The molecule a report is describing
Tirzepatide is a synthetic peptide of 39 amino acids with an amidated C-terminus. Its backbone is modelled on the GIP peptide sequence and it was designed as a dual GIP and GLP-1 receptor agonist analogue. For the analytical chemist, three structural facts matter far more than its pharmacology:
- Length. At 39 residues it is more than twice the length of BPC-157 (15). Every extra coupling step in solid-phase synthesis is another chance for a residue to be skipped.
- Two Aib residues. Positions 2 and 13 carry alpha-aminoisobutyric acid (Aib), a non-proteinogenic residue with two methyl groups on its alpha carbon. Aib is sterically crowded and couples slowly, which makes it a natural place for a deletion to occur.
- A lipidated lysine. The lysine at position 20 carries a C20 fatty diacid (eicosanedioic acid) joined through a gamma-glutamic acid and two short ethylene-glycol linker units. The molecule also has a second lysine, at position 16, that is not modified.
Written out, with the acylated lysine marked: Y-Aib-E-G-T-F-T-S-D-Y-S-I-Aib-L-D-K-I-A-Q-K*-A-F-V-Q-W-L-I-A-G-G-P-S-S-G-A-P-P-P-S-NH2. The side chain alone accounts for a large share of the molecule's hydrophobicity, and it is the reason tirzepatide behaves on a column very differently from an unmodified peptide of similar length.
Purity: what the HPLC number can and cannot say
The headline figure on almost every peptide certificate is reversed-phase HPLC purity, usually detected at 214 nm, where the detector responds mainly to peptide bonds. The figure is the main peak's area divided by the total area of peptide-related peaks under that one method. It is a relative number, not an absolute one: water, counter-ions and residual solvents are not in the denominator, and anything that co-elutes with the main peak is counted as main peak.
For tirzepatide that last point carries extra weight. Because the fatty-acid chain dominates retention on a C18 or C8 column, several closely related by-products differ from the target only by a residue or two in the backbone, and they can elute very close to it. A useful report therefore shows the chromatogram itself, not only the percentage. Look for a sharp main peak, a flat baseline on both sides, and any shoulders or small peaks just before or after it. Our guide to reading an HPLC chromatogram walks through what those features mean.
A report should also name the method: column type, detection wavelength, and how purity was calculated. A purity value with no stated method cannot be compared with any other report, including a later report on the same material.
Identity: proving the main peak is tirzepatide
Purity answers "how much of the sample is the main component". Identity answers "is the main component the right molecule". For a peptide of this size, two kinds of identity evidence are common, and they are not equivalent.
| Identity evidence | What it shows | What it can miss |
|---|---|---|
| HPLC retention-time match against a reference standard | The main peak elutes at the same time as an authentic reference run under identical conditions. | A closely related molecule with similar hydrophobicity could elute at nearly the same time. One dimension of evidence. |
| Intact mass by mass spectrometry (LC-MS or MALDI) | The observed mass agrees with the expected mass, which rules out most deletion sequences and missing side chains. | Isomers. Anything with the same formula has the same mass, including a side chain on the wrong lysine. |
| Peptide mapping (digest, then LC-MS) | Locates the side chain on the correct residue and confirms the sequence fragment by fragment. | Rarely offered for research materials; mostly seen in pharmaceutical development. |
Guidance written for pharmaceutical peptides recommends combining at least two orthogonal identification methods for exactly this reason. Research materials are not held to drug standards, but the yardstick is a useful way to judge what a given report does and does not establish. The trade-offs are covered in more depth in HPLC purity and mass-spectrometry identity, explained.
What a tirzepatide mass result should look like
At roughly 4.8 kDa, tirzepatide does not appear in an electrospray spectrum as a single peak at 4813. It picks up several protons and shows a series of multiply charged ions. Working from the average mass of 4813.5, the expected signals fall at approximately m/z 1605.5 for the 3+ ion, 1204.4 for 4+ and 963.7 for 5+. Software then deconvolutes that series back to a single neutral mass, which is the number a report should print.
Two details help when reading one:
- Average versus monoisotopic. At this size the monoisotopic peak is small and the isotope envelope is broad, so many reports quote the average mass. A report should say which one it gives; the two differ by a few daltons for a molecule this large.
