A CAS Registry Number (often written “CAS number” or “CAS RN”) is a unique numerical identifier assigned to a distinct chemical substance. It exists so that one substance can be named unambiguously, regardless of how many common names, brand names, or synonyms it carries. This entry explains, neutrally, what a CAS number is, how it is structured, who assigns it, and where it appears when identifying a research peptide.
What a CAS number is
A CAS Registry Number is a numeric label with no chemical meaning of its own. It is not derived from a molecule’s structure, formula, or mass. Instead, it is a serial identifier: when a substance is first described in the scientific literature, it is entered into the CAS Registry and assigned the next available number. From that point on, the number belongs to that one substance and to no other. Two laboratories on opposite sides of the world referring to the same CAS number are, by definition, referring to the same chemical entity.
This is the whole point of the system. Chemical names are ambiguous – a single compound may be written a dozen different ways, and the same trivial name is sometimes used loosely for related substances. A CAS number removes that ambiguity by providing a single, language-independent handle.
Who assigns CAS numbers
CAS numbers are assigned by the Chemical Abstracts Service (CAS), a division of the American Chemical Society. CAS maintains the CAS Registry, one of the largest curated collections of disclosed chemical substances. A number is issued once, is never reassigned, and is never reused – even if a substance later becomes obsolete in commerce, its number is retired with it rather than passed on.
How the number is structured
A CAS number is written as up to ten digits in three hyphen-separated parts, in the form NNNNNNN-NN-N:
| First part | Up to seven digits, assigned sequentially |
|---|---|
| Second part | Two digits |
| Third part | A single check digit (0-9) |
For example, water carries the CAS number 7732-18-5, and caffeine carries 58-08-2. The parts are not ranges or categories; the split simply makes a long number easier to read. The final digit is where the useful mathematics lives.
The check digit: how a CAS number is validated
The last digit of a CAS number is a check digit, calculated from the digits that precede it. Because it is computed rather than arbitrary, it lets any system detect a mistyped or transposed number without consulting the registry.
The calculation is straightforward. Reading the digits (excluding the check digit) from right to left, each is multiplied by its position – 1, 2, 3, and so on – the products are summed, and the check digit is that sum modulo 10. Taking water (7732-18-5), the digits before the check digit are 7, 7, 3, 2, 1, 8; read right to left they are 8, 1, 2, 3, 7, 7:
(8 × 1) + (1 × 2) + (2 × 3) + (3 × 4) + (7 × 5) + (7 × 6) = 8 + 2 + 6 + 12 + 35 + 42 = 105. And 105 mod 10 = 5, which matches the published check digit. A number whose final digit fails this test is not a valid CAS number.
What a CAS number does – and does not – identify
A CAS number identifies a specific substance as registered, which for many compounds means a specific salt or hydrate form is distinguished from the parent molecule. This detail matters for peptides. A peptide’s free-acid (free-base) form and its salt forms – for instance an acetate or a trifluoroacetate (TFA) salt – are chemically distinct substances and can carry different CAS numbers, as well as different molecular formulae and molar masses. A CAS number on a specification therefore describes the exact form registered, not “the peptide” in a loose sense.
Equally, a CAS number does not measure anything. It does not report purity, identity in a given vial, or quantity – those are measured values that appear on a Certificate of Analysis. The CAS number states which substance a product claims to be; the analytical data states what the laboratory actually found. The two answer different questions and belong together.
Where a CAS number appears for a research peptide
On a research-compound specification, the CAS number sits alongside the other neutral identity fields – chemical name and synonyms, amino-acid sequence, molecular formula, and molar mass. As a concrete example, the peptide BPC-157 in its free-acid form is commonly listed under CAS 137525-51-0. Presenting the CAS number next to the sequence and formula lets a researcher confirm that all the identity fields describe one and the same substance, and cross-check them against an independent record before relying on the material. For how these identity fields are verified in the laboratory, see mass spectrometry and the guide to what a credible COA must contain.
CAS number vs other identifiers
A CAS number is one of several ways to point at a substance unambiguously, and it is helpful to know how it differs from the others. A molecular formula (such as C62H98N16O22) states the atom count but can be shared by different isomers. A PubChem CID is a separate registry identifier maintained by the US National Library of Medicine. An InChIKey is a hashed string derived from structure. The CAS number is distinctive in being a curated, human-readable serial number with a built-in check digit – which is why it is the identifier most often printed on catalogues, safety documents, and certificates.
Key takeaways
- A CAS Registry Number is a unique serial identifier for one chemical substance, assigned by the Chemical Abstracts Service.
- It carries no structural meaning – it exists to remove the ambiguity of chemical names and synonyms.
- The format is up to ten digits in three parts (NNNNNNN-NN-N); the final digit is a computed check digit that validates the number.
- Salt forms of the same peptide (free acid, acetate, TFA) can carry different CAS numbers, molecular formulae, and molar masses.
- A CAS number states which substance a product claims to be; a Certificate of Analysis states what a laboratory measured. Both belong on a credible specification.
You can see identity fields and analytical data presented together for real lots in the COA Vault, and read about the testing pipeline on how we test.
Frequently asked questions
What does CAS stand for?
CAS stands for Chemical Abstracts Service, a division of the American Chemical Society that maintains the CAS Registry and assigns the numbers. A “CAS number” is short for CAS Registry Number.
What does a CAS number identify?
It identifies one specific chemical substance as registered. For many compounds this distinguishes a particular salt or hydrate form from the parent molecule. It is a unique, language-independent label with no built-in chemical meaning.
How is a CAS number formatted?
As up to ten digits in three hyphen-separated parts – the form NNNNNNN-NN-N. The first part is up to seven digits, the second is two digits, and the third is a single check digit.
What is the check digit in a CAS number?
The final digit is calculated from the preceding digits: reading them right to left, each is multiplied by its position (1, 2, 3, …), the products are summed, and the check digit is that total modulo 10. It lets a system detect a mistyped number.
Can the same peptide have more than one CAS number?
Yes. A peptide’s free-acid form and its salt forms, such as an acetate or trifluoroacetate (TFA) salt, are chemically distinct substances and can each carry a different CAS number, along with different molecular formulae and molar masses.
Is a CAS number the same as a purity figure?
No. A CAS number is an identity label; it does not measure anything. Purity, identity confirmation, and quantity are measured values reported on a Certificate of Analysis for a specific lot.
How is a CAS number different from a molecular formula?
A molecular formula counts the atoms in a molecule but can be shared by different isomers. A CAS number is a unique serial identifier for one registered substance, so it removes the ambiguity a formula alone can leave.
