Screenshot of the CAS Common Chemistry database with information aboutcaffeine (58-08-2).
ACAS Registry Number[1] (also referred to asCAS RN[2] or informallyCAS Number) is aunique identification number, assigned by theChemical Abstracts Service (CAS) in the US to everychemical substance described in the open scientific literature, in order to index the substance in theCAS Registry. This registry includes all substances described since 1957, plus some substances from as far back as the early 1800s.[3] It is achemical database that includesorganic andinorganic compounds,minerals,isotopes,alloys, mixtures, and nonstructurable materials (UVCBs - substances ofunknown orvariable composition,complex reaction products, orbiological origin).[4] CAS RNs are generallyserial numbers (with acheck digit), so they do not contain any information about the structures themselves the waySMILES andInChI strings do.
The CAS Registry is an authoritative collection of disclosed chemical substance information. It identifies more than 204 million unique organic and inorganic substances and 69 million protein and DNA sequences,[3] plus additional information about each substance. It is updated with around 15,000 additional new substances daily.[5] A collection of almost 500 thousand CAS registry numbers is made available under aCC BY-NC license at ACS Commons Chemistry.[6]
Historically, chemicals have been identified by a wide variety of synonyms and properties. One of the biggest challenges in the early development of substance indexing, a task undertaken by the Chemical Abstracts Service, was in identifying if a substance in literature was new or if it had been previously discovered. Well-known chemicals may be known via multiple generic, historical, commercial, and/or(black)-market names, and evensystematic nomenclature based on structure alone was not universally useful. An algorithm was developed to translate thestructural formula of a chemical into a computer-searchable table, which provided a basis for the service that listed each chemical with its CAS Registry Number, the CAS Chemical Registry System, which became operational in 1965.[7]
CAS Registry Numbers (CAS RN) are simple and regular, convenient for database searches. They offer a reliable, common and international link to every specific substance across the various nomenclatures and disciplines used by branches of science, industry, and regulatory bodies. Almost all molecule databases today allow searching by CAS Registry Number, and it is used as a global standard.[8]
A CAS Registry Number has no inherent meaning, but is assigned in sequential, increasing order when the substance is identified by CAS scientists for inclusion in the CAS Registry database.
A CAS RN is separated by hyphens into three parts, the first consisting from two up to seven digits,[9] the second consisting of two digits, and the third consisting of a single digit serving as acheck digit. This format gives CAS a maximum capacity of 1,000,000,000 unique numbers.
The check digit is found by taking the last digit times 1, the preceding digit times 2, the preceding digit times 3 etc., adding all these up and computing the summodulo 10. For example, the CAS number ofwater 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.
Stereoisomers andracemic mixtures are assigned discrete CAS Registry Numbers:L-epinephrine has 51-43-4,D-epinephrine has 150-05-0, and racemicDL-epinephrine has 329-65-7
Differentphases do not receive different CAS RNs (liquid water and ice both have 7732-18-5), but different crystal structures do (carbon in general is 7440-44-0,graphite is 7782-42-5 anddiamond is 7782-40-3)
Commonly encountered mixtures of known or unknown composition may receive a CAS RN; examples areLeishman stain (12627-53-1) andmustard oil (8007-40-7).
^National Induscctrial Chemicals Notification and Assessment Scheme."AICS Search Tool". Archived fromthe original on 14 May 2009. Retrieved11 July 2009.
