Lab Techniques · Jul 28, 2026
What a CAS Number Actually Tells You (and What It Does Not)
A CAS Registry Number is a unique identifier assigned by the Chemical Abstracts Service to a specific substance. It looks like `137525-51-0`, and it is the closest thing chemistry has to a universal serial number for a molecule.
It is genuinely useful, and it is routinely over-interpreted.
What it does tell you
Exactly which substance is meant. Peptide naming is chaotic: the same compound appears under trade names, code names, sequence descriptions and abbreviations, often on the same page. A CAS number cuts through that.
This matters most for closely related compounds. PT-141 (189691-06-3) and Melanotan II (121062-08-6) differ by 1 g/mol in molecular weight and are easy to conflate in prose. Their CAS numbers are unmistakably different.
A way to verify independently. You can look a CAS number up against public chemical databases and confirm the formula and mass match what a supplier states. If a listing gives a CAS number whose registered formula disagrees with the formula printed beside it, that is a real discrepancy worth asking about.
What it does not tell you
Nothing about purity. A CAS number identifies a substance in the abstract. It has no opinion about whether a particular vial is 99% or 60% that substance.
Nothing about what is in the vial. Printing a CAS number on a label is free. It is an assertion about intent, not a measurement of contents. Only analysis tells you what arrived.
Nothing about legal status. A CAS number is a chemical identifier, not a regulatory one. Every compound in this catalogue has one, and none of them is approved by Health Canada or carries a DIN.
Nothing about quality of manufacture. Two vials of the same CAS number can come from wildly different synthesis and purification.
The catalogue, for reference
| Compound | CAS | Molecular formula |
|---|---|---|
| BPC-157 | 137525-51-0 | C62H98N16O22 |
| GHK-Cu | 89030-95-5 | C14H24N6O4Cu |
| Ipamorelin | 170851-70-4 | C38H49N9O5 |
| CJC-1295 (no DAC) | 863288-34-0 | C152H252N44O42 |
| Melanotan II | 121062-08-6 | C50H69N15O9 |
| PT-141 | 189691-06-3 | C50H68N14O10 |
| Retatrutide | 2381089-83-2 | C221H342N46O68 |
| Selank | 129954-34-3 | C33H57N11O9 |
| Semaglutide | 910463-68-2 | C187H291N45O59 |
| Semax | 80714-61-0 | C37H51N9O10S |
| TB-500 | 77591-33-4 | C212H350N56O78S |
| Tesamorelin | 218949-48-5 | C221H366N72O67S |
| Tirzepatide | 2023788-19-2 | C225H348N48O68 |
One pattern worth noticing: the CAS numbers roughly track age. Semax (80714-61-0) and TB-500 (77591-33-4) were registered decades ago. Retatrutide (2381089-83-2) is recent, which is why the literature on it is thinner and why supplier quality varies more widely.
Related
Related reading
- Reading a Peptide Molecular Formula
C221H366N72O67S looks like noise. It is not: the sulfur predicts an oxidation risk, the size predicts the price, and the absence of a metal tells you what class of substance you have.
- Why a 5 mg Vial Costs More Than Another 5 mg Vial
Two vials, same mass, ten times the price. The answer is chain length, and it is visible in the molecular formula before you ask anyone.
- Why Two Suppliers Quote Different Purity for the Same Peptide
Synthesis route, deletion sequences, counterion content and the difference between HPLC purity and net peptide content — why 99% from one supplier is not 99% from another.
- How to Spot a Forged Certificate of Analysis
Eleven checks that separate a real third-party certificate from a PDF someone made in an afternoon. Run them on any supplier, including us.