What Does 99% Purity Actually Mean?
Published August 27, 2026 · Artemis Labs
Research Buyer FAQ › Peptide Purity, Testing & Stability › Purity Percentage Explained
A purity figure of 99% means that in one HPLC run, 99% of the total peak area came from a single peak. It is an area measurement from one method on one sample — not a weight, not a completeness claim, and not comparable between labs unless the method is named.
How this page was built. The definitions below describe what the standard analytical methods measure, for material from any vendor. Published findings under References are transcribed from the papers themselves.
What exactly does the percentage measure?
It measures the share of detected peak area belonging to the main peak in a chromatography run. HPLC — high-performance liquid chromatography — pushes a dissolved sample through a packed column under pressure. Different molecules travel at different speeds, so each comes off the column at its own time, and a detector draws the result as a graph of peaks. The tall peak is the target peptide; the small peaks are everything else the method could see. Purity is the area under the main peak divided by the area under all peaks, times one hundred.
Read that definition again with a buyer’s eye and two limits fall out. It is an area figure, not a weight figure — it describes the peaks the detector registered, not the full contents of the vial. And it is a one-method figure: the number is a property of the sample and the method together, which is why a percentage printed with no method beside it is half a number.
What is in the other 1%?
Mostly close chemical relatives of the peptide itself, created during synthesis. Solid-phase synthesis builds the chain one amino acid at a time, and a small fraction of chains come out wrong: truncated sequences that stopped early, deletion sequences missing one residue, and side products from protecting groups — the temporary chemical caps used during assembly — that were not fully removed. Oxidised variants appear too, especially in sequences containing methionine or cysteine, two amino acids that react readily with oxygen. These impurities matter to a careful buyer precisely because they are near-copies: they behave almost like the target molecule, which is what makes them hard to separate and hard to see.
There is also a second, quieter category: impurities the method never resolved. Anything that comes off the column at nearly the same time as the target gets counted inside the main peak — counted, in other words, as purity. Yoshida and colleagues showed in 2025 that two different column chemistries run against the same synthetic peptides separate different impurities, so a single method can leave some hidden under the main peak (PMID 39922152). The other 1% is therefore a floor, not a ceiling: it is the impurity the method saw, plus whatever it did not.
Is 98% purity good enough?
For most laboratory work, yes — and the honest answer depends on the work, not on the number. The distance between 98% and 99% is one part in a hundred; whether that part matters depends on what it is, which is a question about the impurity profile, not the headline figure. A truncated sequence at 1% means something different from an oxidised variant at 1%, and neither is visible in the percentage alone.
The more useful comparison is between documents, not decimals. A vendor printing 98% next to a named method, a lot number, a test date, and the chromatogram itself has given you a checkable measurement. A vendor printing 99% with none of those has given you a marketing claim wearing a decimal point. The first number can be audited; the second cannot, and an unauditable number does not become more true by being higher.
Can the same material produce different purity numbers?
Yes, honestly and routinely. Two labs running different columns, gradients, or detection wavelengths on the same batch can report different percentages, both correctly — each number is true of its own method. That is not a scandal; it is what an area-based, method-dependent measurement is. The practical consequence for a buyer is that purity figures are only comparable when the methods are, so a bare percentage from an unnamed lab cannot be lined up against a documented one from a named lab. The number is only as good as its provenance.
Why does purity vary so much between suppliers?
Because purification costs money and honest measurement is optional. Crude synthetic peptide does not come off the synthesizer at 99% anything; it gets there through purification passes, and every pass costs yield and time. A supplier selling far below the market price has usually bought fewer of those passes — or skipped the measurement that would reveal it.
The measured extremes are wider than most buyers imagine. In a 2024 test-purchase study, researchers bought vials of the peptide drug semaglutide from unlicensed online sellers and measured them: the labels claimed 99%, and the measured purity ranged from 7.7% to 14.37% (PMID 39509151). Those sellers printed the same “99%” a legitimate supplier prints. The difference was never in the claim — it was in whether a document with a named lab, a matching lot number, and a real trace stood behind it.
What questions does a purity number not answer?
The biggest one is weight. Purity says: of the material the method detected, how much is the target sequence? It does not say how much of the powder in the vial is peptide at all — freeze-dried peptide powder also contains counterion salt and bound water, which the purity method was never measuring. That second question has its own name, its own tests, and its own line on a good Certificate of Analysis: net peptide content versus label mg. A vial can show 99% on the purity line while a fifth or more of the powder’s weight is not peptide, with both figures honest at once. A buyer who knows both numbers exist reads a COA differently — and notices when a vendor reports only the flattering one.
References
- Yoshida K, et al. “Impurity profiling of synthetic cyclic peptides based on orthogonality between hydrophilic-interaction and reversed-phase liquid chromatography.” J Chromatogr A. 2025;1745:465748. PMID 39922152 · doi:10.1016/j.chroma.2025.465748
- Ashraf AR, Mackey TK, Vida RG, et al. “Multifactor Quality and Safety Analysis of Semaglutide Products Sold by Online Sellers Without a Prescription: Market Surveillance, Content Analysis, and Product Purchase Evaluation Study.” J Med Internet Res. 2024;26:e65440. PMID 39509151 · doi:10.2196/65440
All compounds sold by Artemis Labs are for laboratory research use only. Nothing on this page is medical advice, and no statement has been evaluated by the FDA.