Behind the Purity Number: What HPLC Misses, What Half a Percent Means, and How to Read Our Results
A purity percentage only means something once you know how it was measured. Even then it says nothing about whether the molecule is the right one, it can’t rule out every contaminant, and differences that look trivial on paper, 98% against 99.5% for instance, can be larger than they seem.
What a trustworthy figure looks like
Analytical results worth relying on are specific (99.42%, say) and come with a chromatogram, the method, the detection wavelength and a batch number. A rounded marketing number is a different thing altogether.
Why half a percent matters
Impurity of 2% is four times the contaminant load of 0.5%, and that is the real gap between 98% and 99.5%. In receptor assays, truncated sequences may act as weak agonists or antagonists at the same receptor as the parent compound, putting noise into the very signal being measured. In dose-response experiments, impurities also change the effective molar concentration and compound mistakes in the molarity math.
What HPLC can’t see
One clean peak proves the sample is homogeneous, not that it is the right molecule; a completely wrong peptide can produce a single tidy peak too. Identity takes mass spectrometry, which should find a molecular weight within about one dalton of the sequence’s theoretical mass. Credible certificates carry both results, and reading lab results flags HPLC-only certificates for that reason.
Endotoxin is invisible to HPLC as well and needs its own LAL assay, so a 99.9% pure peptide can still fail on endotoxin. Heavy metals are a job for ICP-MS.
What makes up the impurity fraction
- Truncated sequences. Incomplete synthesis cycles leave peptides short of one or more residues. Being chemically close to the target, they can have biological activity that is hard to predict.
- Deletion and modification products. Examples include methionines that have oxidized, asparagines that have deamidated and residues that have racemized.
- Synthesis leftovers. Every synthetic peptide carries TFA (trifluoroacetate) counter-ions; scavengers and solvents should be present only in trace amounts.
How the percentage is produced
Reverse-phase HPLC pushes the dissolved sample through a column that sorts molecules by how hydrophobic they are. Different species emerge at different retention times, and a UV detector, usually reading at 214 or 220nm where peptide bonds absorb, records each as a peak. The arithmetic is then simple: target peak area as a percentage of total detected peak area. A sample at 99.48% purity is one where the intended compound made up 99.48% of the UV-absorbing material.
The purity figures on our COAs page
Our COAs page lists 31 passed Finnrick tests of Welon Peptide Solutions products, with reported purity between 99.5% and 99.9%. The same arithmetic applies to them. Retatrutide tested on 4 Feb 2026 at 99.5% leaves 0.5% of detected material that is not the target; the 11 May 2026 retatrutide result of 99.9% leaves 0.1%, one-fifth as much. Some rows give one decimal place (99.9%) and others two (99.84%, 99.78%, 99.86%), as Finnrick reported them.
Hold these rows to the questions above. They show a pass and a purity figure but no batch number, so they aren’t matched to the vial you receive, and each result covers only the sample tested. They show no endotoxin, heavy-metal or sterility result. For identity, a chromatogram and the method, open the Finnrick certificate linked from each row and see what it reports. And the page lists passed tests only; Finnrick publishes the full history for each product, which can include other results. For research use only. Not for human or veterinary use.
