How We Verify Peptide Purity: HPLC, Mass Spec, and ISO 17025

The companion to this article — What ≥99% Purity Actually Means in Peptide Research — explains what an HPLC purity number is and is not. This article is the operating manual behind that picture: what PepMax actually requires before any lot is released, which analytical methods are used, what the acceptance thresholds are, and how a failed test is handled.

Nothing here is novel chemistry. The methods are USP-referenced and standard for an accredited peptide-testing lab. The contribution is operational rather than scientific — documenting which methods are used, in what order, against what thresholds, and what gets published on the COA versus paraphrased into a marketing summary. A researcher evaluating any vendor — ours or anyone else’s — should be able to read this kind of document and check it against the COA they receive.

Overview of our QC pipeline

Synthesized lots arrive from the contract manufacturing facility under quarantine and are not released for sale until every line of the release file is signed. The pipeline runs in a fixed order, each stage gated on the prior one passing. Re-running an earlier stage to chase a different result is not part of the workflow.

Stages 2 through 8 are performed by an ISO 17025-accredited third-party laboratory. Stage 9 — reviewing the file and authorizing release — is performed in-house against a fixed checklist. Once released, the COA is archived to the public COA library, indexed by lot number, and travels with every vial of that lot for the remainder of its shelf life.

RP-HPLC for purity

The RP-HPLC method is a reverse-phase gradient assay run under USP <621> system suitability conditions^[1]. The parameters below are the default; some compounds (highly hydrophobic peptides, cyclic peptides) require a modified gradient or column chemistry, in which case the modification is noted on that lot’s COA.

The headline number reported on the COA is integrated peak area at 220 nm. The 210 nm trace is recorded for confirmation; if the two wavelengths disagree by more than 0.3 percentage points the chromatogram is reviewed for integration error before the result is committed. Reference-standard system suitability is run at the start of each analytical batch and again every twelve injections; a failed system suitability check invalidates every result downstream of it, regardless of how the test article looked.

The peak-area ratio is the simplest possible math, but it depends on a non-trivial chain of upstream decisions: the column, the gradient slope, the wavelength, the injection load. The companion article covers the “why” of that chain in detail. For our purposes here, what matters is that all four are fixed by the SOP, validated against a reference standard, and printed on every lot-specific COA.

Mass spectrometry for identity

Purity tells us how much of the lot is one compound. It does not tell us which compound. Identity is established separately by liquid chromatography-mass spectrometry — specifically electrospray ionization quadrupole time-of-flight (ESI-Q-TOF) at high resolution. ESI is the standard ionization method for intact peptides because it produces multiply-charged ions in a charge-state envelope that confirms the analyte is the intact molecule rather than a fragment^[2].

The instrument operates at a resolving power of approximately 30,000 at m/z 400, sufficient to resolve the isotope envelope of peptides up to roughly 5 kDa. From the multiply-charged envelope the software deconvolutes a single intact mass, which is then compared to the theoretical monoisotopic mass calculated from the peptide sequence^[3].

For peptides above ~5 kDa or with sequence ambiguities (multiple disulfide arrangements, cyclic forms), MALDI-TOF is run as a secondary confirmation in addition to ESI. Both spectra are attached to the COA. We do not paraphrase the result — the deconvoluted mass and the theoretical mass are both printed, with the ppm error calculated, so that the researcher can audit the identity claim directly rather than trust a checkmark.

Orthogonal contaminant screening

HPLC purity at 220 nm is blind to anything that does not absorb UV at that wavelength. The orthogonal panel exists to cover those blind spots. Each test answers a question that HPLC physically cannot.

Counter-ion content is reported numerically rather than against a pass/fail threshold because it materially affects mass-based dosing in research work. A peptide synthesized with TFA-modified mobile phases routinely carries 8–15% trifluoroacetate by mass; a researcher weighing out 1 mg of lyophilized powder is therefore dispensing roughly 0.85–0.92 mg of the parent peptide, not 1.0 mg. Reporting the number is more useful than asserting compliance with an arbitrary cutoff.

Endotoxin and bioburden testing are research-tier limits, not pharmacopeial product limits. Research peptides are not licensed for human or veterinary use, and the limits we apply are appropriate for in-vitro and analytical work, not parenteral administration.

Why we require ISO 17025 accreditation

Every test described above is performed at a laboratory accredited to ISO/IEC 17025:2017^[8]— the international standard for the competence of testing and calibration laboratories. Accreditation is the floor. Without it the COA is a document the vendor produced about itself.

What 17025 actually changes about a result:

• Method validation files exist.Specificity, linearity, accuracy, precision, and limits of detection / quantitation are documented for every method on the accredited scope — not assumed.
• Calibrated equipment with traceability. Balances, pipettes, pH meters, and HPLC detectors are calibrated against standards traceable to a national metrology institute. The chain of custody for that traceability is auditable.
• Proficiency testing. The lab participates in inter-laboratory comparisons that test whether the same sample produces the same answer when run by someone else. Drift is detected externally, not just internally.
• Document-of-record COAs.The signed COA is the lab’s official finding, not the vendor’s summary of it. A vendor that disagrees with the result cannot quietly edit the document; the underlying analysis exists outside their control.

We do not accept in-house COAs from the synthesis facility as evidence of release. The synthesis facility produces the lot; an independent, accredited lab tests it. These are different organizations with different incentives. Combining them into a single document produced by a single party is a structural conflict of interest that no amount of internal rigor can fully neutralize.

Lot release criteria

The release file pulls every test result from the pipeline above and checks each against a fixed threshold. The thresholds are below. They do not move based on commercial pressure or timeline. A lot either passes the line or it does not.

A held lot is reviewed with the synthesis facility for root cause and one of two things happens: the underlying issue is identified and remediated, in which case a new lot is produced and re-tested from the top of the pipeline, or the lot is rejected and destroyed. We do not retest a held lot against a more lenient method until the result becomes favorable. Re-testing for confirmation under the same method is part of normal QC; testing for “a better number” is not.

The COA you receive is the document the accredited lab signed, archived on the public COA library and indexed by the lot number printed on your vial. Every parameter described above is on it. If something we have committed to is not on the COA, that is a defect in our reporting and we want to know about it — not a permission slip to do less.