How to read a
peptide COA.

Every peptide order arrives with a certificate of analysis. Buyers treat it as proof; suppliers treat it as paperwork. Neither is quite right. A COA is a record of specific tests on a specific lot, and its value depends entirely on which tests were run and whether the method is named.

Two suppliers can both send you a COA that says “Purity: 99%” and be describing materially different products. The difference is not dishonesty — it is that “purity” is a word with more than one meaning in peptide chemistry.

Why COAs mislead

A certificate is generated by whoever released the batch. There is no international standard that decides which lines it must contain. A supplier can issue a perfectly truthful COA that omits the four numbers a formulator actually needs, and still be within industry practice.

The result is that buyers compare certificates as if they were comparable documents. They are not. What follows is the reading order that we use internally, and the questions we would ask if we were the buyer.

The four numbers that matter

1. Purity is not content

HPLC purity is the area of the main peak as a percentage of all UV-absorbing peaks. It tells you how clean the peptide is relative to its own impurities. It says nothing about how much of the white powder in the vial is peptide.

Net peptide content is that second number: the mass of peptide as a share of the total powder, after subtracting water, counter-ion and residual solvent. On lyophilised peptides the two figures routinely sit 10–20 percentage points apart.

2. Identity needs a method, not a word

“Identity: conforms” is not a result. The result is a mass spectrum matching the theoretical mass, or an amino-acid analysis, or both. For a 15-residue peptide with a theoretical mass of 1419.53, you want to see a spectrum, not an adjective.

3. The counter-ion is part of the product

Peptides leave synthesis as a salt — usually acetate or trifluoroacetate. TFA is not inert in cell-based work, and acetate and TFA salts of the same peptide have different masses. If the COA does not state the counter-ion, the mass balance cannot be closed.

4. Water content predicts shelf behaviour

A Karl Fischer figure above specification usually means the material was opened, repacked, or dried inadequately. Water is the leading indicator of everything that goes wrong later — aggregation, colour drift, assay loss on storage.

A worked example

Take a typical 5 mg vial with a COA that reads:

Purity (HPLC, 214 nm)	98.2%
Net peptide content	82.0%
Water content (KF)	4.1%
Counter-ion	Acetate

The vial contains about 4.1 mg of peptide, not 5 mg. If a laboratory weighs 5 mg and assumes it is all peptide, every concentration in the experiment is 18% low — and the error is invisible, because the volume and the balance both read correctly.

This is the single most common quantitative mistake we see. It is not a supplier problem; it is a reading problem. The number was on the certificate.

Red flags worth acting on

• No method named. “Purity: 99%” without HPLC conditions is not reproducible and cannot be compared.
• No chromatogram. The integration should be visible. A single number with no trace hides whether the impurity profile is one 1.5% peak or fifteen small ones.
• Batch number missing or repeated across orders. A COA without a lot number cannot be tied to the goods.
• Identity stated as “conforms”. Conforms to what? Ask for the spectrum.
• Assay by UV alone. A280 or 214 nm absorbance without a standard curve is an estimate, not an assay.
• The COA arrives after the goods. Documentation should travel with the shipment, not follow it.

If a supplier cannot tell you the net peptide content of a lyophilised batch, assume the vial is 15–20% lighter in peptide than its label suggests — and price accordingly.

What to ask for, in order

When you request a quotation, ask for these five items with the price. A supplier who can supply all five is a supplier worth a second conversation.

1. HPLC chromatogram for the lot, with the gradient and wavelength stated
2. Mass spectrum or amino-acid analysis confirming identity
3. Net peptide content, expressed as a percentage of the powder
4. Counter-ion and its limit (acetate, TFA, or none)
5. Water content by Karl Fischer

For anything touching cells, add endotoxin by LAL. For anything going into a regulated market, add the residual-solvent panel and the heavy-metal results by ICP-MS.

None of this is exotic. Every one of these tests is routine; the question is whether the supplier runs them, and whether the certificate says so.

You can see how we state these lines on any of our product pages — for example BPC-157, where the specification lists purity, net peptide content, counter-ion and water separately, or NMN, where the β-anomer and related substances are reported individually.