Peptide Purity Explained: What 99% Purity Means on a Peptide COA

Nearly every research peptide listing carries a purity figure, and the figure is nearly always 98% or 99%. So what does 99% purity mean on a peptide COA? It means something considerably narrower than most buyers assume. The number is a chromatographic ratio describing the relative composition of the detectable organic material in a sample. It is not a statement that 99% of the powder in the vial is peptide, and treating it as one is the most common misreading of an otherwise legitimate analytical report.

This article separates the two ideas that the single word "purity" is doing double duty for: chromatographic purity and net peptide content.

Key takeaways

The number is a ratio, not a mass fraction

Reported purity on a peptide certificate of analysis almost always comes from reversed-phase HPLC with UV detection. The instrument separates the sample into peaks, integrates the area under each one, and expresses the main peak as a percentage of total integrated area. That is the 99%.

Two properties follow directly from how the measurement is constructed.

It is relative. Area-percent compares the target peptide against the other peaks the detector registered. It carries no information about absolute quantity. A vial can report 99.2% purity and contain less material than its label states — purity and fill weight are independent measurements answering independent questions.

It is detector-limited. A UV detector at 214 nm or 220 nm responds to peptide bonds and aromatic residues. Species that absorb weakly or not at all at that wavelength are underrepresented in the peak areas or absent from them entirely. Water, inorganic salts, and residual counter-ions fall into this category. They are physically present in the vial and largely invisible to the measurement that produced the 99%.

The consequence is that HPLC area-percent describes the quality of the peptide fraction, not the composition of the vial.

What else is in a lyophilised vial

A freeze-dried peptide is not a single-component solid. The table below sets out the usual constituents and whether a standard HPLC-UV run accounts for them.

For peptides supplied as trifluoroacetate salts, counter-ion mass alone can account for a meaningful fraction of the powder weight, with the exact figure depending on sequence length and basic residue count. Residual moisture adds further mass. Neither shows up in an area-percent purity figure.

Metal-complexed compounds make the distinction especially visible. A copper-bound tripeptide such as GHK-Cu has a defined stoichiometric metal component that is part of the compound's identity rather than an impurity — a reminder that "what is in the vial" and "what counts as the target species" are questions requiring the chemistry, not just the percentage.

What 99% purity means on a peptide COA — and what it does not

Read strictly, a 99% figure supports a narrow, useful claim: of the UV-absorbing material the chromatographic method resolved, roughly 99% eluted as a single peak, and about 1% was distributed across related substances. Combined with a mass spectrometry result confirming the observed mass matches the theoretical mass for the sequence, that becomes strong evidence of both identity and chromatographic cleanliness. Purity without an accompanying MS identity confirmation is a much weaker claim — a clean peak of the wrong molecule is still a clean peak.

What the figure does not support: any inference about how many milligrams of peptide the vial holds.

Net peptide content: the measurement that answers the mass question

Net peptide content — sometimes called peptide content or assay — is the separate determination of what proportion of the lyophilised powder is peptide by mass. It is conventionally established by quantitative amino acid analysis (AAA), in which the sample is hydrolysed and the liberated amino acids quantified against calibrated standards, or by nitrogen determination.

For a typical TFA-salt peptide reporting 99% chromatographic purity, net peptide content is characteristically lower, because the counter-ion and residual water contribute mass without contributing peak area. The two numbers are not in conflict. They measure different things.

Net peptide content is reported far less often than HPLC purity, largely because AAA is a separate, slower, and more expensive assay than a chromatographic run. Its absence from a report is not by itself a red flag. Presenting a chromatographic purity figure as though it settles the mass question is.

Peptide purity grades and what they are used for

Peptide synthesis suppliers commonly sell material in purity grades. The grade is the minimum HPLC area-percent the batch must reach after purification. The bands below are typical of how the research-chemistry literature and custom-synthesis catalogues describe them; exact cut-offs vary by supplier.

A higher grade narrows the impurity peaks; it does not change net peptide content, counter-ion level or water content, which are separate measurements.

