What a “≥98% purity” claim means — and what it doesn’t

“≥98% purity” is one of the most common lines on a research-peptide product page. Peptid24, #3 in our ranking, states exactly that for its lyophilized products. It’s a useful number — but it is a narrower number than it looks. This post explains what a purity percentage measures, what it leaves out, and how we treat it in our scores.

Where the number usually comes from

In research-peptide documentation, a purity percentage almost always comes from HPLC — high-performance liquid chromatography. The sample is dissolved and pushed through a column; different molecules leave the column at different times and show up as peaks on a chromatogram.

The purity figure is the size of the main peak as a share of all the peaks the detector sees. If the main peak accounts for 98.4% of the total peak area, the report says 98.4% — and a product page might round that to “≥98%”.

So a purity claim answers one specific question: of the material the detector can see, how much is the main compound?

Five things a purity percentage doesn’t tell you

1. Whether the main peak is the right molecule

HPLC separates molecules; it doesn’t identify them on its own. A clean single peak could, in principle, be the wrong compound. Identity is normally confirmed by mass spectrometry (MS), which checks the molecular mass. A COA with both HPLC and MS is stronger than one with HPLC alone.

2. How much peptide is actually in the vial

Purity is a ratio, not an amount. A lyophilized powder also contains water and counter-ions (such as acetate or TFA salts from synthesis) that HPLC doesn’t count as impurities. The share of the powder’s weight that is actually peptide — often called net peptide content — is a separate measurement. A high purity figure says nothing about it.

3. Anything the detector can’t see

HPLC detectors respond to certain molecular features. Contaminants that don’t show up — residual solvents, heavy metals, salts — are invisible in the purity number. They need their own tests.

4. Endotoxins and sterility

Bacterial endotoxins are not measured by HPLC at all. Neither is sterility. Some shops list these as separate test panels, such as endotoxin, sterility and heavy metals, alongside HPLC. We cover endotoxin testing in a separate post.

5. Whether it applies to your batch

A purity figure on a product page is a claim about the product in general. A certificate of analysis tied to a specific batch or lot number is evidence about the material you’d actually receive. Those are different things.

Claim vs. certificate

This is the distinction our spec sheets track:

	What it is	How we show it
Purity claim	A stated figure on the shop’s website	“Purity claim” row — e.g. ≥98% for Peptid24
Public COAs	Certificates the shop publishes for anyone to check	“Public COAs” row — “Yes”, “No” or a dash
Lab testing	What the shop says it tests, and how	“Lab testing” row

For the top three in our ranking right now: Peptid24 publishes a purity claim; Nexo Research and Europa-Peptide don’t publish one. None of the three publish lab reports we could find, so their “Public COAs” and “Lab testing” rows show a dash — “not published”. That’s reflected in the purity & testing scores (8.3 for Peptid24, 8.6 for the other two on our current estimates), and it’s the main reason none of them sits at 9 or above on that criterion.

Questions that turn a claim into evidence

Before ordering from any shop, you can ask support for:

1. The COA for the exact batch you’d receive, with a batch or lot number that matches the label.
2. Which methods were used: HPLC only, or HPLC plus mass spectrometry?
3. Who ran the test: in-house or an independent lab, and whether the certificate can be verified with the lab directly (some labs offer a public lookup key).
4. The test date, so you know the certificate isn’t years older than the batch.
5. Additional panels, if they matter for your work: net peptide content, endotoxin, heavy metals, sterility.

A shop that answers these clearly is giving you more than any percentage on a product page.

How we score purity claims

On our purity & testing criterion (30% of the overall), a published claim counts for something, a batch-level certificate counts for more, and independent, verifiable testing counts for most. Missing information isn’t penalised as if it were bad news — but it can’t be rewarded either. When one of them publishes reports, its spec sheet and score will move.

All products discussed here are sold for laboratory research only. This post explains documentation; it is not advice about using any substance.