Quality peptides come from three sources: custom CROs ($600–$4,000), reference-material houses, and catalog suppliers. Compare cost, lead time, and lot data.
Quality reference peptides come from three sources, and they are not interchangeable. Custom-synthesis CROs (the same shops that supply pharma discovery groups) will synthesize to a specification, run orthogonal analytics, and hand over a lot-specific data package — expect $600 to $4,000 for a 100 mg custom order at >95% purity, with 3 to 6 week lead times. Catalog reference-material houses stock characterized standards at $80 to $500 per milligram-scale vial. Research-use catalog suppliers sell recurring stock items at $25 to $120 per 5 mg vial with same-week shipping. The correct source is whichever one will show you the raw chromatogram and mass spectrum for the exact lot number printed on the vial you receive. Everything below is about verifying that.
A vial of lyophilized powder is indistinguishable by eye from any other vial of lyophilized powder. The material you are actually purchasing is the documentation, and a supplier's willingness to attach a lot-specific analytical package is the single strongest quality signal available before the material arrives.
There is a meaningful difference between three documents that all get called "a COA":
Ask which one is attached before ordering, not after. Suppliers that publish their testing standards openly — see our quality page for the specific methods and acceptance criteria we hold lots to — are making a falsifiable claim. Suppliers that only produce a COA on request, after the sale, are usually generating it on request too.
A purity figure means nothing without the method that produced it. Three details change the interpretation entirely.
Detection wavelength. Most peptide RP-HPLC runs detect at 214 nm, which sees the peptide backbone amide bond and therefore responds to essentially every peptide-related impurity. A run at 280 nm only sees tryptophan, tyrosine, and phenylalanine — a peptide with none of those residues is nearly invisible, and a 280 nm trace can report 99% purity on material that shows 91% at 214 nm. If the wavelength is not stated, the number is unusable.
Gradient and run time. A 5-minute isocratic flush will co-elute deletion sequences that a 30-minute linear gradient from 5% to 65% acetonitrile in 0.1% TFA would resolve on a C18 column. Short methods flatter the sample. Look for a run of at least 20 minutes and a main peak that is not eluting in the void volume.
Area percent is relative, not absolute. 98.5% area purity means 98.5% of the UV-absorbing material that came off the column, not 98.5% of what is in the vial. Salts, water, and residual scavengers do not absorb at 214 nm and never appear in that integration.
The gap between "98% pure" and "980 µg of peptide per 1 mg of powder" routinely runs 15 to 30 percentage points, and it is not a defect. It is a units problem that most product pages never mention.
Solid-phase synthesis with Fmoc chemistry and TFA cleavage leaves the product as a trifluoroacetate salt. For a peptide with several basic residues, TFA counterion mass commonly runs 5% to 20% of gross weight. Lyophilized powder is also hygroscopic; residual and adsorbed water typically measures 3% to 10% by Karl Fischer titration. Add both and the net peptide content of a genuinely high-purity lot often lands between 70% and 85% of the labeled mass.
That is normal, well-documented chemistry. What matters is whether it is disclosed. A supplier reporting 99% HPLC purity and 78% net peptide content by amino acid analysis is telling you the truth twice. A supplier reporting only 99% and staying silent on content is letting you assume the two are the same number, and across a comparative study that assumption produces a systematic mass error you will not detect from the data.
Amino acid analysis to determine net content costs roughly $150 to $350 per sample at a contract lab. Few catalog suppliers run it on every lot. Asking whether it was run at all is a fast way to sort serious operations from repackagers.
This section will cost some orders. It should.
Do not buy from a catalog supplier if your work needs a qualified reference standard. If you are running method validation, generating data for a regulatory filing, or need a certified primary standard with traceability to a pharmacopeial monograph, a research catalog vial is the wrong material at any price. Buy the USP or EP standard, pay the $300 to $900, and stop reading comparison articles.
Do not buy if you cannot analytically verify what arrives. If there is no access to an HPLC or a mass spectrometer and no budget for third-party testing, the purchase rests entirely on the supplier's paperwork. That is a trust position, not a verification position. It can be a reasonable one — but it should be entered deliberately, and the cost of a confirmatory outside test ($120 to $400) should be compared honestly against the price of the vial. On a $45 vial, verification costs 3x the material.
Do not chase the cheapest listing. Price compression below roughly $18 to $25 per 5 mg for common sequences generally means one of three things: repackaged bulk with no per-lot testing, a COA inherited from a different lot, or net peptide content well under 70%. Sometimes it means an honest supplier with low overhead. The three look identical on a product page, which is the problem.
Do not assume purity is stable over storage. Purity is a measurement at a moment. A lot assayed at 98.7% eight months ago, shipped ambient in summer and held in a frost-free freezer that cycles above -15°C, is no longer that lot. Ask for the assay date, not just the assay.
Failure modes worth naming plainly: COA date preceding the manufacture date; a chromatogram with no axis labels or scale; identical trace images reused across multiple products; mass spec reporting only the expected monoisotopic mass with no observed spectrum; "purity ≥99%" as a catalog-wide default across 60 SKUs, which no synthesis operation achieves uniformly; and no stated return path for out-of-spec material — check whether a supplier has a real returns policy for analytical failures before ordering.
Working benchmarks for milligram-scale research quantities, single-vial pricing: common short sequences at $25 to $60 per 5 mg; longer or harder syntheses at $90 to $250 per 5 mg; custom sequences at $3 to $12 per milligram at 100 mg scale. Volume pricing typically starts around 10 to 25 units and cuts 15% to 40% — our wholesale terms list the actual break points rather than "contact us."
Budget separately for verification. A reasonable QC allocation on a research program buying $3,000 to $10,000 of material annually is 5% to 10% spent on independent confirmation of incoming lots — roughly two to four third-party assays a year, run on a rotating basis across suppliers. That spend does more for data integrity than upgrading from a 98% claim to a 99% claim ever will.
For side-by-side supplier comparisons and how these criteria play out against specific vendors, the supplier evaluation library and our high purity peptides review apply the same checklist to named operations.
The short version: a supplier is worth buying from when it publishes lot-specific analytical data you could disprove, states net peptide content alongside area purity, and names its storage and retention policies without being asked. Documentation you can audit beats a number you can only believe.
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.