- Tolerance. A good report states the expected value, the observed value and the acceptance window. "Conforms" with no numbers is weaker evidence than the numbers themselves.
The impurities that are specific to this molecule
Because the structure is known, the likely failure modes are predictable. These are the ones a careful report, or a careful reader of a chromatogram, keeps in mind:
| Impurity class | Why it arises | How it shows up |
|---|---|---|
| Deletion sequences (des-Aib and others) | A slow or incomplete coupling, most likely at the hindered Aib positions. | Mass lower than the target by the missing residue (Aib is 85 Da); usually elutes near the main peak. |
| Non-lipidated backbone | The side-chain attachment step did not go to completion. | Large mass deficit; elutes much earlier than the main peak because it lost most of its hydrophobicity. |
| Side chain on the wrong lysine | Lysine 16 acylated instead of lysine 20. | Same mass as the target. Only chromatography or peptide mapping can tell them apart. |
| Aspartimide-related by-products | The two aspartic acid residues can cyclize during synthesis. | A species 18 Da lighter, plus ring-opened isomers at the same mass as the target. |
| Oxidation | The single tryptophan (position 25) is the oxidation-prone residue. | Peaks 16 or 32 Da heavier than the target, typically eluting slightly earlier. |
The same logic, applied to a different long lipidated chain, is in what a retatrutide report has to show.
Content: why 10 mg of powder is not 10 mg of peptide
A lyophilized peptide is never pure peptide by weight. The solid also holds bound water and counter-ions left from purification, commonly trifluoroacetate or acetate. A long peptide with several basic sites carries a meaningful counter-ion load. Purity says nothing about this: a sample can be highly pure by HPLC and still contain noticeably less peptide than the gross powder weight. A separate net-content line, with its method named, answers that question. Background is in net peptide content and label weight.
A reading checklist for any tirzepatide report
- The report names tirzepatide, the strength on the vial and one lot identifier, and that lot matches the vial label exactly.
- An independent laboratory issued it, with a certificate number you can check on that laboratory's own verification portal.
- The test date makes sense for the lot and is recent enough to describe current stock.
- Purity comes with a named method and a printed chromatogram, and the baseline around the main peak is clean.
- Identity is stated with its method. A retention-time match is real evidence; an observed mass that agrees with 4813.5 g/mol adds a second, independent line.
- Each claim on the report is read as covering the tested sample only. Nothing on it describes a different lot or strength.
How Arctic documents its materials
Arctic Lab Supply publishes lot-matched reports from ILS Laboratories in its COA library for seven products: GLP-3 (RT) 10 mg and 20 mg, BPC-157 5 mg, GHK-Cu 50 mg, Glow 70 mg, and Melanotan 1 and Melanotan II 10 mg. Those reports confirm identity by HPLC retention-time match against a reference standard and measure purity by HPLC, and each carries a certificate number that can be checked on the laboratory's portal. Batch documentation for the Tirzepatide 10mg listing appears in the same library when it is published, so check the library for the lot printed on your vial. Related long-chain materials are listed as Retatrutide 10mg and Semaglutide 10mg.
Questions
What are tirzepatide's CAS number, formula and molecular weight?
CAS 2023788-19-2, molecular formula C225H348N48O68, average molecular weight 4813.5 g/mol. It is a 39-residue synthetic peptide with a C-terminal amide, two alpha-aminoisobutyric acid (Aib) residues and a C20 fatty diacid attached through a linker to one lysine.
Is an HPLC purity number enough to identify tirzepatide?
No. A purity percentage says how large the main peak is relative to other peptide-related peaks under one method. It does not by itself show that the main peak is tirzepatide. Identity needs its own evidence, such as a retention-time match against a reference standard, an observed molecular mass, or both.
Why can a mass result miss some tirzepatide impurities?
Some likely impurities have exactly the same mass as the target. The clearest example is the fatty-acid side chain attached to the wrong lysine: the formula is unchanged, so the mass is unchanged. Isomers like this are separated, if at all, by chromatography or by peptide mapping, not by an intact-mass measurement.
Where is Arctic's batch documentation for tirzepatide?
Batch documentation for the Tirzepatide 10mg listing appears in the COA library at arcticlabsupply.com/coas when it is published. A report should only be applied to the same product, strength and lot printed on the vial.