What a CAS number proves — and what it does not
A CAS Registry Number is a unique identifier assigned to a specific substance. Matching one confirms that you and the supplier are naming the same compound. It does not confirm that the vial contains it: CAS is nomenclature, not analysis.
The failure modes are mundane rather than exotic. A listing carries the CAS of the free base while the product ships as an acetate salt; a related sequence’s CAS is reused for a modified analog; a number is transcribed with two digits transposed. All three are catchable by cross-reference in under a minute.
Average mass versus monoisotopic mass
Two molecular weights exist for every peptide and they are not interchangeable. Average mass accounts for the natural isotopic abundance of each element and is what a spec sheet lists as molecular weight. Monoisotopic mass counts only the lightest isotope of each element, and is what a high-resolution mass spectrometer reports for the lowest-mass peak.
For a compound the size of BPC-157 the two differ by roughly a dalton; for a 5 kDa peptide the gap is several. Comparing a monoisotopic measurement against an average theoretical value produces a mismatch that looks like an identity failure but is only arithmetic — and the reverse comparison can mask a real one.
A cross-check workflow
| Step | What to do | What a mismatch means |
|---|---|---|
| 1 · Sequence | Compare the sequence on the COA against the published sequence for the named compound | A different sequence is a different molecule, whatever the label says |
| 2 · CAS | Search the CAS number in a public chemical database and confirm it resolves to the same compound | A number that resolves elsewhere, or not at all, is a labeling failure |
| 3 · Theoretical mass | Check the COA’s stated theoretical mass against the database value, matching average to average | A large discrepancy usually indicates a salt form or the wrong compound entirely |
| 4 · Observed mass | Confirm the measured value falls within the tolerance the COA states | Outside tolerance with no salt-form explanation is an identity failure |
The workflow takes minutes and requires no equipment. Its value is that steps 1–3 rely on sources outside the supplier’s control, which is precisely what makes them worth doing.
A worked example
BPC-157 is registered under CAS 137525-51-0, with the sequence Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val and an average molecular weight of approximately 1,419.5 g/mol for the free base.
A COA reporting an observed mass of 1,419.5 Da within a stated tolerance of ±0.5 Da against that theoretical value is internally consistent, and the sequence and CAS agree with the published record. Full specifications for each compound are on its research monograph.
Salt forms and counter-ions
Most synthetic peptides are isolated as trifluoroacetate or acetate salts. The counter-ion dissociates during ionization, so it does not shift the mass a spectrometer reports — but it is part of the powder in the vial.
That is why net peptide content is a separate line on a Certificate of Analysis, and why gross vial weight overstates the peptide present. The distinction is covered in how to read a Certificate of Analysis.
Red flags
A sequence that is never disclosed. A CAS number that does not resolve. A single mass figure with no tolerance attached. A theoretical mass quoted to more decimal places than the stated method can support.
And the most telling of all: identical analytical values repeated across multiple lots. Genuine per-lot measurement varies slightly. Numbers that never move are a document being reproduced, not a measurement being made.
Every compound, fully specified
The Ethos Bio research library publishes sequence, CAS number, and molecular weight for all 25 compounds — the values to check a COA against.
Frequently asked questions
Does a CAS number prove a peptide is authentic?
No. A CAS Registry Number identifies which substance is being named; it says nothing about the contents of a particular vial. Authenticity requires mass-spectrometry identity confirmation on that specific lot, cross-checked against the compound’s theoretical mass.
What is the difference between average and monoisotopic molecular weight?
Average mass accounts for the natural isotopic abundance of every element and is what spec sheets list as molecular weight. Monoisotopic mass counts only the lightest isotope of each element and is what a high-resolution mass spectrometer reports for the lowest-mass peak. Comparing one against the other produces an apparent mismatch that is purely arithmetic.
Why does the observed mass on a COA differ slightly from the theoretical mass?
Instrument resolution and calibration produce a small deviation, which is why a COA states a tolerance — typically a fraction of a dalton for a peptide of this size. A result inside the stated tolerance is a pass; a result outside it, with no salt-form explanation, is an identity failure.