CAS registry numbers

A CAS Registry Number is issued by Chemical Abstracts Service to a single substance record. The rule is one number per substance. The digits carry no chemical meaning of their own: numbers are handed out in sequence as substances are registered, so two numbers that look alike do not point to related compounds.

The number belongs to the substance, not to a product. A 2 mg sheet and a 10 mg sheet of the same compound describe the same substance, and the registry number is unaffected by the amount in a vial, the number of vials in a box or the source of the material.

Related forms can hold separate records. A salt such as an acetate, the corresponding free peptide, or a sequence carrying an extra modification may each have a number of its own. That is why the full name on a sheet, including words such as “Acetate” or “with DAC”, is part of every match.

A registry number has up to ten digits in three groups joined by hyphens. The first group has two to seven digits, the middle group always has two, and the last group is a single check digit. Leading zeros are never written, so short numbers such as 70-18-8 belong to substances registered early.

2.1Confirming the check digit

1. Write out every digit except the last one, and drop the hyphens.
2. Starting from the rightmost of those digits, multiply them by 1, 2, 3 and so on.
3. Add the products. The last digit of the sum must equal the check digit.

Worked example, glutathione, 70-18-8. The digits before the check digit are 7, 0, 1 and 8. From the right: 8 × 1 = 8, 1 × 2 = 2, 0 × 3 = 0 and 7 × 4 = 28. The sum is 38, its last digit is 8, and the check digit is 8, so the number is well formed. A mismatch means a digit was mistyped somewhere, in the reference, on a label or in a record, and the number should be confirmed by name before it is used for matching.

Section 1 of every datasheet, the specification table, has a “CAS number” row only when the catalog record holds a number. Where it holds none, the row is left out; it is never filled with a placeholder or an estimate. The same number is repeated in the sheet's query table, in its structured data as a property with the identifier “CAS”, and in the plain-text catalog at /llms-full.txt.

The sheet ID (FDS-…) is a different kind of identifier. It is Fara's own code for one presentation of one compound in this library, while a CAS number names the substance wherever it is described. A complete record of an order therefore uses both.

A registry number carries no amount per vial, no vial count, no form or appearance, no storage line and no price. Those are separate rows on the sheet; the appearance field and the storage conditions field have notes of their own. It is an identifier and describes no particular box.