Purity testing measures how much of a vial is the named compound. See how HPLC area percent works, why mass spec identity is separate, and how to read a COA.
Purity testing is the analytical measurement of how much of a material in a vial is the compound named on the label, and how much is something else. For a reference peptide or research chemical, the standard method is reversed-phase HPLC with UV detection, which separates the target from process-related impurities and reports each as a percentage of total peak area — a result written as "98.7% by HPLC." Identity is a separate question, answered by mass spectrometry confirming the molecular weight matches the theoretical mass within a few daltons. A certificate of analysis pairs both. Purity alone tells you the sample is one dominant substance; it does not tell you which substance. You need both numbers, tied to a specific lot.
Reversed-phase HPLC pushes a dissolved sample through a C18 column — usually 4.6 x 250 mm, 5 µm particles — under a gradient of water and acetonitrile, both with 0.1% trifluoroacetic acid. Compounds exit the column at different retention times based on hydrophobicity. A UV detector set at 214 nm (the peptide bond absorbance) or 280 nm (aromatic residues) records each as a peak.
The purity figure is area percent: the target peak's area divided by the total area of all integrated peaks, times 100. That definition carries three consequences most buyers never think about.
This is why a vial can be 99% pure by HPLC and still be roughly 70–80% target compound by mass. The remainder is water and salt. Peptides purified by preparative HPLC and lyophilized typically carry 4–8% residual water and 5–20% TFA counterion by mass, depending on how many basic residues the sequence contains. The corrected figure is called peptide content or net peptide, and it is determined by amino acid analysis or quantitative NMR — not by HPLC. Most suppliers in this market never report it.
Treat these as separate line items when you evaluate documentation.
A deletion sequence missing a single glycine is 57 Da lighter than the target. It may co-elute closely on a fast gradient and get integrated into the main peak, inflating purity. Mass spec catches it; HPLC alone may not. Conversely, mass spec confirms mass but says nothing about a 3% dimer or an oxidized methionine that HPLC would resolve cleanly. Neither method substitutes for the other, which is the core argument for demanding both on every certificate of analysis.
A COA without a lot number, an injection date, and a legible chromatogram is a marketing graphic, not an analytical record.
Work through this list before you accept a document.
Watch for the round-number tell. Genuine HPLC integration produces figures like 98.63% or 99.14%. A catalog where every product reports exactly "99%" or ">98%" is reporting a specification, not a measurement.
Real numbers, because they explain supplier behavior better than any claim about commitment to quality.
A 1,000-vial lot carrying a $900 full-panel test absorbs about $0.90 per vial. A 25-vial lot absorbs $36 per vial. That arithmetic is the whole story of why small-batch and boutique suppliers test less, and why wholesale pricing structures often come with more complete documentation than single-unit purchases — the fixed cost spreads.
This section costs us sales. Read it anyway.
Independent verification is often not economically rational at small scale. If you buy a single 5 mg vial for $40, sending it out for a $200 HPLC-MS panel means paying 5x the material cost to verify it. For screening work, exploratory assays, or method development where the result is directional rather than publishable, that spend is hard to justify. Order the material, run your experiment, and if the data looks strange, then test. Verifying every low-value purchase is a way to spend a research budget on chromatography instead of research.
A COA does not guarantee the vial in your hand. Testing is destructive and done on a sample. A 1,000-vial lot might have 3 vials tested. Fill-line variation, a stopper seated poorly, a vial that lost vacuum, or 40 hours on a loading dock in July — none of that appears on a document written weeks earlier. The COA describes a lot, and lots are not homogeneous in practice.
Purity is a snapshot with an expiration. Lyophilized peptides stored at -20°C are commonly assigned 24 months; at 2–8°C, closer to 6–12 months; at room temperature, weeks. Sequences containing methionine, cysteine, or tryptophan oxidize faster; Asn-Gly motifs deamidate. A material that tested at 99.2% eighteen months ago may not test at 99.2% today, and a supplier who reships from long-held inventory can be honest and still be shipping degraded material. Ask when the lot was manufactured, not just when it was tested.
Chasing the last percentage point is usually waste. For most binding assays, cell work, and analytical standards, the difference between 98% and 99.5% is invisible in the data — while the price gap can be 30–60%. The exceptions are real: structural work, quantitative reference standards, anything where a 1.5% impurity might be biologically active or spectroscopically interfering. If you cannot name the specific way an impurity would corrupt your readout, you are paying for a number on a page.
Do not buy on documentation alone if you cannot cold-chain it. If your receiving setup cannot get material into a -20°C freezer the day it arrives, or if packages sit unattended at a shared address, the analytical rigor upstream is largely wasted. Check the shipping terms and your own logistics before you optimize the COA.
Third-party testing can be gamed. A supplier can send a purpose-made sample to the lab rather than a lot sample. A lab report can be edited in a PDF editor in ten minutes. Verification means contacting the named laboratory with the report number — and a supplier who will not tell you which lab ran the test has answered the question.
Some buyers should not be in this market at all. These are research-use-only reference materials, and the research use only designation is a legal and practical boundary, not a formality. If you do not have a laboratory, a documented protocol, and institutional oversight, no COA makes the purchase appropriate. That is not a sales-funnel disclaimer — it is the condition under which any of this analytical documentation means anything.
A workable process for a lab buying regularly, without turning procurement into a full-time job.
Comparative writeups in the library walk through how specific vendors' documentation holds up against this checklist, including which ones publish per-lot chromatograms and which recycle a single PDF across an entire catalog.
The short version: purity testing is a measurement of relative peak area under a disclosed method, on a specific lot, at a specific date. It is genuinely informative and genuinely limited. Buy from suppliers who show the chromatogram, name the lab, and stamp the lot — then spend your verification budget where an impurity could actually change your result, and not everywhere else.
Supplied strictly for in-vitro laboratory research by qualified researchers and institutions. Not a drug, food, cosmetic or dietary supplement. Not for human or veterinary consumption. Not evaluated by the FDA.
Purity, identity and lot number, documented for the exact vial you receive.