A CAS Registry Number is a numerical identifier assigned by the Chemical Abstracts Service to registered chemical substances.
This topic is closely related to What is PubChem and what is a PubChem CID? and What is an IUPAC name?, which provide additional context for the interpretation and characterization of chemical materials.
CAS numbers contain three groups of digits separated by hyphens. The final digit is a check digit. Molecular structure cannot be derived directly from the number.
Substances can have multiple names and synonyms. A CAS number helps link database records and documents to the same registered substance.
A salt, free base, hydrate, solvate or specific stereochemical form may have its own CAS number. The name and formula should therefore also be checked.
A CAS number is an important identification tool but does not replace structural or analytical confirmation.
Systematic names can be long and common names can be ambiguous. A CAS Registry Number offers a compact database identifier. It is useful precisely because it does not depend on a particular naming convention or language.
The final digit can be mathematically checked against the preceding digits and helps detect typing errors. A valid check digit only confirms that the number is syntactically consistent; it does not prove that the number belongs to the substance printed beside it.
A parent compound and its hydrochloride, sodium salt, hydrate or stereochemically defined form may be separate registered substances. Copying a CAS number from a related form can therefore create a chemically significant documentation error.
CAS numbers are registry identifiers. PubChem CID, InChI, InChIKey and SMILES serve different database or structural purposes. Cross-checking several identifiers can reveal mismatches that are easy to miss when only a product name is considered.
The CAS number should agree with the chemical name, molecular formula, molar mass and stated form. If one field describes a salt while another describes the free compound, the document needs closer examination.
A CAS number does not encode purity, batch, grade or analytical confirmation. Two containers bearing the same correct CAS number can still differ in purity or other material characteristics. It should therefore be treated as one element of identity documentation.
Analytical data cannot be separated completely from the conditions under which they were produced. Instrument settings, sample preparation, reference materials, calibration and data processing can all influence the final result. Two values that look identical at first glance may therefore not carry exactly the same analytical meaning. When documents are compared, the method and the definition of the reported value matter just as much as the final number.
A reliable reference material can help compare analytical observations with a known substance. Its value depends on the question being asked: retention behaviour, a spectrum or another characteristic can provide strong support under controlled conditions. Reference comparison does not replace critical review of the complete dataset. The identity of the reference, the method used and the similarity of measurement conditions determine how strong the comparison really is.
A technically convincing result consists of more than a percentage or one prominent signal. Clear sample identification, interpretable raw or derived data, logical specifications and a suitable analytical method make a result easier to assess. Internal consistency also matters: chemical name, formula, molar mass, batch information and analytical conclusion should not contradict one another. Where different elements do not align, further verification is preferable to treating one favourable value as decisive.
Information has different functions in an SDS, Technical Data Sheet and Certificate of Analysis. An SDS primarily addresses safety and hazard information, a TDS describes technical characteristics, and a CoA generally reports results for a specific batch. Analytical information has the right meaning only when it is clear which document contains it, which material or batch it concerns, and whether it is a specification, typical value or actual measured result.
