Purity is not content: why a 99% pure vial can still be short

The short version.

Purity tells you what share of the material the detector saw was your compound. Content tells you how many milligrams are actually in the vial. They are different measurements that fail in different ways, and a certificate that prints only purity has answered the easier question. At Veritalys both come together in the identity, content and purity panel, €169 to €399.

The arithmetic

Take a vial labelled 10 mg. The numbers below are an illustration, not a measurement, but the pattern is exactly the one that catches people.

Purity of 99% means that of everything the detector registered, 99% was your compound. It is a ratio. It compares the compound with whatever else is in the vial, not with what the label promised.

Now suppose the vial holds 6.5 mg. Those 6.5 mg can be 99% pure. The purity figure does not move. The powder looks the same, the chromatogram looks excellent, and you are missing 35% of what you paid for. The certificate has not lied. It answered a different question from the one you had.

That is why content is measured separately and reported in milligrams per vial.

What each measurement answers

Your question	The measurement that answers it	What it cannot tell you
Is this the right molecule?	identity, against a reference standard	how much of it is there
How clean is the material itself?	purity, as a percentage	how much is in the vial
Did I get what the label says?	content, in mg per vial	whether it is contaminated
Is it clean enough to work with?	endotoxin in EU per vial, plus microbiology	anything about the next vial

Read down the last column. Every row has a blind spot, and the blind spots do not overlap. That is the case for buying the measurements together instead of one at a time.

Why purity-only certificates are common

Purity is the number buyers ask for, so purity is the number that gets printed.

There is also a practical reason. A percentage comes straight from one chromatogram. A milligram figure has to be tied to a known amount of a reference standard for your exact compound, and the laboratory has to hold that standard. It is more work, so it is easier to leave out.

That is exactly why its absence is worth noticing. An under-filled vial looks normal, passes a purity test, and costs you money.

Read a certificate in four checks

1. Find a milligram figure. If the only number is a percentage, the document cannot tell you whether the vial is full.
2. Check it is per vial. Content as a percentage of powder weight is a different statement from milligrams in the container you hold.
3. Check the batch number matches your vial. A real certificate about another batch is still about another batch.
4. Check you can verify it somewhere the seller does not control. See How to tell if a peptide COA is real.

A Veritalys report prints content in mg per vial next to the declared amount, purity with the chromatogram drawn from the laboratory's peak table, endotoxin in EU per vial where it was tested, the batch number you gave, the date the laboratory received the sample, and a permanent report number anyone can look up.

The limit

One test is evidence about one vial. Filling varies across a run, so a content figure confirms the container that was opened and nothing behind it. Vendors build confidence in a batch by testing each batch. Buyers should ask when a certificate was issued and which batch it names.