How to Read & Verify a COA

Every legitimate COA is tied to a specific lot / batch number, and that number must match the lot printed on the product you received. Look for a manufacture date and a test date. Quality can vary lot to lot, so a prior batch passing does not carry over. If a seller shows a generic COA, an older COA, or one whose lot doesn’t match your product, treat it as no COA at all.

Purity is usually measured by reverse-phase HPLC and reported as area percent — the main peak as a fraction of all peaks detected by UV, typically around 214–220 nm. Its limits matter:

• Area% depends on the method (gradient, column, wavelength) — different methods give different numbers.
• Impurities that absorb weakly at the chosen wavelength can be under-counted.
• It does not identify what the impurities are.
• It is not weight purity, and ignores water, salts, and counterions.

Look for the actual number (98.4% vs 99.6% is a real difference), the method, and ideally the chromatogram. For research peptides ≥98% is common; ≥99% is premium. A number with no method and no chromatogram is a weaker claim.

Mass spectrometry (commonly ESI-MS or MALDI-TOF) confirms the molecular weight of the main component matches the expected weight of the target. A sample can be highly “pure” by HPLC yet be a pure wrong molecule if identity was never confirmed. Look for observed vs. theoretical mass and the method used; sequence confirmation (MS/MS) is stronger still. Purity with no identity testing is only half the picture.

Lyophilized peptides carry water, counterions, and residual salts. The net peptide content assay tells you what fraction of the powder’s mass is truly peptide — frequently 70–90%. So a vial can read 99% pure by HPLC and still be only ~80% peptide by mass. If you quantify by weight, this is critical. Many COAs omit net peptide content — and that omission is itself a gap.

• Water content (Karl Fischer): peptides are often hygroscopic; more water = less peptide per mg and faster degradation.
• Counterion (TFA vs. acetate): TFA residue can interfere with certain sensitive assays; acetate is often preferred. A thorough COA states the counterion and its %.
• Residual solvents: traces from purification (e.g., acetonitrile) should fall within recognized limits if reported.

• Endotoxin (LAL test): relevant for cell-culture and in-vivo research; reported in EU/mg. Often absent from basic research-use COAs.
• Heavy metals, microbial / bioburden, sterility: depend on the research context and are often not performed for bench chemicals.

Honest note: the absence of these tests isn’t automatically “bad” for pure bench work — but you should know they weren’t performed and decide whether your application requires them.

A pass/fail verdict is only as meaningful as the specification behind it — and for research compounds that spec is commonly set by the manufacturer or vendor themselves. Read the specification column, not just the result. Set your own benchmark — for example: HPLC purity ≥99%, identity confirmed by MS, net peptide content reported, tested by a named independent lab — and hold every COA to your standard, regardless of whose checkmark is on it.

In-house testing means the seller tested its own product — a real conflict of interest. Independent third-party testing by an outside laboratory removes that conflict. Look for the lab’s name, an accreditation (such as ISO/IEC 17025), a method reference, and ideally an analyst signature. A COA with no named lab, method, or signature is weak evidence no matter how good the numbers look.

• Does the lot number match the vial you received?
• Is there a manufacture and test date?
• Is the testing laboratory named and reachable?
• Are the methods stated (HPLC conditions, MS type)?
• Is there an analyst / signature?
• Can you cross-check the lot with the vendor or testing lab?

Be cautious of PDFs with no lab identity, mismatched or missing lot numbers, or figures with no supporting method or chromatogram.

Every peptide has a precise molecular mass determined by its amino-acid sequence — a value that can be calculated in advance. A mass spectrometer (commonly LC–MS or MALDI–TOF) ionizes the peptide and measures that mass with high accuracy. When the measured mass matches the theoretical mass for the intended sequence, the molecule is confirmed to be what the label says. Tandem mass spectrometry (MS/MS) can go further, fragmenting the peptide to read its sequence back — confirming not just the weight, but the order of the amino acids.

Chemical synthesis is never perfect — it leaves trace by-products such as truncated or deletion sequences. Reverse-phase high-performance liquid chromatography (RP–HPLC) pushes the sample through a column that separates molecules by how strongly they interact with the packing material, producing a chromatogram of peaks. The intended peptide is the main peak, and its area as a percentage of all peak area is the purity value on a COA (for example, ≥99%). A high number means little else is present besides the target peptide.

A vial is never pure peptide by weight — the remainder is bound water, residual salts, and counter-ions (such as acetate or trifluoroacetate) left from purification. Net peptide content is measured by amino-acid analysis (hydrolyzing the peptide and quantifying its amino acids) or by quantitative HPLC against a reference standard, while water content is measured separately by Karl Fischer titration. Together these tell you how much true peptide a labeled amount actually contains.

A complete COA may also report appearance, counter-ion content, and — for material prepared under sterile conditions — bacterial endotoxin (by the LAL test) and sterility. One important limit: a COA is a snapshot of identity and purity for a single lot. It confirms what the molecule is and how pure it is; it says nothing about whether a compound is safe or suitable for any use. These remain research materials sold strictly for laboratory research.