Most certificates are never read past the purity line. This reads the rest, and tells you where the document disagrees with itself or with published chemistry.
A certificate of analysis is a set of claims that ought to agree with each other. The sequence implies a mass. The mass implies where the ions land in the mass spectrum. The compound named at the top implies all of it. When those numbers were assembled from more than one source, they stop agreeing — and that is something you can see from the document alone, without a laboratory.
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Selecting the compound lets the checker compare the document against published chemistry, not just against itself.
Paste a certificate above to see the checks.
What gets checked
Molecular weight against the reference value
The stated mass is compared with a source-supported reference for the selected molecular form. The tool uses 0.15% and 1% comparison thresholds; these are screening tolerances, not proof of identity. Counterions, modifications and average-versus-monoisotopic reporting can cause differences. Unresolved reference identities are withheld and explained.
Sequence against the published sequence
Length first, then residue by residue. A single substitution at one position is a different molecule, and it is the kind of error that survives every visual check.
The document against itself
An unmodified, linear sequence with free termini implies an approximate average mass. A discrepancy needs review of termini, modifications and mass conventions. This comparison is skipped for known modified or unresolved reference forms.
The mass-spectrometry result against a limited ion model
The checker tests the neutral mass and protonated 1+ through 5+ ions. It does not model every adduct, negative ion, isotope or reporting convention. An unmatched value is a prompt to inspect the original spectrum, not a verdict on the sample.
Purity, peptide content, and traceability
Chromatographic purity uses the tool's 98% and 95% screening thresholds; these are not universal quality grades. Net peptide content measures a different quantity. The checker also reports whether the provided text includes a product, lot and analysis date without authenticating them.
Purity is not peptide content, and the difference is the vial
These two numbers are confused more than any other pair on a certificate, and the confusion costs real material. Purity is the proportion of what was injected onto the column that came off as one peak — it describes how clean the peptide is relative to other peptide-like species. Net peptide content is the proportion of the powder's mass that is peptide at all. The rest is counter-ion, usually acetate, plus bound water.
A powder can have 99% chromatographic purity and 80% net peptide content. If 10 mg denotes total powder mass, that corresponds to about 8 mg peptide; using 10 mg would overestimate the peptide concentration by 25% relative to 8 mg. Some labels already state net peptide mass, so verify the label's basis before applying any correction. A missing content result leaves that basis unresolved.
What a document can never tell you
Every check here is a consistency check. A certificate that passes all of them is internally coherent and agrees with public chemistry — which is exactly what a carefully fabricated one would also do, and what a genuine certificate for a different batch would do too.
The questions this cannot answer are the ones that matter most: was this batch the batch in front of you, was the analysis performed at all, and was it performed by anyone independent of the seller. A lot number and a named laboratory make those questions answerable. Their absence is the finding.
FAQ
What should a peptide certificate of analysis contain?
Product name, lot or batch number, date of analysis, sequence, molecular formula and weight, purity by HPLC with the method stated, a mass-spectrometry identity result, and ideally net peptide content plus water and acetate. A document missing the lot number or the date cannot be tied to a specific vial.
Why does the mass spec number not match the molecular weight?
An m/z value differs from neutral mass. For n added protons, m/z = (M + n × 1.007) / n. The checker tests that limited positive-ion model; other adducts, negative ions, isotope peaks and reporting conventions need interpretation from the original spectrum.
Is 99% purity good?
It is at or above what most research-grade specifications quote. But purity only describes the fraction resolved as a single peak by the method used — it does not identify the remainder, and it says nothing about how much of the powder is peptide rather than salt and water.
Can a certificate of analysis be faked?
Yes, and consistency checks cannot detect a careful fabrication — they detect a careless one. What a document cannot supply is independence: an analysis performed by a laboratory with no stake in the sale, on the batch you actually received.