"Third-party tested" is not one test. It is a label that can mean a single HPLC purity figure or a panel of seven separate assays run against seven separate standards. Purity, identity, sterility, endotoxin, elemental impurities, residual solvents and water content answer entirely different questions, and no one of them implies any of the others. A typical research certificate carries the first two, sometimes with water and net peptide content added.

Purity and identity: what most certificates actually contain

Purity by reversed-phase HPLC is area percent: the share of the peptide material that eluted as the target peak. It compares peptide against peptide. At the wavelengths used it barely sees salt, water or counter-ion, so a high figure and a mostly-non-peptide vial are perfectly compatible, which is the whole subject of purity is not potency and of net peptide content.

Identity by mass spectrometry answers a different question: whether the molecule is the one named on the label. What a ±1 Da match proves covers what identity confirms and where it stops.

Each test below is a separate assay, usually on a separate instrument and often at a separate laboratory.

Sterility is not implied by purity

Sterility is a microbiological lot-release test. In the United States the compendial procedure is USP general chapter <71>, Sterility Tests, portions of which are harmonised with the European and Japanese pharmacopoeias. FDA's aseptic processing guidance names it directly: "USP <71> 'Sterility Tests' is the principal source used for sterility testing methods, including information on test procedures and media."

Two things about it are routinely misread. First, it is a test on samples drawn from a lot, not a property of the lot, and the same guidance is blunt about the consequence:

Because of the limited sensitivity of the test, any positive result is considered a serious CGMP issue that should be thoroughly investigated.FDA, Guidance for Industry: Sterile Drug Products Produced by Aseptic Processing

A pass is evidence, not proof. Second, culture-based sterility work takes time; incubation is measured in days, and no laboratory returns a same-week sterility figure.

A lyophilized research vial is normally not sold as a sterile article, and a purity certificate says nothing about its microbiological state. Bioburden, which counts viable organisms rather than certifying their absence, is a different and cheaper assay again.

Endotoxin is a third question on top of both, measured under USP <85>, and it is covered separately in what EU/mg means.

Elemental impurities: ICP-MS, not "heavy metals"

The trade still says heavy metals. The modern framework is elemental impurities, governed by ICH Q3D(R2), adopted 26 April 2022, and implemented in the US compendia as USP <232> for limits and USP <233> for procedures.

Q3D sorts elements by toxicity and by the likelihood of their turning up:

Q3D's limits are permitted daily exposures that differ by route of administration. Turning one into a concentration limit needs an assumed daily intake, which a research reagent does not have, so a research certificate can report a measured concentration but has no limit to pass or fail it against.

USP <233> sets out two instrumental approaches, one suited to elements detectable by inductively coupled plasma atomic or optical emission spectroscopy and one by inductively coupled plasma mass spectrometry, with validation required for alternatives. ICP-MS is what a peptide sample is usually run on.

Residual solvents: ICH Q3C and why acetonitrile is the one to ask about

Residual solvents are what is left of the chemistry. ICH Q3C(R8), step 4 version dated 22 April 2021, places solvents in three classes:

Class 1 solvents: Solvents to be avoided. Known human carcinogens, strongly suspected human carcinogens, and environmental hazards. Class 2 solvents: Solvents to be limited. Non-genotoxic animal carcinogens or possible causative agents of other irreversible toxicity such as neurotoxicity or teratogenicity. Solvents suspected of other significant but reversible toxicities. Class 3 solvents: Solvents with low toxic potential. Solvents with low toxic potential to man; no health-based exposure limit is needed.ICH Q3C(R8), Impurities: Guideline for Residual Solvents, 22 April 2021

The class 2 table is where peptide chemistry lands. Acetonitrile, the workhorse of preparative RP-HPLC, carries an Option 1 concentration limit of 410 ppm. N,N-dimethylformamide, a standard coupling solvent, is 880 ppm. Dichloromethane is 600 ppm; methanol is 3000 ppm. Class 3 solvents, which include ethanol, acetone, ethyl acetate, heptane and DMSO, are acceptable at 5000 ppm or 0.5 per cent under Option 1, without further justification.

One entry is worth knowing about because it shapes what a certificate can say. Trifluoroacetic acid appears in Q3C's final appendix, among solvents "for which no adequate toxicological data was found". There is no ICH limit for it. That is one reason TFA turns up on peptide certificates as a counter-ion figure or a net peptide content figure rather than as a residual solvent line.

Residual solvents are determined by gas chromatography, usually headspace GC, and the guideline notes that where only class 3 solvents are present a non-specific method such as loss on drying may be used.

Water content, and why it belongs on the page

Water is measured by the Karl Fischer method, in coulometric form for the small water contents typical of a freeze-dried solid. It matters for two reasons that have nothing to do with contamination. Water is part of the mass on the balance, so it sits between the label weight and the peptide weight alongside the counter-ion. And residual moisture in a lyophilized cake governs how the solid behaves over its shelf life, which is why stability programmes track it, and which storage and stability picks up.

What each assay costs, roughly

Public price pages from testing laboratories give the order of magnitude, as of September 21, 2026.

Those figures are from one ISO/IEC 17025 accredited laboratory in San Diego that publishes its price list. A Florida laboratory advertises $150 to $1,500 a sample depending on panel scope, with a nine to eleven business day turnaround. The spread explains the market: an identity-and-purity certificate is an affordable per-lot commitment, and a full panel is a multiple of it.

Reading the phrase "third-party tested"

Ask three questions of it. Which assays. Against which standard, named by chapter or guideline. And by whom, with the report number and the scope of that laboratory's accreditation, which how to verify a peptide lab report explains; how to vet a peptide vendor puts the same questions to the seller.

A certificate that names USP <71>, USP <85>, USP <232>/<233> or ICH Q3C has told you exactly what was measured. A page that says "third-party tested" and stops has told you that a third party was involved.

Sources

• ICH Q3D(R2), Guideline for Elemental Impurities, International Council for Harmonisation, 26 April 2022
• ICH Q3C(R8), Impurities: Guideline for Residual Solvents, International Council for Harmonisation, 22 April 2021
• Guidance for Industry: Sterile Drug Products Produced by Aseptic Processing, U.S. Food and Drug Administration, September 2004
• USP General Chapter <71> Sterility Tests, United States Pharmacopeia
• USP General Chapter <233> Elemental Impurities—Procedures, United States Pharmacopeia
• Pyrogen and Endotoxins Testing: Questions and Answers (Edition 2), U.S. Food and Drug Administration, March 2026
• Peptide testing pricing, Prodigy Labs, San Diego
• Peptide testing services, ACS Laboratory
• Matejtschuk P et al., Impact of formulation choices on the freeze-drying of an interleukin-6 reference material, Frontiers in Molecular Biosciences, 2022