CAS Numbers: What They Identify and What They Do Not

A CAS number identifies a chemical substance. It does not describe the quality, purity, grade, legal status or safety of any sample of that substance. It is a name written in digits, and a name cannot tell you what is in a particular vial. On a peptide certificate it answers one narrow question, which substance is being discussed, and it leaves every other question to the measurements.

That sounds obvious until you read certificates. The number sits in a header box, formatted like a credential, and is easily taken for part of the evidence. It is part of the label. What follows is where the number comes from, how it is built, what it can and cannot distinguish, and how it relates to the tests that actually establish identity and purity.

What a CAS Registry Number is

A CAS Registry Number, often written CAS RN, is a numeric identifier assigned by CAS, a division of the American Chemical Society. CAS maintains the CAS Registry, a database of disclosed chemical substances drawn from the scientific literature and other sources. A number is assigned to a substance when it enters that database.

CAS describes each number as a unique identifier that designates only one substance and has no chemical significance. Numbers are issued in sequential order to new substances identified by CAS scientists. A higher number generally means the substance was registered later. It does not mean the substance is larger, more complex or better characterised.

Two consequences follow. The number cannot be decoded. Nothing about the digits tells you the structure, the molecular weight or the class of compound. And the number does not depend on any naming system. That is its purpose. One substance may carry a systematic name, a trade name, an abbreviation and a laboratory code, and the registry number links them all to the same entry.

The format and the check digit

A CAS number has up to ten digits in three groups separated by hyphens. The first group has two to seven digits, the second has exactly two, and the third is a single check digit. Plain hyphens are used. Water is 7732-18-5.

The check digit is calculated from the other digits. Taking water as the example, drop the check digit and read the remaining digits, 7732-18, as the string 773218. Multiply the rightmost digit by 1, the next by 2, the next by 3, and so on moving left. Add the products. Divide by 10 and keep the remainder.

• 8 x 1 = 8
• 1 x 2 = 2
• 2 x 3 = 6
• 3 x 4 = 12
• 7 x 5 = 35
• 7 x 6 = 42

The total is 105. The remainder after dividing by 10 is 5, which is the final digit of 7732-18-5. The number is internally consistent. Change any one digit and the arithmetic will usually fail. Written as 7732-18-6, the same number is rejected.

Be precise about what this proves. The check digit catches transcription errors, a swapped or mistyped digit. It does not show that a number is registered, and it does not show that the number belongs to the substance named beside it. A fabricated number can be constructed to pass the arithmetic. A correct number can be pasted next to the wrong compound. Validity of format is not validity of meaning.

How finely a number divides substances

A registry number designates one substance as CAS defines it, and CAS makes fine distinctions. Those distinctions are often finer than a product name implies.

Stereoisomers are one example. L-epinephrine is registered as 51-43-4, D-epinephrine as 150-05-0 and the racemic mixture as 329-65-7. The same atoms in the same connectivity, with a different spatial arrangement, give different numbers. For peptides this matters because amino acids are chiral, and a sequence built from D-amino acids is a different molecule from the same sequence built from L-amino acids.

Hydrates are another. Copper(II) sulfate anhydrous is 7758-98-7 and the pentahydrate is 7758-99-8. Same copper sulfate, different water content, different numbers, and a molecular weight that differs by about 90 g/mol.

Salts follow the same principle. A free base and a salt of that base are different substances, and a salt is commonly registered separately from its parent. A peptide supplied as an acetate salt, a trifluoroacetate salt or a free base can therefore be listed under different numbers in catalogues and databases. Which number a supplier quotes depends on which form it believes it is selling, and on how carefully it looked.

The reverse occasionally appears too. The same substance can show up under more than one number in older or third-party listings, usually through listing mistakes, so check against a primary source. CAS also registers mixtures and substances of unknown or variable composition. A number can refer to a defined molecule or to a loosely defined material. The digits do not say which.

Not every distinction is drawn. CAS does not give different phases of the same substance different numbers, so liquid water and ice share 7732-18-5.

What this means for a peptide

A peptide is usually supplied as a lyophilised salt. The sequence determines the parent molecule. The counterion, the water content and any modification such as amidation or acetylation determine the form actually in the vial. Each of these can change the registry entry that applies.

