Buying an impurity standard that has no CAS number
The enquiry starts the same way every time: a pharmacopoeia monograph lists impurities A to H, the customer looks up the CAS numbers, and half the column is empty. The assumption is that the supplier has been careless. Usually it is not carelessness — the numbers do not exist, and they were never going to.
A CAS number records that something was published
A CAS Registry Number is an index entry, not a certificate. It is assigned when three conditions are met at the same time: the substance must be compositionally definable, it must be distinguishable from what is already registered, and it must have appeared in the open scientific literature. The third condition is the one that decides most impurity cases, and it is the one buyers never consider.
An impurity is not usually published. It is found in a forced-degradation study, or in a batch that went off-specification, or in a spike experiment during method development. It is characterised by LC-MS and NMR inside one company, assigned an internal number, and offered to customers. Nothing about that process requires a paper, a patent or a public disclosure. If nobody published it, no index entry was created, and there is no number to look up.
This leads to a conclusion that runs against intuition: a missing CAS number is often a sign the impurity is new rather than doubtful. The well-known degradants have numbers because they have been studied for twenty years. The one your analytical team found last quarter does not, and it may be the only reason you are looking for it.
It also runs the other way. A number tells you a substance has been described somewhere. It says nothing about whether a supplier can make it, whether the batch is characterised, or whether the material in the vial is the same thing the number refers to. Identity has to be established separately, in both directions.
Four reasons the number is missing
The four cases below are not equally common. The distribution can be measured: of the 13,678 reference standards in our own catalogue, 7,654 — 56% — carry no CAS number, and they fall into these groups.
| Case | What it means | Share of our catalogue |
|---|---|---|
| Numbered process or degradation impurity | Characterised in-house, never published, so never indexed | 6,684 of 10,479 (64%) |
| Pharmacopoeia letter impurity | Official in the monograph, but the monograph assigns no number | 33 of 49 (67%) |
| Positional isomer of a known impurity | Same formula as a published compound, different site | Common among NDSRIs |
| Unreported salt or hydrate form | The free base is indexed; this form of it is not | 55 of 246 (22%) |
Note what these figures do not say. Salt forms are mostly indexed — 78% of them carry a number, which is why the salt case is the smallest group and also the most misunderstood. The gap is concentrated almost entirely in the first row: impurities that were never published because there was no reason to publish them.
The practical consequence is that the CAS number is a weak filter for this catalogue. It works for the compounds that have been in circulation for years, and it fails precisely on the ones a development project needs first.
Pharmacopoeia letters do not fill the gap
The natural next move is to treat the monograph designation as an identifier — if the CAS is missing, use “EP Impurity F” instead. It feels equivalent. It is not, for three reasons.
A letter is a position in a document, not a property of a molecule. Impurity A in one monograph and Impurity A in another are unrelated compounds. The letter marks where the peak appears in that specific method, and it is reused across hundreds of monographs with a different meaning every time. Quoted without the parent drug, it identifies nothing.
The two pharmacopoeias do not translate into each other. EP assigns letters in its own sequence; USP uses a Related Compound series in another. The same degradant can be Impurity D in one system and Related Compound A in the other, and the numbering does not shift by a constant offset that could be corrected. If a specification was written against EP and the supplier quotes against USP, the letter alone cannot tell you whether you are buying the right substance.
The monograph does not guarantee a number exists. In our catalogue, 33 of the 49 standards named by pharmacopoeia letter have no CAS number at all. Being officially recognised as an impurity that must be controlled, and having an index entry, are two independent facts.
The safe habit is to always carry the parent drug alongside the designation, and to convert a letter into a structure before it goes into a purchase order. A letter is only unambiguous when the structure it points to is written next to it.
One formula, several substances
A molecular formula is the fallback identifier when the CAS is absent, and it is a good one — every one of the 7,654 standards without a CAS number in our catalogue still carries a formula and a calculated molecular weight. But a formula narrows a search; it does not establish identity.
