What “99% pure” actually means.
Purity is the number every peptide vendor leads with and almost none explain. It's a real, useful measurement — once you know exactly what it counts, and what it quietly leaves out.
Where the number comes from.
Purity is measured by high-performance liquid chromatography. The peptide is dissolved and pumped through a column that separates molecules by how they interact with it; each species emerges at its own retention time and registers as a peak on a detector. The purity figure is the target peptide's peak area divided by the total area of all peaks — the fraction of detected material that is the intended molecule. When we publish BPC-157 at 99.2%, that is the fraction its batch chromatogram showed.
What the other peaks are matters as much as their size. Peptide synthesis builds the chain one amino acid at a time, and each step is fractionally imperfect — so the classic impurities are deletion sequences (chains missing a residue) and truncated chains (syntheses that stalled early). These near-miss molecules are chemically similar to the target, which is exactly why a high-resolution separation is needed to see them at all.
Purity is a fraction, not an amount.
The most common misreading: purity says nothing about how much peptide is in the vial. A 10 mg vial at 99% purity is not a promise of 9.9 mg of peptide — purity describes the composition of the peptide material, while the fill is a separate measurement entirely (the quantity assay on a proper certificate of analysis). A vendor can print a genuine 99% on a vial containing 6 mg of material. Both numbers, independently measured, are the only honest description of a vial.
Net peptide content: the part almost nobody mentions.
Even a perfectly pure lyophilized cake is not 100% peptide by weight. Peptides are freeze-dried with bound water molecules, and they carry counter-ions — most commonly trifluoroacetate (TFA), a residue of the purification chemistry that pairs with the peptide's positive charges. Together, water and salts routinely account for 10–30% of a cake's mass. Net peptide content is the weight fraction that is actually peptide, and it's why careful quantitative work starts from a content assay rather than the label mass.
TFA deserves its own sentence: beyond diluting the mass, residual trifluoroacetate is known to interfere with some sensitive cell-culture systems, which is why the literature on cell-based assays occasionally specifies TFA-removed material. For most receptor and model work it's a non-issue — but a lab should know it's there, and a vendor should never pretend the cake is 100% peptide by weight.
Why two 99%s can describe different vials.
- Different methods see differently. A short, low-resolution HPLC run merges neighbouring peaks into the main one and flatters the number; a longer gradient on a better column resolves them and tells the truth.
- Detection wavelength matters — impurities that absorb weakly at the chosen wavelength are undercounted.
- The claim may describe a different batch. Synthesis quality varies run to run; only a lot-matched, batch-specific report describes your vial.
- And sometimes the number is simply copied from a supplier's paperwork — the vendor never measured anything. This is depressingly standard practice.
This is why the credible unit of trust is not the number but the measurement: an independent laboratory, a batch-specific chromatogram, and a lot number that ties the report to your vial. Every Sentir batch is measured that way before sale — the printed purity is the measured purity of your batch, never a supplier's claim. The full protocol is on how we test.
What purity can't tell you.
Purity is a chromatographic statement about peptide species. It cannot see bacterial endotoxin, which is measured by LAL assay; it cannot confirm identity, which is mass spectrometry's job; and it cannot vouch for the fill, which is the quantity assay. A 99% figure standing alone answers one of the four questions a vial raises. The other three are why a full certificate of analysis exists.
Common questions.
For most preclinical and receptor work, the difference is marginal — what matters more is that the number is real, batch-specific, and accompanied by identity and endotoxin data. A genuine 98.5% with a chromatogram beats an unverifiable 99.9% every time.
Lyophilized peptide cakes include bound water and counter-ions such as trifluoroacetate from purification — typically 10–30% of the mass. That's chemistry, not shortchanging; the honest response is to measure and disclose actual content, which is what a quantity assay does.
Yes — slowly, and faster with heat, light, moisture and repeated freeze-thaw cycles. Degradation products are new impurities, so mishandling erodes the measured purity over time. Proper storage — see our [storage guide](/learn/peptide-storage-handling) — is how a batch's numbers stay true.
