The One Field Worth Checking Before the Purity Number
Two certificates cross your desk bearing the same product name. The measured masses on them differ by a factor of several. Nobody has made a mistake, and nothing on either page is false — the name simply was never a specification. This is the gap a CAS Registry Number closes, and understanding what it closes means understanding exactly what it designates.
Where a product name runs out
Trade names are marketing designations. They answer to no structural authority. One supplier may attach a given name to a free base while another attaches it to a salt; with fragment products, the same name has been applied to two materials whose masses are separated by a factor of several. Thymosin beta-4 and the fragment works through that case.
So when a certificate carries a registry number, it settles which substance was characterized in a way no trade name is capable of settling. When it carries only a trade name, the measured mass is the field left doing that job.
Assignment follows structure, not vocabulary
The number designates one specific substance, defined by its structure. It is not a name, not a class, and not a product. Its format runs to three groups of digits, the final one a check digit computed from the rest — which is why transposing a digit tends to produce an invalid number rather than a valid number belonging to something else.
Because structure governs assignment, every distinction that matters chemically earns a separate entry:
| Salt form | A peptide as free base and the same peptide as its acetate or trifluoroacetate salt are different substances carrying different numbers. Why that shifts the mass in the vial is covered in TFA and acetate counter-ions. |
| Terminal modification | An amidated peptide and its free-acid counterpart have separate numbers. The one-dalton difference behind that is set out in terminal modifications. |
| Complexation | A peptide and its metal complex are distinct substances. GHK and GHK-Cu are the example in this catalog; the two registry numbers and what each mass refers to walks through the arithmetic. |
| Stereochemistry | Where it is defined, an enantiomer or a specified diastereomer gets its own entry. |
The fields the number leaves empty
Purity, batch, manufacturer, quality of any description: none of it travels with a registry number. Two materials sharing one can differ enormously across everything a purity test is capable of measuring. What the number fixes is the identity the substance is supposed to have. Establishing what actually arrived is the work of the rest of the document.
Nor is registration approval. A CAS number is an index entry and nothing more, and possessing one carries no regulatory implication whatsoever. That ground is covered in regulatory status of research peptides in the USA.
All material is supplied for laboratory research use only. It is not a drug, not a supplement, and not for use in humans or animals.