So a certificate that says only the peptide name and one CAS number has made a choice about form without explaining it. The number may refer to the free base while the material is an acetate. It may refer to the salt while the sample was not characterised as one. The reader cannot tell from the number alone.

Care is also needed in the other direction. Do not assume that a number quoted by a supplier is correct because it looks authoritative. Registry numbers for research peptides are copied between catalogues, and errors travel with the copies. The reliable route is to check the number against a primary database such as CAS Common Chemistry, which is a free CAS resource covering a subset of the registry, or against the full registry through a subscription. A substance absent from the free subset may still be registered.

CAS numbers and identity testing

This is the central point. Identity testing measures properties of the sample. A CAS number is not one of those properties.

Mass spectrometry measures the mass of the molecules present. Chromatography against a reference standard compares how the sample behaves with how a known material behaves. Both produce data that can be checked. Neither produces a CAS number as output. The laboratory compares the measured mass with the theoretical mass calculated for the claimed sequence and form, and reports whether they agree within instrument tolerance. Our note on molecular weight and identity covers that comparison and its limits.

The CAS number enters afterwards, as a label. It tells the reader which substance the laboratory was asked about. It is a way of making the compound name unambiguous, and it is useful for that. It is not a result.

There is a further limit worth stating. Mass spectrometry usually reports the peptide itself. A counterion such as acetate often separates from the peptide in the instrument, so a mass reading often cannot say whether the vial held a salt or a free base. The salt form is better addressed by other tests and by the supplier records. A CAS number that names a salt form therefore claims something the identity result may not have examined.

Common misuse on certificates

The same handful of errors appear again and again. None needs bad intent. Several are simple habit.

• The number as a quality mark. A certificate or product page presents the CAS number in the same box as the purity figure, as though both were measurements. One is a label and the other is a result.
• The number as an approval. Having a CAS number means a substance was disclosed and registered. It does not mean the substance is approved, licensed, safe or permitted for any use. CAS assigns numbers to substances that have been described. It does not assess them.
• The number as proof of authenticity. Quoting a valid-looking number makes a document look technical. It cannot be used to confirm that the document is genuine. That requires a record held by the issuing laboratory.
• The wrong form. A free base number is printed for an acetate salt, or the reverse. The sample and the label then describe different substances.
• One number for a blend. A product containing several compounds carries a single number, usually that of one component. The certificate covers whatever the number names and nothing else.
• A number that does not match the name. A transposed digit or a number copied from a neighbouring catalogue row. The check digit may still pass.
• Phrases such as CAS verified or CAS certified. CAS assigns registry numbers. It does not verify or certify samples, and a phrase implying otherwise should be read with suspicion.

How to read the number on a certificate

Treat the CAS number as a cross-reference. Four short checks are enough.

First, confirm that the number is arithmetically valid by running the check digit. This takes seconds and removes typing errors. Second, look the number up in a primary source and confirm that it belongs to the substance named on the certificate. Third, establish which form the number refers to, free base, a specific salt, a hydrate, and compare that with the form the certificate says was tested. Fourth, put the number aside and read the measurements: the method, the purity with the method that produced it, the identity data, the batch identifier and the date. Our guide to reading a peptide Certificate of Analysis goes through each field.

The language on a certificate matters here too. A result that conforms is conforming to a stated specification, not to a registry entry. See what conforms means on a certificate of analysis.

What a certificate can and cannot say

Novagen is a third-party analytical laboratory. It reports what was measured in the sample submitted. A certificate describes that sample and nothing beyond it. Purity is a statement about composition and is not a statement about safety, and every measurement has limits. A purity figure depends on the method used and its detection settings, and a different method can give a different value.

A CAS number on any certificate identifies the substance the sample was submitted as. It is not a finding. The findings are the measured values. Novagen publishes a verifiable certificate register at novagenlaboratories.com. Current quoted turnaround is 21 days from receipt of samples. Questions can go to admin@novagenlaboratories.com.

The short version is this. A CAS number tells you what a document says it is about. The measured values on the certificate tell you what was found in the sample submitted. Read the first for orientation and the second for what was measured, alongside its limits.