The clearest illustration in our own inventory is a set of three nitrosamine standards, all with the formula C19H31N3O6 and a molecular weight of 397.47, catalogued separately as impurity P, impurity U and impurity Z. Three substances, one composition, three different nitrosation sites on the same parent molecule. None of them is interchangeable with the others, and none of them has a CAS number, because the site that makes each one distinct is exactly the part that has not been described in the literature.
This is the normal situation for NDSRIs rather than an exotic one. A molecule with two secondary amines has two distinct nitrosation products before any stereochemistry is considered. They co-elute on some methods and separate on others, they have different acceptable intake values if their potency categories differ, and a supplier who quotes “the nitroso impurity of X” without stating the site may send either.
So the formula is the search key and the structure is the confirmation. Searching by formula is genuinely useful — it returns every catalogue entry with that composition, and you then pick by structure. It will not tell you which one you need.
The number you do find may belong to another form
A search that succeeds can be more misleading than one that fails. The number in the pharmacopoeia, the number in the supplier catalogue and the number on the certificate do not always refer to the same physical material, and the differences are systematic.
- Free base against salt. Most indexed impurity records belong to the free base. If the standard is supplied as a salt, the mass you weigh contains the counter-ion, and every concentration calculated from the weight is low by the salt factor. This is the single most common arithmetic error in impurity quantitation, and it produces no warning: the chromatogram looks perfect.
- Hydrate and solvate forms. A hydrate carries water in the crystal lattice, and a solvate carries solvent. Neither is visible in the chromatographic purity figure. Both change the mass fraction of the analyte.
- Two records for one substance. A compound registered more than once can carry more than one number, which is why a pharmacopoeia citation and a supplier catalogue entry for the same material sometimes disagree without either being wrong.
- A near neighbour's number. The failure mode with no innocent explanation: a positional isomer or a close analogue quoted with the CAS of the compound it resembles. It matches the search, it looks plausible, and it is the wrong substance.
Only the last of these is a supplier error. The first three are normal chemistry that has to be handled in the specification rather than argued about after delivery.
What to send instead of a number
An enquiry without a CAS number does not have to be a slow enquiry. Four fields replace it completely, and each one closes a specific ambiguity.
| Field | What it resolves | Example |
|---|---|---|
| Parent drug | Which molecule the impurity is a variant of | Perindopril |
| Site or designation | Which isomer, where the modification sits | N-nitroso at the ring nitrogen |
| Molecular formula | Composition, and the check on everything else | C19H31N3O6 |
| Structure or SMILES | The only unambiguous statement of identity | Drawing, or a SMILES string |
| Quantity and pack | Stock versus custom synthesis | 25 mg, one vial |
| Documentation | COA content, MSDS, TSE/BSE statement | Assigned purity by qNMR |
A drawing is worth more than all of the text. A formula plus a molecular weight still admits several isomers; a structure does not. If the structure exists only as a peak in a chromatogram, say so — that changes the request from a catalogue lookup to a feasibility question, and it is better to establish that on day one than after two rounds of quotation.
If a formula is all you have, search the catalogue by formula before writing the enquiry. The lookup returns every entry with that composition, and if one of them is the compound you need, the enquiry becomes a stock question that can be answered the same day.
How to check what comes back
Three checks catch almost every identity problem, and all three can be done before the material is weighed.
Recalculate the molecular weight from the structure. A formula has one molecular weight to within rounding. If the structure a supplier sends and the molecular weight in the quotation disagree by more than a rounding error, one of them is describing a different substance — and it is usually faster to find that out from the paperwork than from the chromatogram.
Check whether the number exists, and what it refers to. CAS maintains a free public subset of its registry at Common Chemistry, covering roughly half a million substances. It is not the full registry, so a blank result proves nothing, but a hit tells you the exact substance the number belongs to, its formula and its synonyms. That is enough to catch a number borrowed from a near neighbour.
