Most people learning how to read a peptide COA (certificate of analysis, the lab's report on a batch) stop at one number: purity. It's the biggest number on the page, and for research peptides it's usually above 95%. So a 99% must mean the vial is 99% peptide, right? It doesn't.
A 99% pure vial can be 78% peptide
Both can be true at once. Purity tells you how clean the peptide is. Net peptide content tells you how much of the powder is peptide at all. Not every certificate shows that second line. It's the one that tells you what you're actually weighing out.
The proof: how to read a peptide COA past the purity line
Here's what's actually going on.
Purity only grades the peptide part. The lab runs a sample through RP-HPLC, a test that spreads the sample out into its parts, like runners strung out over a race. A detector watches them cross the line. Purity is the right peptide's share of what it saw, and salt and water don't count.
And the powder always carries salt and water. Peptides come out of purification paired with a small charged partner called a counterion, usually acetate or trifluoroacetate (TFA for short). Think of it as packing peanuts that ship with the peptide. The powder also pulls a little moisture out of the air. Bachem, a Swiss peptide maker that's been doing this since 1971, says research peptides are typically more than 95% pure by HPLC, while the powder itself can range from 70% to 90% peptide (Bachem).
Those peanuts aren't always harmless. In 1999, a University of Auckland team grew rat bone cells in the lab and compared the same peptides in two salt forms, TFA and hydrochloride. The TFA versions consistently showed less cell growth. Even a trace of TFA on its own slowed the cells. The authors warned that this is enough to miss a real effect, or report one that isn't there (Cornish et al., Am J Physiol 1999). Same peptide. Different peanuts. Different result.
So why should you care? If the powder is 75% peptide and you count it as 100%, a quarter of what you weighed was never peptide. That's why every PUŪRA certificate reports net peptide content, right next to LC-MS identity (is it the right molecule), RP-HPLC purity (how clean it is) and LAL endotoxin (a check for bacterial residue).
"No net content line on the certificate. Is the vial bad?" Not necessarily. Plenty of certificates skip it. But one that shows it is telling you how much peptide you're really working with.
The big learning lesson
Keep reading: What "third-party tested" really means · How to choose a research peptide supplier · How to store freeze-dried peptides
Frequently asked questions
How do I read a peptide certificate of analysis?
Start with the header: compound name, lot number, test date and the name of the lab. Then read each result as its own question: identity (is it the right molecule), purity (how clean the peptide is), net peptide content (how much of the powder is peptide) and endotoxin (bacterial residue).
What's the difference between purity and net peptide content?
Purity tells you what share of the peptide material is the right peptide. Net peptide content tells you what share of the whole powder is peptide at all, after counting salt and water.
Why isn't peptide powder 100% peptide?
Peptides come out of purification paired with counterions like acetate or TFA, and the powder holds a little water. For research-grade material, net peptide content commonly falls between 70% and 90%.