What a CAS Registry Number is
A CAS Registry Number is a unique numerical identifier assigned to a chemical substance by the Chemical Abstracts Service, a division of the American Chemical Society. Each registered substance receives exactly one number, and each number points to exactly one substance, which makes it a precise way to refer to a compound without depending on a name. Names vary between catalogues, languages and conventions, and a single peptide is often written several different ways; the registry number does not move. More than 200 million substances are recorded in the CAS REGISTRY, each with its own number, molecular formula and, where a structure is defined, a recorded structure. For a laboratory sourcing a reference material, the registry number is the anchor that ties a label to one specific, documented substance rather than to a name that several different materials might share.
How the number is built, and the check digit
A CAS number is written in three hyphen-separated parts: a first block of up to seven digits, a second block of two digits, and a single final digit. BPC-157, for instance, is registered as 137525-51-0. The parts carry no chemical meaning. A number is sequential, assigned in the order a substance enters the registry, so it encodes nothing about structure, size or composition; it is a serial label, not a formula. The final digit is a check digit, calculated from the digits before it, which lets software catch a mistyped number. To compute it, the preceding digits are read from right to left and multiplied by 1, 2, 3 and so on, and the products are added; the check digit is that sum modulo 10, meaning the remainder after dividing by ten. For 137525-51, reading the eight digits before the check digit from right to left and weighting them gives 1, 10, 15, 8, 25, 42, 21 and 8, which sum to 130; 130 modulo 10 is 0, matching the final digit of 137525-51-0. A number that fails this test has been transcribed incorrectly and should be rechecked against a primary registry before it is recorded.
What a CAS number confirms, and what it cannot
A CAS number fixes which substance a document is talking about. It does not, on its own, confirm what is actually inside a particular vial. The number is an identifier for a substance in the abstract; a specimen on a bench is a physical material that has to be measured. A label can print a CAS number, but only analysis of that specific lot shows whether the material in the vial matches it. This is the gap that identity testing closes, and it is why a registry number on a certificate carries weight only when it sits next to a measurement.
A second limit is that one name can map to more than one CAS number. A peptide is frequently registered both as the net peptide, sometimes called the free base, and as a salt, most often the acetate salt, since acetate is a common counterion. The two forms share a sequence but differ in mass, because the salt carries additional counterion atoms, and each has its own registry number and its own molecular weight. Stereochemistry can split a name the same way, where a D-configuration residue and its L-configuration counterpart are different substances with different numbers. Because of this, the exact registry number recorded for a lot, read together with the molecular formula, says more than the name alone.
Molecular formula and molecular weight as identity anchors
Alongside the registry number, two chemical facts anchor a substance’s identity: its molecular formula and its molecular weight. The molecular formula is the count of each kind of atom in one molecule; for BPC-157 it is C62H98N16O22, meaning 62 carbon, 98 hydrogen, 16 nitrogen and 22 oxygen atoms. From the formula a mass can be calculated in two ways, and the distinction matters when a measurement is compared against it. The average molecular weight sums the standard atomic weight of each element, which is itself an abundance-weighted average across that element’s natural isotopes; this is the figure most catalogues and certificates print. The monoisotopic mass instead sums the mass of the single most abundant isotope of each element, and it is the figure a high-resolution instrument reports for the intact molecule. For a peptide these two numbers are both correct but not identical: they differ by roughly one Dalton at this size, because heavier isotopes such as carbon-13 pull the average upward. A careful comparison therefore matches like with like, a monoisotopic measurement against a monoisotopic calculation, rather than reading one figure against the other.
Verifying identity against a Certificate of Analysis
Identity verification joins these pieces into a single chain. It begins with the intended substance: from a primary registry such as PubChem or CAS Common Chemistry, the CAS number and molecular formula are confirmed, and the theoretical mass is calculated from the formula. The laboratory then measures the material. High-performance liquid chromatography (HPLC) separates the specimen into its components and records each as a peak; the area of the main peak relative to the total is reported as an area percentage, and the absence of unexpected peaks is part of the reading. Mass spectrometry supplies the identity step: liquid chromatography-mass spectrometry (LC-MS) and high-resolution mass spectrometry (HRMS) measure the intact mass and set it against the theoretical value. Lower-resolution LC-MS places the mass to within about a Dalton, enough to tell one peptide from another; HRMS places it to within a few parts per million, enough to distinguish substances that differ only slightly. When the measured mass matches the mass calculated from the registered formula, within the tolerance of the instrument, the specimen’s identity is confirmed.
A Certificate of Analysis records this chain so that someone else can retrace it. A complete certificate names the compound, its CAS number and molecular formula, the theoretical and observed masses, the main-peak area, a batch or lot number tying the figures to one production run, and the method and instrument used. A reader can take the CAS number printed on the certificate, look it up in a public registry, confirm the formula and the mass that formula implies, and check that the observed mass on the certificate agrees. That is what makes the document verifiable rather than merely asserted. The practice of reading one is set out in the guide on how to read a peptide Certificate of Analysis, and the two measurement techniques are compared in HPLC versus mass spectrometry.
University-core testing and an independently checkable record
The identity work behind every batch supplied here is carried out at the USC Alfred E. Mann Multi-Omics Mass Spectrometry Core, a university laboratory, using LC-MS and HRMS. The Certificate of Analysis for each batch is published rather than kept on request, which is what lets the chain above be followed from the outside: the registry number, the molecular formula, the calculated mass and the measured mass are all on the page, open to anyone who wants to check them against a primary source. How each run is documented is described in how every batch is documented, and the collected certificates are held in the COA library. A worked single-compound example is the BPC-157 research profile, whose registry number, 137525-51-0, is the one checked against the check-digit rule earlier in this guide. The full range of reference materials is on the shop page.
For laboratory research use only. Not for human or veterinary use.
Frequently asked questions
What is a CAS Registry Number?
A CAS Registry Number is a unique numerical identifier assigned to a chemical substance by the Chemical Abstracts Service, part of the American Chemical Society. Each registered substance has exactly one number, and each number refers to one substance, so it identifies a compound precisely regardless of how its name is written. BPC-157, for example, is registered as 137525-51-0.
Does a CAS number prove what is inside a vial?
No. A CAS number identifies a substance in the abstract; it does not show what a particular lot actually contains. Confirming that the material in a given vial matches its stated number requires analysis of that lot, typically HPLC for the main-peak area and mass spectrometry for identity, recorded on a Certificate of Analysis.
Why can one peptide name correspond to more than one CAS number?
A peptide is often registered both as the net peptide (free base) and as a salt, most commonly the acetate salt, and these forms differ in molecular weight because the salt carries extra counterion atoms. Stereochemistry can also split a name, since a D-configuration residue and its L-configuration counterpart are different substances. Each form has its own registry number, so the exact number recorded for a lot, with the molecular formula, is more specific than the name.
How is molecular weight used to confirm identity?
The molecular formula gives a theoretical mass, which is compared against the mass a laboratory measures by mass spectrometry. The average molecular weight (summed from standard atomic weights) and the monoisotopic mass (summed from the most abundant isotope of each element) differ by about a Dalton for a peptide, so a careful comparison matches a monoisotopic measurement to a monoisotopic calculation. When the measured and calculated masses agree within the instrument's tolerance, identity is confirmed.
How can I check a CAS number against a Certificate of Analysis myself?
Take the CAS number printed on the certificate and look it up in a public registry such as PubChem or CAS Common Chemistry. Confirm that the molecular formula matches, read or calculate the mass that formula implies, and check that the observed mass on the certificate agrees with it. Because livvmore publishes its certificates, every figure in that chain is on the page and open to this kind of independent check.