The CAS Registry Number — assigned by the Chemical Abstracts Service of the American Chemical Society — is the standard global identifier for a defined chemical substance. For peptide buyers, it is the most reliable cross-reference between catalogues, safety data sheets, customs paperwork and inventory systems.
What a CAS number identifies
A CAS number identifies a specific molecular entity in a specific salt and isotopic form. Two peptides with the same primary sequence but different counter-ions (acetate vs TFA) can be assigned different CAS numbers. A peptide acid and its corresponding amide are different substances and may carry different CAS numbers.
The number itself has no chemical meaning — it is purely an ascending registration index.
When a peptide has one
A CAS number is assigned when a defined substance is registered, typically because:
- It has been described in the published literature with a structure.
- It has been registered for commercial sale.
- It has been registered for regulatory or pharmacopeia listing.
Most named research peptides — semaglutide, tirzepatide, BPC-157, TB-500, melanotan-2, GHK-Cu, semax, selank, epitalon, the bradykinins, angiotensins, GHRPs, and so on — have CAS numbers. The number on the COA is the one assigned to the free base unless the salt form is explicitly noted.
When it does not
Several legitimate categories of research peptide do not have a CAS number:
- Custom synthesis sequences that have never been published or commercialised under that exact identity.
- Mass-spec calibration mixtures and protein digests (e.g. the BSA tryptic digest) that are mixtures rather than single substances.
- Newly disclosed analogues whose registration has not yet been processed.
- Internal research codes used before a public name has been assigned.
The absence of a CAS number is not a quality signal. A custom-synthesis peptide at ≥99% HPLC purity with a full COA is a defined research substance whether or not CAS has issued a registration.
Practical use
| Use | Notes |
|---|---|
| SDS / safety | Match the CAS on the SDS to the CAS on the COA before relying on the SDS hazard class. |
| Customs declaration | The CAS number, not the trivial name, is the customs-broker-friendly identifier. Peptide acetate salts often clear under their CAS even when the trivial name is unfamiliar. |
| Inventory / LIMS | Use the CAS as a primary key in inventory; trivial names drift, CAS numbers do not. |
| Cross-supplier comparison | Same CAS = same defined substance, regardless of vendor or salt notation. Use it to confirm two suppliers are quoting the same compound before comparing price. |
On Peptide Shop COAs
Every Peptide Shop COA lists the CAS number where one exists. Where it does not, the field is marked “N/A — defined substance, sequence and identity confirmed by ESI-MS” rather than left blank.
Cross-references
- How to read a peptide COA
- Peptide research glossary — entries on CAS number and counter-ion.