How to read a peptide COA: HPLC purity, MS identity, and what suppliers leave out

Most peptide buyers make their decision based on a single number: “purity ≥99%”. That number is the least informative value on the entire document. This guide explains what each section of a certificate of analysis actually proves — and where a well-formatted COA can still tell you nothing.

1. What a COA is — and what it is not

A Certificate of Analysis is a batch-level test report. Its whole value rests on one property: the document must describe the physical material in the vial you are holding. Everything else — layout, logos, batch numbering style — is decoration.

Three limits are worth stating plainly, because procurement teams routinely over-read the document:

• A COA is not a certification. It does not mean the facility is audited, licensed, or accredited. It is a report of measurements on a sample.
• A COA is not transferable between batches. A COA from batch A tells you essentially nothing about batch B. If a supplier sends one undated COA “for this product”, the document has no batch meaning at all.
• A COA is not independent by default. Most COAs are produced by the seller's own QC. That is normal in this industry, but it means the document is a claim, not third-party evidence.

2. HPLC purity: what “area %” really measures

Purity in a peptide COA almost always means HPLC area percentage by UV detection, typically at 214 nm (peptide bond absorbance) or 220 nm. The calculation is simple: the area of the main peak divided by the total area of all peaks in the chromatogram, expressed as a percentage.

This number is useful, but it measures relative chromatographic cleanliness, not absolute peptide quantity. Four consequences follow:

Note the third row in the left column: purity is method-dependent. A 20-minute shallow gradient on a good C18 column will resolve impurities that a 10-minute fast run merges into the main peak. Two labs can report 95% and 99% for the same material and both be honest. This is why a COA without the chromatographic conditions (column, mobile phase, gradient, wavelength, injection volume) is only half a document.

3. Reading the chromatogram, not the headline number

If the COA includes the actual HPLC trace — and a serious supplier will — you can extract far more than the percentage. Look at four things:

Peak shape and symmetry

A symmetric main peak with a tailing factor near 1.0–1.5 indicates a clean single species. Heavy tailing often signals aggregation, residual trifluoroacetate interactions, or column overload. Fronting usually means the injection was too concentrated to be quantitative.

Shoulders on the main peak

A small shoulder immediately before or after the main peak is the classic signature of a near-identical impurity: a deletion sequence, an oxidized methionine, or a diastereomer. These are exactly the impurities that matter for assay reproducibility, and they are the ones most often “absorbed” into the main peak integration by widening the baseline.

Where the impurity peaks sit

Impurities eluting well before the main peak are usually truncated sequences (more polar). Peaks after the main peak are often more hydrophobic — protected fragments, aggregation, or acetylated variants. The pattern tells you something about the synthesis, which is more diagnostic than the total.

The y-axis scale

A chromatogram plotted with the main peak at full scale can make 1–2% impurities look negligible. Zoomed insets or a stated “impurity threshold, ignore below X%” are legitimate practices, but you should know they are there.

4. Mass spectrometry: identity, not quality

LC-MS or MALDI-TOF confirms molecular mass. It answers “is this the molecule we ordered?” — it does not answer “is it pure?” or “is it the right quantity?”.

When reading an MS section, check these points:

• Expected vs observed mass, both stated. A COA that only prints one number is not a comparison. You want to see the theoretical monoisotopic (or average) mass alongside the measured value, and the delta.
• Charge states and adducts. For larger peptides, the spectrum shows multiple charge states ([M+2H]²⁺, [M+3H]³⁺). Sodium and potassium adducts (+22, +38) are normal; they should be labelled, not silently ignored.
• Sequence coverage is not the same as mass match. A mass match confirms composition, not order. Two isomers with the same residues in a different sequence have identical mass. Only MS/MS fragmentation (or sequencing) distinguishes them — almost no commercial COA includes this, so know that the gap exists.
• Counter-ion and salt form. Acetate vs trifluoroacetate vs hydrochloride changes the mass by a fixed offset. If the COA reports a mass that matches the free base but ships the TFA salt, the numbers can appear “off by 114 Da” and still be correct. Ask which form is supplied.

5. Net peptide content — the figure most buyers never check

This is the single most consequential number on a peptide COA, and it is frequently absent. Net peptide content (also called peptide content or net peptide) is the mass fraction of the powder that is actually peptide, as opposed to water, residual solvents, and counter-ions.

Same purity, ~12% difference in the amount of material you can actually dissolve and use. For an assay where you weigh out a fixed mass, that difference propagates straight into your concentration calculations and can look like a failed experiment when nothing was wrong with the peptide.

A complete COA therefore includes: HPLC purity, identity by MS, water content, and net peptide content, plus the analytical methods used. If net peptide content is missing, you cannot convert a weighed mass into a molar concentration without guessing.

6. Three fields suppliers routinely omit

① Test date versus shipment date

Peptides degrade. A COA dated three years before your shipment describes material that no longer exists. Look for a test date that is recent relative to the dispatch date, and ideally a re-test or stability statement for long-stored batches.

② The identity of the testing party

“Third-party tested” is a phrase, not a fact. A verifiable claim names the laboratory, provides a report number, and — critically — gives you a way to check the report independently, such as a lab-run verification page that is not hosted on the seller's own domain. If the “independent lab” exists only as a logo on the seller's website, the independence is unproven regardless of what the report says.

③ Storage and handling conditions actually used

Was the batch stored at −20 °C or on a shelf? A COA that reports excellent purity but omits storage conditions tells you the material was good on the test date and nothing about the months since. Ask for the storage condition and the dispatch temperature regime.

7. A seven-point checklist you can use today

Send this to any supplier before your first order. A serious one will answer all seven within a day; a reseller working from a template will stall by item three.

1. 1
   Batch linkage. Does the COA number match the lot number on the vials that will ship to me?
2. 2
   Test date. When was this specific batch tested, and how long has it been in storage since?
3. 3
   Full chromatogram. Can you send the HPLC trace with method conditions (column, mobile phase, gradient, wavelength)?
4. 4
   Identity data. Is the observed mass shown against a stated theoretical mass, and which salt form is supplied?
5. 5
   Net peptide content. What is the peptide content, water content, and counter-ion fraction of this batch?
6. 6
   Testing party. Which laboratory ran the tests, and how can I verify the report through them directly?
7. 7
   Storage and dispatch. Under what conditions was the batch stored, and how is it shipped (ambient, gel pack, dry ice)?

What this means practically

None of the above requires a laboratory to interpret — only the willingness to read past the first line. In practice, the buyers who ask for items 1, 3 and 5 get better material at the same price, because suppliers know which customers check. The document is not a formality attached to the shipment; in this category it is the most reliable signal of how the supplier operates.