Purity and identity sound like they should be the same question, but they're not — and the two analytical techniques that answer them work on entirely different physical principles. Between them, HPLC and mass spectrometry cover nearly all of what a legitimate peptide Certificate of Analysis reports.
HPLC: separating the sample to measure purity
High-Performance Liquid Chromatography physically separates a sample's components by pumping it through a column — typically a reverse-phase C18 column for peptides — under high pressure, using a mobile phase gradient (commonly water and acetonitrile with a trifluoroacetic acid modifier). Different molecules travel through the column at different rates depending on their hydrophobicity and interaction with the stationary phase, so they exit at different times and register as separate peaks on a UV detector. Purity is calculated as the target compound's peak area relative to the total peak area detected — a measurement of how clean the sample is, not what it actually is. See our deeper look at understanding HPLC purity analysis.
Mass spectrometry: confirming what the molecule actually is
Mass spectrometry works on a completely different principle: it ionizes the sample — commonly via electrospray ionization for peptides — and measures the mass-to-charge ratio of the resulting ions. Peptides frequently pick up multiple charges during ionization, producing a series of related peaks that software deconvolutes back into a single molecular mass. That measured mass is then compared against the mass predicted by the compound's stated amino acid sequence. A match confirms identity; a mismatch means the vial doesn't contain what the label says, regardless of how clean the HPLC trace looks.
Why one without the other is an incomplete picture
A high HPLC purity result on the wrong compound is meaningless — you'd simply have a very clean sample of something you didn't order. Conversely, confirming identity by mass spec on a sample loaded with synthesis byproducts doesn't tell you how usable that material actually is for a controlled protocol. The two tests are complementary by design: HPLC answers how much, mass spec answers what.
What this means for documentation
A Certificate of Analysis that only reports a purity percentage, with no identity confirmation, is telling half the story. The stronger standard — one independent laboratories generally follow — is to report both a measured HPLC purity value and a mass-spec-confirmed identity, tied to a specific batch/lot number. That combination is what separates genuine analytical documentation from a number printed on a label with nothing behind it, and it's a core item on our supplier evaluation checklist.
Instrumentation, in practice
Neither technique requires exotic equipment by modern analytical-chemistry standards — HPLC systems and single-quadrupole or time-of-flight mass spectrometers are standard instrumentation in any properly equipped analytical laboratory. What varies between suppliers isn't access to the instruments, but whether the testing is actually performed, on the actual batch shipped, by a lab with no incentive to round a result upward. That's the distinction our guide to third-party testing covers in more depth.
Research Use Only. Supplied strictly for laboratory research and development — not for human or veterinary use, consumption, or any therapeutic or diagnostic purpose. This article is research education, not usage guidance.
See a third-party–tested compound: BPC-157 (batch COA available) →