"Identity: conforms" is the shortest line on a certificate and the one that carries the most weight. If the material is not what it says it is, nothing else on the page matters — purity of the wrong compound is not a useful number.
This post is about what that line rests on for PEP-RTA-3 specifically: the formula, the mass, how an instrument actually sees it, and — the part usually skipped — what a mass measurement cannot rule out.
The two numbers that define the molecule
Molecular formula: C221H342N46O68 Molecular weight: 4731.2 g/mol
That is a 39-residue acylated peptide. For scale, it is roughly a third the size of insulin and about forty times the size of a small-molecule drug. The formula is the authoritative identifier — it resolves in PubChem under CID 171390338 — and it is what the shorthand discussed in what "reta" means on a label does not give you.
All eleven of our published PEP-RTA-3 certificates report the same confirmed mass of 4731.2, across three batches and four vial sizes. In the two blend certificates two masses appear — 4731.2 for PEP-RTA-3 and 4409.1 for PEP-CAG-A — because a blend is two molecules, not one.
What the instrument actually measures
Here is the part that trips people up. A mass spectrometer does not measure mass. It measures mass-to-charge ratio, m/z.
To be measured at all, a neutral molecule has to be given a charge. Electrospray ionisation does this by adding protons, and a peptide of this size readily accepts several. Each added proton contributes about 1 Da of mass and one unit of charge, so:
| Ion | Charge | Approximate m/z |
|---|---|---|
| [M+3H]³⁺ | 3+ | ≈ 1578 |
| [M+4H]⁴⁺ | 4+ | ≈ 1183 |
| [M+5H]⁵⁺ | 5+ | ≈ 947 |
So a 4731 Da peptide shows up not as one peak at 4731, but as a series of peaks well below 1600 — which is also why a mass spectrum of a peptide looks nothing like what people expect from the molecular weight.
On certificate COA7580 those three ions are labelled on the spectrum. The arithmetic is straightforward: for the 4+ ion, (4731.2 + 4 × 1.008) ÷ 4 ≈ 1184.
Why the series is the evidence
A single peak at 1183 proves very little — plenty of things land near any given m/z. The confirmation is the consistency of the envelope.
Three independent ions, at three different charge states, each back-calculating to the same neutral mass, is a far stronger claim than any one of them. If the material were something else, the peaks would not space themselves correctly, and the deconvoluted mass would not land on 4731.2 from all three directions.
This is why "confirmed via LC-MS/MS" is meaningfully different from "identity: PEP-RTA-3". The first names a method that produces checkable structure; the second records that somebody wrote a name on a form.
The LC half matters too
The method on our certificates is LC-MS/MS — liquid chromatography feeding tandem mass spectrometry — and the chromatography is not decoration.
Running the sample through a column first separates components in time, so the mass spectrometer examines one thing at a time rather than everything at once. A blend makes this visible: certificate COA7594 reports two retention times, 11.268 and 11.783 minutes, because the two components came off the column separately and were measured separately. Had they co-eluted, the spectra would overlap and attributing masses would be guesswork.
The MS/MS part adds fragmentation: ions are broken apart and the fragment masses examined. Fragments carry sequence information in a way that an intact mass cannot, which is what moves the evidence from "a molecule of the right size" toward "a molecule with the right sequence".
What identity testing cannot tell you
This is the honest limit, and it is the reason identity is one field among five.
Mass cannot distinguish isomers. Two compounds with identical molecular formulas have identical masses. A peptide with two residues transposed has the same formula as the correct sequence. Intact mass will not separate them; only fragmentation, and only if the fragments differ in the right places.
Mass says nothing about stereochemistry. Swap an L-amino acid for its D form and the formula, the mass and the charge envelope are unchanged. Chirality is invisible to this method.
Identity is not quantity. A confirmed mass says the right molecule is present. It does not say how much — that is the content field, and as eleven certificates shows, content runs above the label on every lot we publish.
Identity is not purity. Confirming the main component says nothing about what else is in the vial. That is area percent by RP-HPLC, and it has its own blind spots.
Put together: identity, content and purity answer three different questions, and a certificate needs all three. Reading a PEP-RTA-3 certificate field by field covers how they fit together.
A specification check, in order
If you are checking material against a specification rather than reading a certificate for interest:
- Formula — does the document state C221H342N46O68, and does it match the compound you ordered?
- Mass — is 4731.2 reported, and is it confirmed rather than quoted from a catalogue?
- Method — is LC-MS or LC-MS/MS named? Tandem is stronger than intact mass alone.
- Charge states — does the spectrum show a consistent envelope, or a single unlabelled peak?
- Chromatography — is a retention time given, and for a blend, is there one per component?
- Lot linkage — does the lot on the certificate match the lot on the vial?
Fail any of the first three and you do not have an identity test. Fail the last and you have an identity test for someone else's vial.
Frequently asked questions
What is the molecular weight of PEP-RTA-3?
4731.2 g/mol, with the molecular formula C221H342N46O68. That figure appears on all eleven PEP-RTA-3 certificates published here and matches PubChem CID 171390338.
Why does the mass spectrum show peaks near 1183 instead of 4731?
Because the instrument measures mass-to-charge, not mass. A peptide this size picks up several protons during ionisation, so it appears as a series of multiply charged ions — around m/z 1578, 1183 and 947 for the 3+, 4+ and 5+ states. All three back-calculate to the same 4731.2.
Does a confirmed mass prove the sequence is correct?
Not on its own. Intact mass confirms composition; two sequences with the same amino acids in a different order have the same formula and the same mass. Tandem MS fragmentation is what begins to distinguish them.
Can mass spectrometry detect a D-amino acid substitution?
No. Stereoisomers have identical formulas and identical masses, so chirality is invisible to mass measurement. Detecting it requires a chiral separation or a method that is sensitive to three-dimensional structure.
Why do blend certificates list two masses and two retention times?
Because a blend contains two separate molecules. The reta/cagri blend reports 4731.2 for PEP-RTA-3 and 4409.1 for PEP-CAG-A, each with its own retention time, because the column separated them before the mass spectrometer measured them.
All material discussed is supplied for laboratory research use only. It is not for human or animal consumption, and nothing above describes a use in people.
References
- PubChem, National Library of Medicine. CID 171390338; molecular formula C221H342N46O68, molecular weight 4731. pubchem.ncbi.nlm.nih.gov/compound/171390338
- PubChem, National Library of Medicine. CID 171397054; C194H312N54O59S2, 4409. pubchem.ncbi.nlm.nih.gov/compound/171397054
- Coskun T, Urva S, Roell WC, et al. LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist. Cell Metabolism 2022;34(9):1234-1247.e9. doi.org/10.1016/j.cmet.2022.07.013
- Bioviridian Inc. Certificate of analysis COA7580, PEP-RTA-3 10 mg, lot RETA10-0803. coa.html
Every product mentioned is sold for laboratory research use only and is not for human or animal use. Nothing on this page describes or recommends use of the material sold here in humans or animals.
