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PubChem is a public chemical database maintained by the US National Center for Biotechnology Information. It combines structures, names, identifiers and linked information from many sources.
This topic is closely related to What is a CAS number? and What are InChI and InChIKey?, which provide additional context for the interpretation and characterization of chemical materials.
CID means Compound Identifier and identifies a standardised Compound record in PubChem. It is not the same identifier system as a CAS Registry Number.
PubChem distinguishes deposited Substance records from standardised Compound records. Several source records can point to the same standardised structure.
Records can contain formula, molecular weight, IUPAC name, SMILES, InChI, InChIKey, synonyms and external references. Available fields vary by compound.
A defined stereoisomer or salt can be represented differently from an unspecified or neutral structure. The drawn structure should therefore be checked alongside the CID.
Chemical identity involves more than a name. Formula, molar mass, salt or free form, stereochemistry and, where relevant, hydration or solvation should all refer to the same chemical entity. A mismatch between these fields is an important reason to recheck source data.
Databases describe chemical entities and collect reference information. A Certificate of Analysis, by contrast, generally reports measurements for a specific batch. Correct database identity therefore does not automatically prove the composition or purity of physical material.
A robust check compares name, CAS number, PubChem record, formula, mass and structural identifiers. When several independent fields consistently describe the same form, documentation becomes more reliable. Contradictions deserve additional attention.
A measurement gains meaning from the method, sample and conditions. HPLC, LC-MS, NMR, FTIR and solid-state techniques provide different kinds of information and should not be treated as interchangeable evidence.
A frequent error is combining data for closely related but non-identical forms, such as a free base with the molar mass of a hydrochloride or an unspecified structure with a stereospecific identifier. Cross-checking is particularly useful for finding such errors.
SDS, TDS and CoA serve different functions. An SDS primarily addresses safety, a TDS technical characteristics and a CoA batch-specific test results. The same type of value can therefore have a different context in different documents.
No database, identifier or analytical technique automatically describes every aspect of a material. Molecular structure, chemical purity, water content and solid-state form are different information levels. Strong assessment uses information appropriate to the actual question.
Most errors are avoided by not relying on one prominent number or one name. Check whether the different data are mutually consistent, distinguish structural identity from experimental batch data, and use complementary analysis when the question requires it.