unverifiable.
• No lot number, or a lot number that doesn't match your vial.
• Only HPLC or only MS. Purity without identity, or identity without purity, is half the picture.
• Recycled certificates — the same document reused across different products or dates.
• No test date or method, making the result impossible to place in time or context.

A good COA is boring in the best way: a named compound, a matching lot number, an HPLC purity figure with its chromatogram, a mass-spec identity confirmation, and a date. When all five line up and match the vial in your hand, you can trust what you're working with.