Purity is an HPLC measurement, not a kitchen test. Learn what you can actually check at home — cake, seal, net weight — and how third-party assays work.
Short answer: you can't meaningfully test peptide purity at home. Purity is a chromatographic measurement, and the equipment that produces a defensible number — a reversed-phase HPLC with a UV detector, or an LC-MS — starts around $25,000 new and requires a C18 column ($450–$900, good for roughly 500–1,500 injections), HPLC-grade acetonitrile at $90–$200 per 4 L, and a validated method. What a non-instrumented setting can do is limited to identity and integrity checks: visual inspection of the lyophilized cake, seal and crimp integrity, net content by differential weighing on a 0.01 mg balance, and cross-checking a certificate of analysis against an independent third-party assay costing $80–$250 per sample. Everything else — TLC kits, color reagents, "cloudiness" heuristics — resolves nothing below about 5% impurity.
A bench without instruments still produces useful information, provided nobody mistakes it for a purity figure.
Thin-layer chromatography gets recommended constantly on forums. A TLC setup costs about $200 in plates, chambers, and solvent, and its practical limit of detection for a co-eluting related substance is roughly 1–5%. A deletion sequence missing a single glycine residue differs from the target by 57 Da and will not separate on a TLC plate. If your acceptance criterion is 98%, TLC cannot see the 2% you care about.
Researchers occasionally price an in-house analytical capability. The honest arithmetic:
A lab already running LC for other reasons absorbs this easily. A lab building it solely to verify purchased reference material spends $30,000+ and a quarter of a year to replicate a $150 assay.
This is the point most purity discussions never reach, and it is the one that changes purchasing decisions.
HPLC purity is reported as area percent at 214 nm, the wavelength where the peptide bond absorbs. A 99.1% figure means 99.1% of the UV-absorbing material eluting in that run was the target species. It says nothing about material that doesn't absorb at 214 nm.
Lyophilized peptides synthesized by Fmoc SPPS and purified by prep RP-HPLC carry counterion and solvent load:
A vial can be simultaneously 99% pure by HPLC and 80–85% peptide by mass. Both numbers are true. Only one of them tells you how much target compound is in the vial.
A certificate reporting 99% purity without a net peptide content figure is describing the shape of a chromatogram, not the contents of a vial.
Net peptide content is determined by amino acid analysis ($150–$300) or quantitative NMR, and counterion by ion chromatography or elemental analysis ($75–$150). Most suppliers, including reputable ones, don't run these by default. Asking for them is a fast way to separate a company that manufactures to a specification from one that resells and relabels. Our own approach to what gets tested and reported is documented on the quality page.
Sending a sample out is the correct move, and the pricing is not prohibitive:
Send 1–5 mg and specify the lot number on the submission form so the result is traceable back to a batch rather than to an anonymous sample. Ask the lab to return the raw chromatogram and the mass spectrum, not just a summary line. A PDF that shows only "Purity: 99.2%" with no baseline, no retention time, no gradient conditions, and no injection volume is unverifiable and worth roughly what it costs to fabricate — which is nothing.
Plainly, and against our commercial interest:
Anyone whose actual question is "did I receive what I paid for" is usually better served by supplier due diligence — documented lots, published methods, a stated return policy — than by building an analytical capability. Comparative write-ups in our library, including the high purity peptides review, cover what that documentation looks like across vendors.
Purity degrades after receipt, and this is where a non-instrumented lab has real leverage.
Standard ground transit runs 2–5 business days, and a short unrefrigerated excursion is generally tolerated by a sealed lyophilizate — the details of how material leaves our facility are on the shipping page. Storage conditions after delivery are where most real-world purity loss happens, and they cost nothing to get right.
1. Request the lot-specific certificate of analysis before ordering, with the chromatogram and mass spectrum attached.
2. Confirm the CoA lists gradient conditions, column dimensions, detection wavelength, injection volume, and analysis date. Missing method parameters are the tell.
3. Check whether net peptide content and water content are reported. If not, ask. The answer is informative either way.
4. On a new vendor, send one vial from the first lot for independent HPLC-MS at $200–$400. Compare against the supplied CoA, not against expectation.
5. Verify the lot number is consistent across vial, CoA, and invoice.
6. Repeat the independent check on a spot-check basis — one lot in five is a defensible cadence for most programs.
All material discussed here is sold for laboratory research use only, under the terms described on our research use only page. Questions about methods, lot documentation, or retained samples for a specific batch go through contact.
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.