“99% pure” is the headline number in peptide marketing, and it is the wrong number to lead with. Not because it is false — it is usually accurate — but because it answers a question most buyers are not actually asking.
Purity is a ratio. It describes the composition of whatever material is present: of everything detectable in the vial, what fraction is the target compound. A 99.4% HPLC purity means that among the material that is there, almost all of it is the right molecule. That is genuinely good, and it is worth reporting.
But purity says nothing about how much material is there. That is a different measurement — net content, or peptide fill — and it compares the actual mass of peptide in the vial against the label claim. The two numbers are independent. A vial can be 99.4% pure and contain 86% of its labeled amount at the same time. Both statements are true. The material is excellent; there is meaningfully less of it than you paid for.
This is not a hypothetical failure mode. Underfill is common precisely because purity is easy to showcase and net content is easy to omit, and because a buyer who sees a high purity figure tends to assume the quantity question has been answered too. It has not.
The practical takeaway is simple. When you evaluate a COA, treat purity and net content as two separate gates, and be suspicious of any report that clears the first without mentioning the second. A supplier confident in their fill will report it. A report that shows you only the ratio, and stays silent on the quantity, has told you which number it would rather you look at.