A certificate of analysis is only as trustworthy as the peptide testing lab that produced it. Before you accept any purity figure, it helps to know what a competent laboratory should be doing behind the scenes: which accreditation it holds, which analytical methods it runs, how it handles your sample, and how transparently it reports the result. This guide turns that into concrete questions to ask a peptide testing lab, and the red flags that should give you pause.
Start with accreditation
The single most useful signal is formal accreditation. ISO/IEC 17025, whose full title is "General requirements for the competence of testing and calibration laboratories," is the primary standard against which testing and calibration laboratories are accredited. It specifies requirements for competence, impartiality, and consistent operation. The current version is the 2017 edition (earlier editions were published in 1999 and 2005), and the standard is published jointly by ISO and IEC (ISO/IEC 17025:2017).
Accreditation is not self-declared. It is granted by a third-party accreditation body that formally attests to a laboratory's technical competence and quality-system adherence within a defined scope. That last phrase matters: a lab can be accredited for some tests and not others, so the relevant question is whether peptide purity testing sits inside its accredited scope. In many cases suppliers and regulatory authorities will not accept results from a laboratory that is not accredited at all.
A concrete example shows the ceiling this sets. Janoshik Analytical, an independent testing laboratory based in Prague, Czech Republic (the entity Janoshik s.r.o. was registered in Prague in October 2022), is not ISO/IEC 17025 accredited for peptide purity testing (Janoshik Analytical). Its data can still orient you, but by definition it is not intended for, and would not be accepted in, formal regulatory submissions. Know which category of result you are getting.
The methods that matter: HPLC and LC-MS
Reversed-phase HPLC for purity and content
Reversed-phase HPLC (RP-HPLC) is the workhorse. It is used to determine a peptide lot's homogeneity, stability, identity, content, and purity. A purity assay is typically run on a C18 reversed-phase column with an acetonitrile-water gradient and trifluoroacetic acid (TFA) as the acidic modifier, with UV detection at 215 nm. That wavelength is optimum for detecting the peptide bond, so it detects all peptide species present rather than only the target (peptide analysis method notes, AltaBioscience). One nuance to note when reading a number: the reported HPLC purity value excludes water and trifluoroacetate salt in the dried material.
LC-MS/MS for identity
Purity alone does not tell you whether the sample is the right molecule. Electrospray LC-MS/MS is used to confirm the mass of the peptide and to verify its amino acid sequence. Ask a peptide testing lab whether identity confirmation is included in the standard panel or only run on request or on flagged samples. In the Janoshik example, HPLC with UV detection is the primary method, with LC-MS/MS used for identity confirmation on request or on flagged samples. A purity figure with no identity confirmation answers "how clean," not "what is it."
What a purity number actually means
Rigorous purity and content assignment is more involved than a single injection. The reference-standard approach uses a two-step mass-balance method: bulk material is first characterized for all impurities (peptide-related impurities, counter ion, water, residual solvents, and non-combustible residues), and the purified bulk then serves as the standard for HPLC quantification of the vialed material (reference-standard characterization, NIH PMC). The practical takeaway is that a headline "99 percent" from one chromatogram is not the same claim as a content value anchored to a full impurity accounting. Ask what the number is measured against.
What a clean COA does not cover
This is where many readers over-interpret a certificate. A standard HPLC-purity-plus-MS-identity panel does not cover endotoxin, sterility, heavy metals, residual solvents, or bioburden. A clean COA confirms the sample is the correct molecule at roughly the stated strength; it does not establish injection safety.
Endotoxin is a separate, specific test. USP <85> Bacterial Endotoxins Test (the LAL test) detects and quantifies bacterial endotoxins in parenteral products using lysate from the amebocytes of the horseshoe crab (Limulus polyphemus), performed by one of three methodologies: gel-clot, turbidimetric, or chromogenic (USP General Chapter <85>). Under USP <85>, the endotoxin limit for most injectable drugs is 5 EU/kg of body weight per hour, with a stricter limit of 0.2 EU/kg for intrathecal products. If those attributes matter to you, they are line items a purity panel will not include, and you should not read them into a clean result.
Turnaround, sample handling, and report transparency
Three operational questions round out the decision.
Turnaround: ask for the expected timeframe up front, and be wary of promises that seem implausibly fast for the methods involved.
Sample handling: understand how the sample reaches the lab and how it is stored. A result only ever speaks to the specific sample that was tested, so who controlled that sample changes what the certificate actually proves.
Report transparency: a strong report states its method, shows the underlying chromatogram, and lets you verify the document independently. Some labs issue a unique verification reference on each certificate that can be checked on a public portal (Janoshik, for instance, publishes certificates with a unique reference checkable at public.janoshik.com). A certificate that shows only a bare number, with no method, wavelength, or chromatogram, is thin by comparison.
Red flags to watch for
- A vague "ISO certified" claim with no accreditation body, scope, or standard number. Accreditation to ISO/IEC 17025 is always scoped.
- Purity claims presented without a chromatogram, stated method, or detection wavelength.
- No way to independently verify the certificate: no reference number and no portal.
- Identity confirmation (mass spectrometry) not offered at all, only a purity percentage.
- Safety language such as "safe to inject" attached to a panel that only measured purity and identity.
- A round purity number with no impurity accounting behind it.
The regulatory backdrop
Even if a routine panel is narrow, it is worth knowing what full characterization looks like. The FDA guidance "ANDAs for Certain Highly Purified Synthetic Peptide Drug Products That Refer to Listed Drugs of rDNA Origin" (finalized May 2021 and published in the Federal Register on May 20, 2021) addresses characterization of product- and process-related impurities in synthetic peptides (FDA guidance). More broadly, regulatory guidance treats purity as a critical quality attribute and relies on chromatographic and mass-spectrometric methods, including HPLC, LC-MS, headspace GC-MS for residual solvents (per ICH Q3C), and ICP-MS for heavy metals (per ICH Q3D), to characterize impurities. The regulatory landscape around therapeutic goods is covered at the Coalition for Better Health.
The bottom line
The right peptide testing lab is the one whose scope, methods, and reporting you can actually inspect. Accreditation status sets the ceiling on what a result can be used for, the methods (RP-HPLC for purity and content, LC-MS/MS for identity) define what the number means, and transparent reporting is what lets you verify the claim rather than take it on faith. Anything about how the sample was analyzed, or how you can check it, should be answerable. When it is not, treat that silence as the most important red flag of all.