Most people choosing a research peptide supplier look for one thing: a big purity number. 99%? Done. Except a certificate can say 99% pure and still describe the wrong molecule. It happens.
With any research peptide supplier, identity comes before purity
Purity tells you how much of the peptide material is one thing. Identity tells you what that one thing is. Different questions, different machines. A certificate that only answers the first has done half the job.
The proof: a 99% pure wrong answer
Here's what's actually going on.
Purity is a race. HPLC, short for high-performance liquid chromatography, pushes the sample through a packed tube. Different molecules come out at different times, like runners crossing a finish line. The biggest group's share of the total is your purity number. On its own, though, that race can't tell you who any of the runners are.
Identity is a scale. Mass spectrometry, or mass spec, weighs the molecule. Every peptide is a chain of building blocks called amino acids, so you can work out its weight in advance from the order of the chain. BPC-157, for example, comes to about 1,419.5 daltons, the unit chemists use to weigh molecules (PubChem). If the measured weight matches, you've got the right molecule's mass. If it doesn't, it isn't the right molecule, however pure it is.
The usual mistakes are tiny. A 2014 review from Ghent University in Belgium mapped the impurities most often seen in peptide medicines. First on the list: deletions and insertions, a chain missing one link or carrying an extra one, left behind when a step in making the peptide doesn't go perfectly (D'Hondt et al., J Pharm Biomed Anal 2014). Drop a single glycine, the smallest amino acid, from BPC-157 and the chain gets about 57 daltons lighter. That's exactly what the scale catches.
So why should you care? Before you buy, open the certificate and look for two separate results. One is an identity result from mass spec, often listed as LC-MS, which sorts the sample first and then weighs it. The other is a purity percentage from HPLC. Purity alone answers one of four questions. The other three are identity, net peptide content and contamination.
"So is 99%+ the number to look for?" Careful. Real results wobble a little from lot to lot (a lot is one production batch), like 98.4% or 99.2%. The same "99%+" on every product, every lot, forever, is a marketing number, not a lab number.
The big learning lesson
Keep reading: How to read a peptide COA · What "third-party tested" really means · Made in USA vs imported peptides
Frequently asked questions
How do I choose a research peptide supplier?
Start with the certificate of analysis. Look for an identity result by mass spec, a purity figure by HPLC, net peptide content and an endotoxin result, along with a lot number, a test date and the name of the lab that ran the tests.
What's the difference between peptide purity and identity?
Purity, measured by HPLC, is the main compound's share of the peptide material the detector sees. Identity, confirmed by mass spec, is whether that main compound is the right molecule.
Why do purity numbers change from lot to lot?
Every production run is slightly different, so real test results vary a little. A figure that never changes across lots is likely not a per-lot result.