What does 99% purity on a peptide COA mean?

In an HPLC purity result, it usually means the main peak accounts for about 99% of the total peak area that the detector saw at the measured wavelength. It is a share of what was detected, not a share of the powder by weight, and it does not say how much peptide is in the vial.

Why can the quantity found differ from the amount on the label?

Assay methods have their own variability, lyophilised peptides contain counter-ions and water, and fill amounts differ slightly. Small gaps are common; a large gap in either direction is worth asking the seller or the laboratory about.

Is purity or identity more important?

They answer different questions and neither replaces the other. Identity says the right molecule is present; purity says how much of what was detected is that molecule; quantity says how much is there.

Purity, quantity and identity: what each number on a COA means

By PeptideProof · Updated 12 October 2026 · 6 min read

Three numbers on a peptide COA are often treated as one: identity, purity and quantity. They are measured in different ways, answer different questions and fail in different ways. Knowing the difference is the quickest way to read a report more accurately.

Result	Question it answers	Usual method
Identity	Is it the right molecule?	Mass spectrometry (LC-MS)
Purity	Of what was detected, how much is the main peptide?	HPLC with UV detection (often around 220 nm)
Quantity	How much peptide is actually there?	Assay against a reference standard

Identity: the right molecule

Mass spectrometry measures the mass of molecules. For a peptide, the result is compared with the mass expected from its sequence. Peptides pick up several charges, so reports often show a charge state, for example [M+4H]4+, together with the measured value. If the measured mass does not fit the expected one, the sample is not what the label says, whatever the purity is.

Purity: a share of what was detected

An HPLC run separates the components of the sample and a detector records them as peaks. Purity is typically the area of the main peak divided by the total area of all peaks. Two limits matter:

It only counts what the detector sees at the chosen wavelength. Some substances, such as salts and water, absorb little or no UV light and don’t appear.
It is relative. A report can show a pure peak for a vial that contains much less peptide than declared — purity says nothing about how much material there is.

Quantity: how much is there

Quantity (content or assay) compares the sample against a reference standard of known amount. This is the number to set against the amount on the label. Freeze-dried peptides are not 100% peptide by weight — counter-ions and residual water make up part of the powder — so how the laboratory defines “content” matters, and the method should be named.

A worked example

A vial is labelled 10 mg. The report shows purity 99.1% and quantity 9.3 mg. Read it as: the detected material is almost entirely the main peptide, and the vial holds about 93% of the declared amount. That is a gap of roughly 7% — within the range many assays vary by. If the quantity read 6.4 mg or 13 mg, purity would not explain the gap, and the question to ask is why.

How much of a gap is normal?

Assay results commonly vary by several percent between methods and laboratories, and fill amounts differ slightly. As a rule of thumb, a gap under about 10% is rarely a concern on its own, while a larger gap deserves a question. PeptideProof flags a difference of 10% or more between the quantity found and the quantity declared as an observation — for example in the example report — never as a conclusion about a product.

For the whole document, not just these three numbers, see how to read a peptide COA.

Frequently asked questions

What does 99% purity on a peptide COA mean?: In an HPLC purity result, it usually means the main peak accounts for about 99% of the total peak area that the detector saw at the measured wavelength. It is a share of what was detected, not a share of the powder by weight, and it does not say how much peptide is in the vial.
Why can the quantity found differ from the amount on the label?: Assay methods have their own variability, lyophilised peptides contain counter-ions and water, and fill amounts differ slightly. Small gaps are common; a large gap in either direction is worth asking the seller or the laboratory about.
Is purity or identity more important?: They answer different questions and neither replaces the other. Identity says the right molecule is present; purity says how much of what was detected is that molecule; quantity says how much is there.

See it on a real document

PeptideProof reads a COA for you and shows what is only reported, what has no evidence and what looks inconsistent. See an example first, or check your own — new accounts include free analyses.

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Educational information about reading laboratory documents. It is not medical, dosing or purchasing advice, and it doesn’t say that any product is safe, legal or suitable for any use. See the medical and research-use disclaimer.