A CAS number is a label, not a description. It carries no information about structure, formula or mass, and it cannot be derived from any of them. Its value is that it is unique and unambiguous: one registered substance, one number, no synonyms to argue about.
How the number is built
The format is up to ten digits in three parts separated by hyphens, as in 137525-51-0. The parts have no chemical meaning. Numbers are assigned in sequence as substances are registered, so a high first block indicates a recent registration and nothing else.
The last digit is the exception. It is a check digit calculated from the others, which means a mistyped CAS number usually fails arithmetic rather than pointing at a different compound. That is a small but real safeguard when a number is copied by hand between a label, a certificate and a purchase order.
What the number identifies
It identifies a substance as registered, and the registry treats forms of the same compound as separate substances. A free acid and its sodium salt have different numbers. A hydrate and the anhydrous form have different numbers. Stereoisomers have different numbers.
The practical consequence is that a CAS number on a label makes a claim about the form, not only the molecule. If a specification quotes the number for a free base and the vial contains a salt, the label and the material disagree even though the peptide is the same.
Where this gets awkward for peptides
Peptides purified by reversed-phase chromatography are usually isolated as salts, carrying trifluoroacetate or acetate from the purification. The peptide and its salt forms are different registered substances, so more than one number can be correct depending on which form is being described.
Metal complexes are worse. A copper peptide exists as a species in equilibrium whose charge and counterion depend on conditions, so the registry number attaches to one particular way of writing it. Our notes on copper peptides and on GHK-Cu cover why the name does not pin down a formula there.
Not every compound has one
Registration is not automatic. A compound can be well characterised in the literature, catalogued elsewhere, and still have no CAS number attached to it in public sources. Newer research compounds sit in this position more often than older ones.
The absence of a number is not evidence of anything about the material. It means the public chemical record is thinner, which shifts the burden onto the batch documentation.
Catalogues compound the problem by renaming things for commercial reasons. A compound sold under a house name still has whatever registry number its structure earned, and the number is the thread back to the chemistry when the name has drifted.
The other identifiers you will meet
- PubChem CID: a database record number, assigned by the database. It links to a structure, a formula and computed properties.
- ChEMBL ID: the same idea in a different database, oriented toward bioactivity data.
- InChI and the InChI Key: an identifier derived from the structure itself by a published algorithm, so two people with the same structure generate the same string without consulting any registry. (Reference: Heller et al., 2015)
The last one is the important contrast. A CAS number has to be looked up because it was assigned. An InChI can be computed, so it carries the structure inside it.
Why a unique label was needed at all
Chemical names are not unique. One compound can carry a systematic name, several trivial names, a research code, an international nonproprietary name and whatever a catalogue decides to print. Some of those names are ambiguous on their own, and some are shared between compounds that differ in ways a name does not capture.
Nomenclature rules exist and they help. The conventions for naming amino acids and peptides, for instance, are set out by the joint IUPAC and IUB commission. (Reference: IUPAC-IUB, 1984) But a systematic name for a forty-residue peptide is unusable in practice, which is why a short arbitrary number earns its place.
The number is not the measurement
One habit worth resisting is treating a matching CAS number as a verification step. It confirms that the label names the substance the specification intends. It cannot confirm that the material is that substance, because nothing about the number is measured from the sample.
Every identifier shares this limitation, including the computed ones. An InChI derived from a drawn structure describes the structure someone drew, not the powder someone weighed.
Checking one
A CAS number resolves in the public chemical databases. Entering it should return a compound whose name, formula and mass match what the specification says. Three things are worth comparing rather than one: if the number resolves but the formula does not match the label, the disagreement is real and worth asking about.
This is also the fastest way to catch the difference between a free peptide and its complex or salt, because the two resolve to different records with different masses.
One more comparison is worth making while you are there. The mass in the database record is calculated from the structure, and the mass on a certificate is measured from the batch. Two numbers arrived at by different routes agreeing is a stronger statement than either alone.
What a CAS number does not establish
It says a substance with that identity has been registered. It says nothing about the vial: not the purity, not whether the material is that substance, not which batch it came from.
Those questions are answered by measurement. A purity figure from HPLC and an identity result from mass spectrometry, reported on a certificate of analysis whose batch number matches your vial and which you can look up, are what connect a registry entry to the powder in front of you.
Check this against a real batch
Every compound in the catalog lists its formula, molecular weight and CAS number, and every batch we ship has a certificate you can look up by number.
Frequently asked questions
What is a CAS number?
A unique registry identifier for a chemical substance, up to ten digits in three hyphenated parts. It carries no structural information and cannot be derived from a formula; numbers are assigned in sequence as substances are registered.
Does the last digit mean anything?
Yes. It is a check digit calculated from the preceding digits, so a mistyped CAS number usually fails the arithmetic instead of resolving to a different compound.
Do a peptide and its salt have the same CAS number?
No. The registry treats a free acid, its salts, hydrates and stereoisomers as separate substances with separate numbers. Since peptides are usually isolated as salts after purification, more than one number can be correct depending on which form is described.
Does every compound have a CAS number?
Registration is not automatic, so a well characterised compound can lack one in public sources. The absence says nothing about the material; it means the public record is thinner and the batch documentation carries more weight.
How is an InChI different from a CAS number?
An InChI is computed from the structure by a published algorithm, so anyone with the same structure produces the same string. A CAS number is assigned by a registry and has to be looked up.
Does a CAS number on a label prove what is in the vial?
No. It states which substance the label claims. Purity and identity come from measurement, reported on a certificate of analysis tied to a specific batch.
References
- Heller, S.R., McNaught, A., Pletnev, I., Stein, S., & Tchekhovskoi, D. (2015). InChI, the IUPAC International Chemical Identifier. Journal of Cheminformatics, 7, 23.
- IUPAC-IUB Joint Commission on Biochemical Nomenclature (1984). Nomenclature and symbolism for amino acids and peptides. Biochemical Journal, 219(2), 345-373.
- Senko, M.W., Beu, S.C., & McLafferty, F.W. (1995). Determination of monoisotopic masses and ion populations for large biomolecules from resolved isotopic distributions. Journal of the American Society for Mass Spectrometry, 6(4), 229-233.
- Wieser, M.E. (2006). Atomic weights of the elements 2005 (IUPAC Technical Report). Pure and Applied Chemistry, 78(11), 2051-2066.