A certificate of analysis is the most useful document a peptide supplier publishes, and the fastest way to misread one is to assume it covers everything. It reports the tests within its stated scope. Everything outside that scope is simply absent, and absence is not a failure.
Five things are commonly assumed to be covered and usually are not.
Water content
Freeze-dried material is not anhydrous. Water remains bound in the cake after drying, typically a low single-digit percentage, and it matters because water is a reactant in most peptide degradation routes and a plasticiser that lowers the temperature at which the solid softens.
It is rarely on a certificate because it needs a dedicated determination. Residual moisture covers what the number would tell you and why its absence does not affect the content figure.
Residual solvents
Synthesis and purification use organic solvents, and traces survive into the finished solid. A panel for them is a separate gas-chromatography method with its own cost, and most research-grade certificates do not carry it. Heavy metals and residual solvents covers the two tests together.
Counter-ion content
A lyophilised peptide is a salt, usually trifluoroacetate or acetate, and the counter-ion is part of the powder's mass. It is almost never quantified on a research certificate, which is one reason the powder weighs more than the content figure. TFA and acetate salt forms covers what that mass is and when it matters.
| Not usually tested | Why it matters | What it would change |
|---|---|---|
| Water content | Drives degradation and softening of the solid | How confidently you can set a storage window |
| Residual solvents | Carried over from synthesis | Suitability for solvent-sensitive assay work |
| Counter-ion | Part of the powder mass | Nothing in the content figure; it is already peptide-specific |
| Sequence order | Two orders share one mass | Whether identity is proven or only consistent |
| Stability over time | A certificate is one day's reading | Whether the figures still hold months later |
Five absences, and what having the number would actually let you decide.
Sequence order, which mass cannot establish
Two peptides containing the same residues in a different order have the same molecular formula and the same mass. A certificate reporting identity by a single mass cannot separate them. Only a method that fragments the molecule and reads the ladder can, which is why the identity method named on the document matters as much as the result.
This is not a theoretical concern for short sequences, where swapped residues are a realistic synthesis outcome.
Stability, which no certificate claims
Every figure on a certificate describes the sample tested on the date tested. Nothing on the page describes how the material behaves three months later, or how it was stored between the laboratory and you. How peptides degrade covers the routes, and retest dates and expiry dates covers what the date on the vial actually represents.
Why the scope is what it is
Each additional test is a separate method with its own instrument, standards and analyst time, and research-grade material is priced against a panel that answers the questions most buyers actually have: what is it, how pure is it, how much is there, and is it pyrogen-free enough for cell work.
Widening that panel is possible and is paid for. A supplier who adds residual solvent or moisture testing on request is behaving normally; one who claims those tests are already included without showing them on the certificate is not.
What to do about an absence that matters to you
- Decide whether the work genuinely depends on it, because most research applications do not need residual solvent data.
- Ask the supplier whether the test can be added, and what it costs. The answer is usually yes and a number.
- If the material is already in hand, an accredited laboratory will accept a single sealed vial and run the test directly, which third-party testing covers.
- Record the decision either way, so the gap is a documented choice rather than an oversight nobody noticed.
What the absences do not undermine
Identity, purity and content are unaffected by any of this. Content in particular is an assayed mass of peptide rather than a weight of powder, so water and counter-ion do not inflate it. The absences narrow what a certificate can support; they do not weaken what it does support.
This sits inside the wider picture of what gets tested and why, which why peptide testing matters sets out across the whole analytical panel.