Behind a Purity Percentage: How HPLC Works and Why Mass Spec Has to Back It Up
Two certificates, one reading 98% and the other 99.5%, can look nearly identical at a glance. In an assay they may behave very differently. And neither number, on its own, proves you have the right molecule or rules out every contaminant. This guide starts with why small purity gaps matter, then explains how the figure is produced and where its blind spots are.
Small gap, large difference
Turn the two figures around and look at what is not the target. A 98% sample is 2% something else; a 99.5% sample is 0.5% something else. The first carries four times as much contaminant as the second. In a receptor assay that matters, because truncated sequences can act as weak agonists or antagonists at the same receptor the parent compound binds, which puts noise right where the signal is meant to be. In dose-response experiments, the impurity load also shifts the true molar concentration and magnifies any slip in your molarity arithmetic.
What the non-target fraction is made of
- Short chains. When a synthesis cycle fails to finish, the product is missing one or more residues. These truncated sequences look chemically like the target and are sometimes biologically active in ways no one can predict.
- Altered residues. Deletion and modification products include methionines that have oxidized, asparagines that have deamidated, and residues that have racemized.
- Process leftovers. Every synthetic peptide contains TFA (trifluoroacetate) counter-ions. Scavengers and solvents should be present only at trace levels.
How the percentage is calculated
In reverse-phase HPLC, the sample is dissolved and forced through a column that sorts molecules by hydrophobicity. Different species come off the column at different moments, each at its own retention time, and a UV detector logs each one as a peak. The detector usually reads at 214 or 220nm, where peptide bonds absorb. From there it is simple division: purity is the target’s peak area over the summed area of every detected peak, times 100. A hypothetical reading of 99.48% would say that this share of the UV-absorbing material was the intended compound.
Blind spots
Identity. A single clean peak means the sample is homogeneous, nothing more. A completely different peptide could give the same tidy trace, so it does not prove you have the correct molecule. For that you need mass spectrometry: the molecular weight it measures should sit within about one dalton of the theoretical mass of the sequence. Credible certificates therefore report HPLC and MS together, and the COA reading guide counts an HPLC-only certificate as a red flag.
Endotoxin and metals. HPLC cannot see endotoxin, which requires a separate LAL assay; even a sample at 99.9% purity can fail on endotoxin. Heavy metals need ICP-MS. A purity percentage is silent on both.
Applying this to the reports we link
A result worth trusting is precise, a value like 99.42% rather than a round marketing number, and it arrives with the chromatogram, the method, the wavelength and the batch. Run that checklist on the reports listed on our COAs page. It links 16 reports that Freedom Diagnostics Testing issued for LiliPeptide samples, each identified by tested sample, batch, test date and report number; for instance, Tirzepatide 30mg, batch LIL812026BLK, tested 23 Sep 2026, report LILI2609210551. The page itself quotes no purity numbers. Open the report to see which methods it lists and what figure it gives, and keep in mind that the result applies only to the sample that report identifies.
For research use only. Not for human or veterinary use.
