The journal
Purity is not peptide content
Two numbers describe a vial of lyophilised peptide, they mean entirely different things, and almost every supplier quotes only the one that flatters them. This is the distinction worth understanding before you compare any two certificates.
Purity answers: is this the right peptide?
Purity is what fraction of the peptide present is the peptide you ordered. It is measured by HPLC: separate everything in the sample, measure the area under each peak, and express the main peak as a percentage of the total.
The impurities it catches are the ones synthesis produces — deletion sequences a residue short, truncations, oxidised forms, cleavage fragments. All of them are peptide. All of them are the wrong peptide. That is what a purity figure is counting.
Peptide content answers: how much of this powder is peptide at all?
Now weigh the vial's contents. Not all of that mass is peptide. The rest is:
- Counter-ions. Peptides are purified by reverse-phase HPLC using trifluoroacetic acid, and the basic residues — lysine, arginine, histidine, the free N-terminal amine — come out of that process paired with trifluoroacetate. Each TFA carries 114 daltons, and a peptide with several basic sites carries several of them.
- Residual water. Lyophilised powders are hygroscopic and retain bound water even after drying.
Peptide content — sometimes called net peptide content — is the fraction of the total mass that is actually peptide. It is measured separately, by amino acid analysis or by nitrogen determination, not by HPLC.
Both numbers can be honest and far apart
A vial can be 99% pure and 80% peptide by mass at the same time, and neither figure is a lie. They are answering different questions:
- 99% of the peptide in there is the right peptide.
- 80% of the powder in there is peptide.
The practical consequence is arithmetic. If you weigh out material against a nominal mass and the peptide content is 80%, the concentration you calculate is roughly a fifth higher than the concentration you have. For work where the absolute figure matters, that is the difference between a result and a footnote.
Why this is the most-abused pair of numbers in the industry
Because purity is cheap to measure and flattering, and content is neither. A supplier that runs one HPLC and prints the result has a 99% to advertise. A supplier that also determines net peptide content has a second, smaller number to explain.
The tell is what happens when you ask. A supplier who can tell you which figure they are quoting, and how it was determined, is doing the work. A supplier who treats the question as hostile is answering it.
What we specify
Our catalog specification is 99%+ purity by HPLC, documented per lot, verified first by an independent third-party laboratory and then again in-house before anything ships. That is a purity figure and we say so plainly rather than letting it stand in for a content figure it is not.
The certificate for your lot is filed against your order and searchable in the COA lookup. If the distinction above matters for your work, ask us about the lot in front of you — that is a question about the material, and questions about the material are the ones we answer.
All products supplied by Alpha Aminos are for laboratory research use only and are not for human or animal consumption. Nothing on this page describes or is intended to describe an effect of any compound. No statement here has been evaluated by the Food and Drug Administration, and nothing here is intended to diagnose, treat, cure, or prevent any disease.