How peptide purity is tested: HPLC-UV vs HPLC-MS

HPLC with UV detection gives the area-percent figure. Adding mass spectrometry (HPLC-MS or LC-MS) confirms that the main peak has the expected mass, so the purity figure is attached to the right molecule. A purity number with no identity confirmation tells you the sample is mostly one thing, not that it is the compound on the label. The difference is covered in HPLC vs mass spectrometry peptide testing.

Peptide purity on Hermetica's published certificates

Each figure below is the HPLC-MS result printed on the third-party certificate for that lot, as listed on the COA index. Each one describes the tested lot only.

Read each figure with the cautions above: it is a ratio of detectable peaks for that lot, not a statement of fill weight.

Practical questions for evaluating a purity claim

• Does the report name the analytical method, column, wavelength, and gradient, or does it state a bare percentage?
• Was it issued by an independent laboratory, and can it be verified against that laboratory's own records?
• Is the purity figure tied to a specific lot number that matches the vial in hand?
• Does an MS result accompany the purity figure and confirm the expected mass?
• Is purity being described in language that implies mass content when the underlying measurement is area-percent?

Hermetica Research Labs publishes the batch-level third-party reports behind its listings on the certificate of analysis page, with lot numbers traceable to individual products such as BPC-157 10mg and TB-500 10mg. Reports are issued by Janoshik Analytical and Freedom Diagnostics and can be checked against the issuing laboratory rather than taken on our word.

The short answer to the question, then: 99% purity is a well-defined chromatographic statement about relative composition, valuable when the method is disclosed and the identity independently confirmed, and silent on the quantity of peptide present. Read it for what it measures.

For a worked example, the Retatrutide Canada page links a Freedom Diagnostics certificate reporting 99.86% by HPLC-MS, and the GHK-Cu peptide Canada page covers a lot reported at 99.76%.

All products supplied by Hermetica Research Labs are intended strictly for laboratory research use only. They are not drugs, foods, cosmetics, or medical devices, and are not approved by Health Canada for the diagnosis, treatment, cure, or prevention of any condition. They are not for human or veterinary consumption or administration, and are to be handled exclusively by qualified professionals in an appropriate research setting. This article is provided for informational and educational purposes concerning analytical chemistry and laboratory practice, and does not constitute guidance on the use of any compound.

Frequently asked questions

What does 99% purity mean on a peptide COA?

It means that 99% of the material detected by the HPLC method is the target peptide, measured as a share of peak area. It is a ratio among detected components, not a weight percentage of the powder.

Does 99% purity mean 99% of the powder is peptide?

No. A lyophilized vial also holds water and counter-ions such as trifluoroacetate, which HPLC purity does not count. Net peptide content is a separate measurement.

Is 99% purity better than 98%?

It means fewer detected impurities in the tested sample, but only if both figures come from comparable validated methods. A purity figure is also only meaningful for the lot that was tested.

What purity do Hermetica's certificates report?

The five published certificates report 99.576% for BPC-157, 99.496% for TB-500, 99.558% for MOTS-c, 99.76% for GHK-Cu and 99.86% for Retatrutide, all by HPLC-MS.

References

1. D'Hondt M, Bracke N, Taevernier L, Gevaert B, Verbeke F, Wynendaele E, De Spiegeleer B. (2014). Related impurities in peptide medicines. Journal of Pharmaceutical and Biomedical Analysis, 101, 2–30. doi:10.1016/j.jpba.2014.06.012
2. Roux S, Zékri E, Rousseau B, Paternostre M, Cintrat JC, Fay N. (2008). Elimination and exchange of trifluoroacetate counter-ion from cationic peptides: a critical evaluation of different approaches. Journal of Peptide Science, 14(3), 354–359. doi:10.1002/psc.951
3. International Council for Harmonisation. (2023). ICH Q2(R2) Guideline on validation of analytical procedures, Step 5 (EMA/CHMP/ICH/82072/2006). ema.europa.eu (PDF)

Research Use Only. Not for human or veterinary use. Purchasers must be 21 or older.