"≥99% pure" is the most repeated phrase in this market and the least examined. Purity is a real, measurable property, but only when there's a named method and a verifiable document behind the number. Here's what the testing actually measures and how to tell a measurement from a marketing line.
Quality testing answers two separate questions, and a serious COA covers both:
A sample can be 99% pure and still be the wrong peptide; purity alone won't catch that. That's why the two tests go together.
High-Performance Liquid Chromatography dissolves the sample and pushes it through a separation column. Different compounds travel at different speeds and emerge at different times, each producing a peak on a graph called a chromatogram. The target peptide is the big peak; impurities and byproducts are the small ones. The area of the main peak against the total gives the purity percentage.
A trustworthy result shows a clean chromatogram with one clearly dominant peak. A number quoted with no chromatogram, or a messy trace with several large peaks, tells a different story than the headline figure. This is also the step where purity is actually created: chromatographic purification is what removes the byproducts of synthesis, long before any drying or packaging happens.
Mass spectrometry ionizes the molecule and measures its mass-to-charge ratio, which gives its molecular weight. That weight is compared to the theoretical weight for the intended amino acid sequence. A match confirms the sample is the peptide it claims to be; a mismatch is a red flag no purity number can paper over. Purity says "mostly one thing"; MS says "and that thing is correct."
A purity percentage is only as good as the method and document behind it. The same sample can report slightly different numbers under different HPLC conditions, so the value is meaningful in context, not as a bare figure on a label. Three things separate a measurement from a claim:
Without those, "99% pure" is a sentence, not a spec.
The most important word in "third-party tested" is third. In-house testing asks a vendor to certify its own product; even with honest intentions, that's a conflict of interest, and it's impossible to distinguish an honest in-house lab from a dishonest one from the outside. An independent testing lab has no stake in the outcome. The strongest labs go further and issue a verification link, so anyone can confirm the certificate came from them and wasn't edited after the fact.
This is the single clearest signal of a serious vendor in this space: not a purity number, but a purity number you can independently verify. Learn to read the document itself in how to read a COA.
Purity and identity are chemistry. They describe what's in the vial, not what it does. No amount of analytical data speaks to biological or health effects, and any vendor using a purity number to imply an outcome has left chemistry and entered claims they can't support. Good testing earns trust precisely by staying in its lane.