Ask what the assigned purity is measured against. For a compound with no monograph and no primary standard, the certificate has to state how the content was assigned — typically by quantitative NMR against an internal standard, with an uncertainty. A chromatographic purity of 98% with no assigned value is a qualitative statement wearing a quantitative label, and it will not support a validation.
None of these require the number. All of them work on a compound that has never been indexed.
Why a missing number slows the quotation down
It is worth knowing what happens at the other end, because it explains a delay that looks like indifference.
Catalogue and inventory systems are keyed on identifiers. A CAS number resolves to a record, a location and a price in one step. An enquiry that arrives as “the degradant that elutes after the main peak, formula C19H31N3O6” has to be matched by hand: a chemist reads it, searches the structure database, checks whether the same compound is listed under a different name, and only then can confirm availability. That is not a pricing task, it is a identification task, and it is why the reply takes days instead of hours.
Sending the structure does not just help the supplier — it moves the enquiry from the slow queue to the fast one. The most efficient enquiries we receive contain the parent drug, the structure and the quantity, in that order, and no CAS number at all.
Frequently asked questions
Can a legitimate impurity standard really have no CAS number?
Yes, and it is the majority case in our catalogue: 7,654 of 13,678 entries have none. A CAS number is created when a substance has been described in the open literature. Many impurities are characterised in-house from forced-degradation or spiking experiments and never published, so no index entry was ever created. The absence of a number says nothing about whether the compound is real or whether it can be supplied.
Is an EP impurity letter the same as a CAS number?
No. A letter identifies a peak's position within one specific monograph and is reused across hundreds of monographs with unrelated meanings. Quoted without the parent drug it identifies nothing, and EP and USP lettering do not translate into each other. Always carry the parent drug name and the structure alongside the letter.
Two suppliers quote different CAS numbers for the same impurity. Who is wrong?
Possibly neither. A substance registered more than once can carry more than one number, and a number may refer to the free base while the material supplied is a salt or hydrate. It is also possible that one of them has quoted the number of a positional isomer. Check what substance each number actually resolves to, and compare that against the structure you need.
If I only have a molecular formula, can you still quote?
Yes, but expect a question back. A formula fixes the composition and not the isomer, so we will ask for the modification site or a structure drawing. If the formula matches an existing catalogue entry, the enquiry becomes a stock question and is answered the same day. Searching by formula on our catalogue page is the quickest way to find out which case you are in.
What does the certificate say when there is no CAS number?
Identity is established by the characterisation data rather than by an identifier: the structure as confirmed by mass spectrometry and NMR, the molecular formula, the calculated molecular weight, and the assay assigned by a quantitative method such as qNMR with a stated uncertainty. The certificate describes the lot that was shipped, not a reference lot.
Does a missing CAS number affect customs or transport documents?
It can add a step rather than block a shipment, because some declarations expect a registry number in a mandatory field. Tell us at the quotation stage if the material has to clear customs under a specific regime, and we will provide the identifier that applies to the form being shipped, or the composition data needed where no number exists. It is much easier to resolve before dispatch than at a border.
Sources
- CAS, CAS Registry — registration criteria for a chemical substance, including the requirement that it be described in the open scientific literature.
- CAS, Common Chemistry — free public subset of the registry, searchable by name, CAS number or formula.
- European Pharmacopoeia and USP–NF monographs — impurity designations and the nomenclature systems used by each pharmacopoeia. Current editions are subscription publications and should be consulted directly for any specification.
- ICH M7(R2) — framework for assessing and controlling mutagenic impurities, including the treatment of impurities without a full toxicological dataset.
- Catalogue figures quoted in this article are computed from the CATO reference standard inventory as of November 2026 and are stated as counts, not estimates.
Send the structure, not the number
13,678 reference standards are catalogued, and 7,654 of them have no CAS number at all. Send the parent drug and a drawing and we will confirm feasibility, state whether it is in stock, and tell you if the isomer you need is the one we hold.