Buying and sourcing

What a certificate of analysis does not test for

This page is about the shape of a document, not about any product. It names no threshold as acceptable, because the relationship between any figure on a certificate and any outcome in a person has not been established in this market. It recommends no laboratory and no seller.

The default certificate is two lines long

Most certificates circulating in this market report a chromatographic purity percentage and a mass consistent with an expected molecular weight. Two attributes, two lines, one page. Whatever else a reader takes from the document is supplied by the reader.

The rule for reading it is one sentence: a test that does not appear was not performed, or was performed and not reported, and from outside the two are indistinguishable. What follows is the list of what commonly does not appear.¹

Sterility

Sterility is an attribute of a preparation and a process, established by a defined test and — in manufacturing — by a validated aseptic process with environmental monitoring behind it. FDA's guidance on sterile drug products produced by aseptic processing describes the apparatus involved: facility design, air classification, personnel practice, media fills, process simulation.³

Nothing about a purity chromatogram touches any of that. A material can be chromatographically pure and not sterile, and the two questions are not merely different — they are answered by different disciplines.³

Bacterial endotoxin

Endotoxin is the second question people collapse into sterility, and it is a third thing again. Endotoxins are heat-stable bacterial cell-wall components; a preparation can be sterile — containing no viable organisms — and still carry endotoxin left behind by organisms that are no longer alive. FDA maintains a dedicated questions-and-answers document on pyrogen and endotoxins testing precisely because it is a distinct assay with distinct pitfalls.²

Counterion and water: the mass you did not buy

This is the omission with the largest practical consequence, and it is the least discussed. Synthetic peptides are commonly isolated as salts, and the counterion is part of the powder. Residual water is also part of the powder. Neither is peptide.⁴

A chromatographic purity figure does not address this at all, because it reports the main peak as a fraction of detected peak area — and counterion and water do not produce peaks. A vial can be truthfully described as highly pure and still contain a mass whose peptide fraction is unknown, unless water content and counterion content are separately reported. Where a certificate omits both, no statement about how much peptide is present can be derived from it.⁴

Residual solvents and elemental impurities

Solid-phase peptide synthesis and the purification that follows use organic solvents, coupling reagents and scavengers, and the process runs on equipment. Residues of both classes — organic solvents from the process, elemental impurities from reagents and hardware — are recognised impurity categories with their own analytical methods, and neither is visible on a UV chromatogram run to measure purity.⁴

The impurities that do show up, and are not separated

Even inside the purity number, one figure conceals several distinct populations. The peptide-impurity literature describes related substances in peptide preparations as including deletion and insertion sequences, truncated and elongated chains, diastereomers, oxidation and deamidation products, residues of incompletely removed protecting groups, and aggregates. These behave differently from one another and a single percentage does not distinguish them.⁴

Regulators treat that granularity as consequential. FDA has published material on assessing peptide impurities specifically to inform immunogenicity risk, and has issued revised draft product-specific guidances for generic peptide products covering impurity thresholds, higher-order structure and biological activity. Those are the questions a regulatory dossier answers. A one-page certificate does not attempt them.⁵⁷

And the questions no laboratory can answer

Two more absences are structural rather than analytical. A certificate describes the sample submitted, so it cannot describe the vial you hold unless the lot matches and the chain of custody is known. And it cannot vouch for its own authenticity, because a PDF is trivially editable and an altered certificate looks exactly like an unaltered one.¹

Storage and transport sit in the same category. FDA has recorded complaints that unapproved GLP-1 products arrived warm or with inadequate ice packs — an event that happens entirely after any testing, and that no certificate is in a position to describe.⁸

The checklist, stated as absences

• Sterility — a separate test, with a validated aseptic process behind it in manufacturing.
• Bacterial endotoxin — separate again, and not implied by sterility.
• Water content — part of the mass, not part of the peptide.
• Counterion identity and content — likewise, and commonly a large fraction.
• Residual solvents — from synthesis and purification, invisible to a UV purity method.
• Elemental impurities — from reagents and equipment, requiring their own method.
• Separation of related-substance classes — a single purity figure reports none of them individually.
• Method validation status — naming a technique is a weaker claim than establishing that the procedure was validated for the material tested.²³⁴⁶

The phrase to distrust is "tested", unqualified. Tested for what, by which method, against which acceptance criterion, on which lot. Four questions, and a certificate that answers fewer than four has left the rest to the reader's imagination.⁶