How to Read a Peptide Certificate of Analysis

A peptide certificate of analysis (CoA) is the written record of what was measured on a released lot. It is not a guarantee that the vial will solve your biology, and it is not a complete chemical encyclopedia. Reading it well means knowing which fields are identity, which are purity, which are quantity, and which are simply blank because the test was never performed.

ChinaPeptides documents research lots through peptide quality control. This article is a field-by-field walkthrough so you can accept a lot, question a lot, or write a better specification the next time you order custom peptide synthesis. Companion pieces on HPLC and LC-MS explain the underlying measurements in more depth.

The header: lot, sequence, and the molecule you think you bought

Match the catalog or project code, the lot number, and the sequence character-for-character to your order, including termini and modifications. A missing amide, a silent D-residue, or a biotin spacer that does not match the drawing is a different product. If the CoA sequence is truncated in a table, look for an attachment. Do not release the peptide into a shared freezer until that match is done.

Molecular weight on the header should state monoisotopic or average. The mass result later in the document must use the same convention. Formula-level identity is “consistent with,” not “proves order of residues,” unless tandem MS or another sequencing method is listed.

HPLC purity: the number everyone quotes

Find the method: column, gradient, ion-pair reagent, wavelength, and integration policy. Then look at the chromatogram, not only the percent. A 98.2% on a steep gradient is weaker evidence than a 96.0% on a shallow, well-chosen method. Shoulders under the main peak matter more than a tiny early peak that is clearly unrelated if your assay cares about close analogs.

HPLC area percent is not peptide content. It is not “percent active.” It is not comparable between two suppliers without method context. The purity selection guide explains when 95 versus 98 was the right specification; the CoA only tells you whether the lot met whatever was specified.

CoA field	What it supports	What it does not support
Sequence / modifications	Intended structure as submitted	That the biology will work
HPLC area %	UV homogeneity under that method	Weight percent peptide; activity
MS / LC-MS	Formula-level identity (usually)	Purity; residue order unless MS/MS listed
Appearance / amount	What was bottled	Net peptide micromoles unless content is given
Salt form	Counter-ion if stated	Residual TFA percent unless assayed
Blank test names	Nothing; test was not done	A hidden pass

Mass spectrometry: assignment, not a sticker

The reported mass should match the calculated mass within the stated tolerance. Check charge-state or deconvolution notes. Look for comments on adducts (Na, K) versus covalent modifications (oxidation). If the CoA shows only a rounded integer with no spectrum, you have less evidence than a labeled figure with [M+2H]2+ and [M+3H]3+ assigned.

For cyclic and labeled peptides, confirm that the mass is the cyclic or labeled formula, not the linear unlabeled precursor. If both appear, ask which peak was released. This is a common confusion on lots that were cyclized or dyed in a second step.

Quantity, water, TFA, and net peptide content

The label milligrams are usually the weight of the lyophilized solid as bottled—peptide plus counter-ion plus residual water and residual solvent. Net peptide content, when present, tells you how much of that weight is peptide. Without content, a 5 mg vial of a TFA salt is not 5 mg of peptide backbone. For quantitative biochemistry, either request content or measure concentration after dissolution (UV extinction if you have a reliable ε, AAA, or a calibrated HPLC).

Water and residual TFA are not always listed. Their absence means they were not reported, not that they are zero. If your NMR or cell assay is TFA-sensitive, the specification should have requested a salt exchange; the CoA should then state acetate or HCl, not merely omit the word TFA.

What is usually not on a research CoA

Amino-acid analysis, residual solvent by GC, endotoxin, bioburden, elemental impurities, and enantiomeric purity are typically absent unless the project paid for them. Do not infer a pass. If a protocol requires endotoxin for a research cell experiment, that requirement belongs on the purchase specification, not in a complaint after delivery.

Solubility is almost never certified. A CoA cannot promise that 10 mM in PBS will be clear. That is a handling and design problem.

How to accept or query a lot

Accept when sequence, mass, and HPLC method-plus-trace match the order and look internally consistent. Query when the mass is unexplained, the chromatogram disagrees with the tabulated percent, a modification is missing from the header, or a required extra test is absent. Send the lot number and the specific field, not a general claim that “purity feels low.” If you re-run HPLC in-house, attach your method; see why traces differ in the HPLC article before you reject the lot.

Archive the PDF with the remaining solid. Future users in the group will thank you when the original email thread is gone. For a new project, use request a quote and list the CoA tests you will need, including content or salt exchange, so the certificate can be written to the experiment.

A CoA is documentation for a research chemical. It is not a clinical certificate and it does not authorize diagnostic or human use.