"99% purity, HPLC tested" appears on nearly every research compound listing in this market, ours included. It is worth explaining what the phrase actually refers to, because a claim everyone makes is only worth something if it can be interrogated.
HPLC: what it separates
High-performance liquid chromatography does one thing well: it separates a mixture into its components.
The sample is dissolved and pushed under high pressure through a column packed with a fine stationary material. Different molecules interact with that packing to different degrees, so they travel through at different speeds and emerge separately. A detector at the far end records what comes off and when, producing a chromatogram — a trace with a peak for each component.
The compound of interest produces the main peak. Related substances, residual reagents from synthesis and degradation products produce smaller ones.
Where the percentage comes from
Purity by HPLC is calculated by area. The software integrates the area under every peak, and the purity figure is the main peak's area as a proportion of the total.
So 99.640% means the target compound accounted for 99.640% of the total detected peak area, and everything else together accounted for 0.360%.
Two consequences follow, and both matter.
First, this is a relative measure, not an absolute one. It describes the proportion of what was detected. It is not the same as saying the vial is 99.640% compound by mass — water content in particular is not captured this way.
Second, HPLC only reports what its detector can see. Most peptide analysis uses UV detection, which responds to molecules that absorb at the chosen wavelength. A contaminant that does not absorb there does not produce a peak and does not appear in the calculation. This is a real limitation and it is why purity alone is an incomplete answer.
Why mass spectrometry is the other half
HPLC tells you how much of the material is one dominant thing. It does not, on its own, tell you what that thing is. A different compound of similar chemistry could produce a clean single peak and a purity figure of 99% — while being entirely the wrong molecule.
Mass spectrometry answers identity. It measures mass-to-charge ratio, which for a peptide corresponds to its molecular weight and therefore its amino acid sequence. If the observed mass matches the expected mass for the sequence, the identity is confirmed.
The two techniques are complementary and neither is sufficient alone:
- HPLC — how pure is it?
- Mass spectrometry — is it the right compound at all?
A supplier claiming purity without identity confirmation has answered the easier question.
The quantity figure is separate again
A certificate typically reports quantity alongside purity — the measured mass of compound in the vial. That is a third, independent question: not what it is, and not how pure, but how much.
All three can diverge. Material can be correctly identified, highly pure, and underfilled. Purity of 99% in a vial containing half the stated mass is still a bad outcome. Read all three figures together.
What the claim is worth without a certificate
Nothing, in practical terms.
"99% purity" as a line of website copy is an assertion the seller has written about themselves. It costs nothing to type and carries no more weight than any other marketing sentence. The claim only becomes meaningful when attached to a specific batch, a named laboratory, a dated analysis and a verification route you can follow without the seller's involvement.
That is the standard to hold suppliers to, including this one. Ours are published at Certificates of Analysis, and each carries the key to verify it directly with the laboratory. If you want to know how to read what you find there, we have written a field-by-field walkthrough.
All compounds are supplied strictly for laboratory research and analytical purposes. Not intended for human or veterinary use.