99% purity doesn't mean you got 5mg

Look at any peptide website in the UK and you will find a number somewhere near the top of the page. Ninety-nine per cent. Sometimes 99.9%. It is printed in large type, usually next to a laboratory-looking graphic, and it is doing a great deal of persuasive work.

It is also, on its own, close to meaningless. Not false — meaningless. It answers a question most buyers are not asking, while leaving the question they actually care about untouched.

The two numbers

A proper analysis of a lyophilised peptide produces two separate figures, and they measure completely different things.

Purity tells you whether you got the right thing. Content tells you whether you got enough of it. A certificate quoting only purity has answered the first question and skipped the second entirely.

How a 99% pure vial ends up short

This is not necessarily anyone cheating. Some of it is simply what lyophilised peptide is.

What settles in the bottom of the vial is not pure peptide by weight. It also contains water the freeze-drying did not remove, counterions left over from purification — trifluoroacetate or acetate, depending on the process — and residual salts. Weigh the contents and you are weighing all of that together.

So a vial filled to five milligrams by gross weight may hold appreciably less than five milligrams of actual peptide. The industry term for the fraction that is peptide is net peptide content, and where it is not measured, nobody involved knows what it is. Including the seller.

Then there is the ordinary commercial explanation, which is that somebody filled it short.

Purity analysis cannot distinguish between these, because purity analysis is not looking at quantity. You can run an immaculate chromatogram on a vial containing three milligrams and it will return the same 99% as a vial containing five.

Why the market settled on the flattering number

Partly cost. Purity by HPLC is the cheaper, faster, more routine test, and a supplier's own certificate almost always leads with it. Quantifying content properly means additional work against a reference standard.

Mostly, though, it is because purity is the number that behaves. It is reliably high and it looks like precision. Content is the number that occasionally embarrasses the person publishing it.

A seller printing 99.9% on a banner is not lying to you. They are answering a narrower question than the one you thought you asked, and letting you complete the sentence yourself.

What to look for instead

• —Both figures, stated separately. Purity as a percentage, content as a mass in milligrams, on the same report for the same lot.
• —A lot number, so the figures attach to a specific batch rather than to the product in the abstract.
• —A named laboratory, so the figures can be traced to whoever produced them.
• —A result that resolves on the laboratory's own system rather than in a document hosted by the seller.

If a seller publishes only purity, that is worth asking about rather than assuming the worst — but it is worth asking. The answer, and how readily it comes, tells you most of what you need to know.

What we publish, and what we don't

Our first independent result was BPC-157, lot ZEN-2608-001, tested by Janoshik: 99.892% purity and 5.71mg measured content on a 5mg vial. Both numbers, because one without the other is half an answer.

The rest of the range is a work in progress and the batch ledger says so per lot. Some batches carry an independent result; most currently carry the supplier's certificate only, and the page marks which is which rather than letting one good number stand in for the catalogue. Independent retesting is running batch by batch, and results are published as they come back regardless of what they say.

That is a less impressive claim than a banner reading 99.9%. It has the advantage of being checkable.