CAS registry numbers^[1] are unique numerical identifiers for chemical elements, compounds, polymers, biological sequences, mixtures and alloys.^[2] They are also referred to as CAS numbers, CAS RNs or CAS #s.

Chemical Abstracts Service (CAS), a division of the American Chemical Society, assigns these identifiers to every chemical that has been described in the literature. The intention is to make database searches more convenient, as chemicals often have many names. Almost all molecule databases today allow searching by CAS number.

As of September 2009, there were more than 50 million organic and inorganic substances and more than 61 million sequences in the CAS registry.^[3] Around 50,000 new numbers are added each week.

CAS also maintains and sells a database of these chemicals, known as the CAS registry.

Contents 1 Format 2 Isomers, enzymes, and mixtures 3 Ownership Issues 4 CAS number search engine 5 See also 6 Notes 7 External links

[edit] Format

A CAS registry number is separated by hyphens into three parts, the first consisting of up to 7 digits, the second consisting of two digits, and the third consisting of a single digit serving as a check digit. The numbers are assigned in increasing order and do not have any inherent meaning.

The check digit is calculated by taking the last digit times 1, the next digit times 2, the next digit times 3 etc., adding all these up and computing the sum modulo 10. For example, the CAS number of water is 7732-18-5: the checksum 5 is calculated as (8×1 + 1×2 + 2×3 + 3×4 + 7×5 + 7×6) = 105; 105 mod 10 = 5.

[edit] Isomers, enzymes, and mixtures

Different stereoisomers of a molecule receive different CAS numbers: D-glucose has 50-99-7, L-glucose has 921-60-8, α-D-glucose has 26655-34-5, etc. Occasionally, whole classes of molecules receive a single CAS number: the group of alcohol dehydrogenases has 9031-72-5. An example of a mixture with a CAS number is mustard oil (8007-40-7).

[edit] Ownership Issues

CAS information is copyrighted by the American Chemical Society. Users who wish to incorporate CAS numbers into their own databases should see the details of the CAS usage policy:^[4]

A User or Organization may include, without a license and without paying a fee, up to 10,000 CAS Registry Numbers or CASRNs in a catalog, website, or other product for which there is no charge

[edit] CAS number search engine

• CHEMINDEX Search via Canadian Centre for Occupational Health and Safety ^[5]
• Common Chemistry ^{TM [6]} via Australian Inventory of Chemical Substances ^[7]
• European chemical Substances Information System ^[8] via the website of Royal Society of Chemistry ^[9]
• HSNO Chemical Classification Information Database via Environmental Risk Management Authority ^[10]
• Search Tool of Australian Inventory of Chemical Substances ^[11]
• EASE ^[12]

This section requires expansion.

[edit] See also

• Academic publishing
• Beilstein Registry Number
• Chemical database
• Chemical file format
• Dictionary of chemical formulas
• EC number (Enzyme Commission)
• EC# (EINECS and ELINCS)
• Identifier
• International Chemical Identifier (InChI)
• MDL number
• PubChem
• SMILES
• UN number

[edit] Notes

[edit] External links

• CAS registry description, by the Chemical Abstracts Service

To find the CAS number of a compound given its name, formula or structure, the following free resources can be used:

• Chemical Synthesis Database
• PubChem
• NIH ChemIDplus
• NIST Chemistry WebBook
• R&D Chemicals
• NCI Database Browser
• Chemfinder
• ChemSub Online (Multilingual chemical names)
• European chemical Substances Information System (ESIS) - useful for finding EC#
• NIOSH Pocket Guide to Chemical Hazards, index of CAS numbers