Tirzepatide testing: purity, impurities, mg per vial and how published samples fail

What a tirzepatide test shows

A useful tirzepatide test gives three separate numbers: an identity result by LC-MS, a purity result by HPLC, and mg of tirzepatide per vial. Each answers a different question, and none of them stands in for the others. Endotoxin and sterility are extra tests that answer questions about contamination.

Test	What it shows	What it cannot tell you
Identity (LC-MS)	Measured mass against tirzepatide’s calculated monoisotopic mass, 4810.5249 Da	Purity or amount
Purity (HPLC)	Main peak as % of UV peak area, with the main impurities by retention time	Water, salts, mg in the vial
Content per vial	mg of tirzepatide as free base, against a reference standard, as % of label	Whether other vials hold the same
Blind GLP-1 identification	Which GLP-1 family peptide is present when the lab is not told the label	Peptides outside the candidate list
Endotoxin (LAL)	Endotoxin in EU per mg and EU per vial	Whether live microbes are present
Sterility (USP <71>, 14 days)	Microbial growth from the units tested over 14 days of incubation	That the whole batch is sterile

The tirzepatide compound page lists the formula (C225H348N48O68) and the ions an LC-MS run looks for: m/z 1604.5156 (3+), 1203.6385 (4+) and 963.1122 (5+), all calculated.

Tirzepatide test results in published data

Finnrick, a US testing platform that publishes community-submitted results, holds a large public set. In its data to 20 September 2026, tirzepatide had 2,478 published tests: 1,930 passed, 522 failed and 26 were incomplete. About 21% of classifiable samples failed. Retatrutide, with 3,858 tests, is the only product in its published table with more.

Finnrick’s default standard, Purity Focus, passes a sample only if the identity is right, purity is at least 99.5%, and the fill is within ±35% of the label. Overfill fails too. The counts do not say which of these each failed sample missed. The guide to peptide fail rates puts tirzepatide next to other products.

The FDA warns that illegally marketed tirzepatide and semaglutide could contain the wrong ingredients, too little, too much or none of the active ingredient. Identity and content tests are the ones that catch those failures.

Same batch, different mg

Content results depend on the lab’s reference standard and calculation basis. Krysia’s Substack (2 August 2026) sent one tirzepatide batch labelled 60 mg to three labs. One lab reported 62.3 mg. The second reported 70.99 and 71.63 mg on two vials, and the third 73.72 and 75.44 mg. That is a spread of about 20% on one batch. One of the labs pointed to method differences and to the lack of USP reference standards. This is one published case, not proof that any one lab is wrong. The guide on why labs disagree covers the causes.

Tirzepatide purity: what “99% pure” means

HPLC purity is the tirzepatide peak’s share of all UV-absorbing peaks, at a stated wavelength. A tirzepatide HPLC purity method separates the sample on a reversed-phase column, detects peaks by UV, and integrates their areas. A good report states the wavelength, shows the chromatogram and lists the peaks it integrated.

Three limits matter:

• Purity is not content. A vial at 99.6% purity can hold 30% less tirzepatide than the label says.
• Purity ignores what UV does not see. Water, salts and counter-ions such as TFA or acetate do not count.
• The threshold is a choice. Finnrick’s public standards use 99.5%. Its members-only strict standards use 99.8%, and its Accuracy Focus standard uses 98%. The same 99.3% sample passes one standard and fails another.

The guide on what purity means goes deeper.

Tirzepatide impurity analysis

Tirzepatide impurities show up as extra peaks on the HPLC trace. LC-MS can then put a mass on each one. A mass 16 Da above the parent suggests oxidation, and 1 Da above suggests deamidation. Truncated sequences sit below the parent mass.

Mass spectrometry has one blind spot. Isomers, such as a D-amino acid in place of an L-amino acid, have exactly the same mass as the parent peptide. A 2022 study in the Journal of Pharmaceutical and Biomedical Analysis examined such D-amino acid isomers in semaglutide. That is why identity and purity are two tests: LC-MS confirms the molecule, and the chromatography separates what has the same mass.

ELISA and other tirzepatide tests

Some searches ask about a tirzepatide ELISA test. ELISA is an antibody method, often used to measure a drug in blood samples. It does not separate impurities, so it gives no purity figure. Antibodies can also bind related molecules. For a vial of powder, LC-MS, HPLC and a content assay answer the questions buyers ask.

Identity by retention time alone has the same weakness in a different form. ICH Q6A says retention time alone “is not regarded as being specific”. Tirzepatide and retatrutide are long, lipidated peptides that elute in similar regions, so a peak at the expected time does not prove which one it is.

Mix-ups with retatrutide

Tirzepatide is 82 Da heavier than retatrutide (4810.52 against 4728.47 Da) and 699 Da heavier than semaglutide. One LC-MS run tells all three apart by mass. The FDA warning about wrong ingredients covers this risk, and a mass check is the direct way to catch a swap. The blind GLP-1 identification guide explains how a lab identifies the peptide without seeing the label.

How to get tirzepatide tested

1. Request a quote. List the product, the tests and the label claim, or leave the claim out for a blind test.
2. Send the number of vials the quote asks for. Endotoxin and sterility need their own unopened vials.
3. Keep vials sealed and numbered. The guide on how to get peptides tested walks through each step.
4. When the report arrives, open it on the lab’s own site, not from a forwarded copy.

Published prices from several labs are in the guide to peptide testing cost.

What HK Analytical does for tirzepatide

We test tirzepatide with any mix of identity (LC-MS), purity (HPLC), content per vial, blind GLP-1 identification, endotoxin (LAL), Sterility (USP <71>, 14 days), residual solvents, counter-ion (TFA, acetate), water content (Karl Fischer) and vial-to-vial consistency, in one order. Results within days of receipt. Sterility (USP <71>, 14 days) is the one test that takes 14 days, because the method incubates for 14 days. Every number comes with its raw data: the chromatogram behind purity and the mass spectrum behind identity. Each report is a signed record you can check at hkanalytical.com/verify. It shows our photo of the vial as received and states how the client is related to HK Analytical. Quotes are on request through the quote form.