What “99% purity” does and doesn’t tell you about a peptide

"99% purity" is the most common claim in research peptide marketing, and it's easy to see why. It sounds like a complete description of quality: a single number that settles the question. It is a useful measurement. It is also a narrow one, and it's easy to read far more into it than it contains.

The short answer: purity is the share of the detected material that is the main compound. It says nothing about how much material there is, nothing about water and salts, nothing about contamination, and nothing about whether the main compound is the right one.

How the number is made

Peptide purity is almost always measured by HPLC with a UV detector. The dissolved sample runs through a column that separates its components, and the detector records each one as a peak. The purity figure is the area of the main peak divided by the total area of all the peaks the detector saw, expressed as a percentage.

So purity answers exactly one question: of the material this detector could see, what share was the main compound?

What it leaves out

How much is in the vial

Purity is a ratio. It says nothing about the absolute amount. A vial labeled 10 mg that contains 5 mg of material, 99% of which is the right compound, reports as 99% pure. The measurement is correct. The vial holds half the label amount. Both things are true at once.

The amount is a separate measurement, called content. It is harder to do, which is part of why it's reported less often. For a buyer it can matter more than purity does.

Water and salts

Freeze-dried peptide powder isn't pure peptide. It also contains water and counter-ions, usually trifluoroacetate or acetate salts left over from manufacturing. Together these commonly make up 10 to 20 percent of the powder's weight, sometimes more. They aren't counted in a typical HPLC purity figure, so they don't lower it.

That's why "10 mg" can mean two different things: 10 mg of powder (gross) or 10 mg of peptide (net). Both are sold, and the label often doesn't say which.

Anything the detector can't see

UV detection at the usual wavelength picks up the peptide bond. Impurities that absorb weakly, or not at all, at that wavelength contribute little or nothing to the total, so they barely move the percentage. A clean-looking chromatogram is clean at one wavelength.

Contamination and sterility

A purity run doesn't test for bacteria, endotoxin, heavy metals or residual solvents. A very high purity figure is not evidence that any of those is absent. Each needs its own test.

Which molecule it is

Purity measures the share of the main peak. Whether that peak is the declared compound is a separate question, answered by an identity test such as mass spectrometry. A sample of the wrong compound can still produce a beautiful single peak.

How precise is a purity number?

Two competent laboratories testing the same material by slightly different methods, a different column, gradient or way of integrating small peaks, can report purity figures that differ by more than the decimals on a typical certificate suggest. A figure given to two or three decimal places implies a precision that comparisons between laboratories rarely support.

Where results from different laboratories are going to be compared, the better practice is for them to run a common method, and to test identical material first to measure how closely they agree. Publishing that measured agreement next to a purity figure tells a reader how much a difference between two numbers can actually mean.

Reading purity well

• Read it next to an identity result. Purity of an unknown compound tells you very little.
• Look for content, and for the basis it's reported on.
• Check the method, and look at the chromatogram if there is one.
• Treat very precise figures with caution.
• Don't read purity as a statement about sterility or safety. It isn't one.

A purity figure is most useful when it is published as measured, with identity and content beside it, the method stated, and no pass or fail attached.