Which GLP-1 is in my vial? Blind GLP-1 identification for semaglutide, tirzepatide and retatrutide

How GLP-1 peptide testing names the peptide in a vial

To find out which GLP-1 is in a vial, a lab dissolves a sample and runs it by LC-MS: liquid chromatography followed by a mass spectrometer. The instrument measures the mass of the peptide molecules. The lab then compares that mass with the calculated mass of each candidate peptide. In a blind test the lab is not told what the label says, so it must check every candidate on its list.

This works because the common GLP-1 peptides differ in mass by at least 82 Da, and a label cannot change what a molecule weighs.

GLP-1 vials make up much of what buyers send for testing. In an interview published in September 2026, Finnrick’s chief executive said GLP-1 peptides make up 63% of its test requests. The FDA warns that counterfeit and illegally marketed GLP-1 drugs “could contain the wrong ingredients”. A blind identification checks one vial for that risk.

GLP-1 masses side by side

The table gives each peptide’s monoisotopic mass, calculated from its formula, and the ion a mass spectrometer sees when the molecule carries four extra protons. Cagrilintide is an amylin analogue. It is in the table because it is sold beside the GLP-1 peptides and blended with semaglutide.

Peptide	Class	Formula	Monoisotopic mass (Da)	[M+4H]4+ (m/z)	Closest in this table
Liraglutide	GLP-1 family	C172H265N43O51	3748.9465	938.2439	Semaglutide, 362.17 Da
Semaglutide	GLP-1 family	C187H291N45O59	4111.1154	1028.7861	Cagrilintide, 295.14 Da
Cagrilintide	Amylin analogue	C194H312N54O59S2	4406.2515	1102.5702	Semaglutide, 295.14 Da
Retatrutide	GLP-1 family	C221H342N46O68	4728.4718	1183.1252	Tirzepatide, 82.05 Da
Tirzepatide	GLP-1 family	C225H348N48O68	4810.5249	1203.6385	Retatrutide, 82.05 Da

The closest pair is retatrutide and tirzepatide, 82.05 Da apart, about 17,000 ppm of retatrutide’s mass. A high-resolution instrument measures within a few ppm. In a technical report from HALO, a column maker, semaglutide read 4111.1027 Da against 4111.1154 Da calculated: an error of 3.09 ppm, or about 0.013 Da. The smallest gap in the table is thousands of times larger.

So the mass reading is rarely the hard part. The weak points are whether the lab looks for every candidate, whether the list covers what might be in the vial, and whether the report shows the evidence. Retention time alone is weaker evidence: ICH Q6A does not accept a single retention time as specific identification. The guide to LC-MS peptide identity explains why.

What blind means in GLP-1 testing

Labs use the word blind for two set-ups:

• Label withheld. The lab is not told which peptide or how many mg the label claims. It must identify the peptide from the data. This is what a blind GLP-1 test means.
• Source withheld. The lab is not told who sold the vial. ACS Peptide Testing Labs, for example, describes blind testing as withholding the source vendor from the analyst, and its page says the analyst is also blinded to the expected result.

Withholding the label matters for two reasons. First, a targeted method only finds what it looks for. If a lab checks only the claimed mass, the report can say the claimed peptide is missing without saying what is there. Second, a known claim invites anchoring, because a result close to the label looks right. A result produced without the claim cannot lean on it.

Worked example: a vial sold as semaglutide that holds cagrilintide

Semaglutide and cagrilintide are often sold side by side, and both are long, lipidated peptides that can leave an HPLC column close together. Take a vial labelled semaglutide that in fact holds cagrilintide:

• A report that only checks the label can be wrong. A method that looks for semaglutide’s retention time, or that is told the label, can be pulled towards the claimed answer.
• The mass gap is large. Cagrilintide is 295 Da heavier than semaglutide. A report that prints the observed mass would show about 4406 Da where 4111 Da was expected, which no rounding explains.
• The candidate list decides what a blind test can name. A mass-based blind test that looks only for semaglutide, tirzepatide and retatrutide would report semaglutide as not detected, so the vial would not pass as semaglutide. It would not name cagrilintide unless cagrilintide was on its list.

Check that a report you rely on is its latest version. The guides to semaglutide testing and cagrilintide testing cover each side of a swap like this.

Labs that list a common GLP-1 peptide blind test

Janoshik and Ozcanium Analytics sell a blind test with a fixed list of three candidates; Uzorak lists one without naming its candidates. The table shows what each lab publishes on its own site, checked in September and October 2026.

Lab	Test as listed	Candidates named	What the result gives
Janoshik	Common GLP-1 peptide blind test (Semaglutide, Tirzepatide and Retatrutide)	Semaglutide, tirzepatide, retatrutide; a mixture is flagged	The peptide found, with mg and purity; the reports we read print no mass reading
Ozcanium Analytics	Common GLP-1 Blind Testing	Semaglutide, tirzepatide, retatrutide	A blind version of its identity, purity and milligram content test
Uzorak	BLIND GLP-1, in its standard mass and purity service	Not stated in the listing	HPLC-UV purity and amount

Janoshik lists no separate semaglutide, tirzepatide or retatrutide test, so those vials go through its blind test; a GLP-1 aggregate analysis can be added, and each report opens on its site with a QR code and key. Ozcanium’s sample report prints the ions it monitored next to the ions it detected. Uzorak includes raw data and a public report with the standard service.

How to read GLP-1 test results from a blind test

A blind result has four possible outcomes:

What the report shows	What it means	Next step
One candidate detected, the one on the label	The main peptide has the labelled peptide’s mass	Read purity and mg; identity is one question of three
One candidate detected, a different one	The vial holds a different peptide from the label	Keep the vial and report; test other vials from the lot
Two candidates detected	A mix: a product sold as a blend, or a mix-up	Check how the product was sold; see peptide blend testing
No candidate detected	None of the listed peptides is present above the stated limit	The vial may hold another peptide, or very little peptide; a standard identity test for a named suspect can follow

A complete blind result prints:

• every candidate on the list, with its calculated mass;
• for each detected candidate, the observed mass, the error in ppm and the charge states matched;
• for each candidate not detected, the limit below which it would not be seen;
• a statement that the label claim was withheld;
• mg of the peptide found, with its basis, if the test includes content.

What a blind GLP-1 test cannot tell you

• Peptides off the list. Mazdutide, survodutide, cagrilintide or a non-GLP-1 peptide will not be named unless the lab looks for them.
• Same-mass forms. D-amino-acid isomers of semaglutide have the same mass as the parent peptide. Chromatography is needed to separate them, so purity needs HPLC.
• Purity, endotoxin and sterility. These are separate tests unless the order includes them.
• Other vials. The result covers the vial tested. Vial-to-vial consistency is its own test.
• Safety. A test shows what is in the vial. It cannot say whether a product is safe to use.

Some searches for a GLP-1 test mean a test on a person. A vial test analyses the product and says nothing about the person who bought it.

How to order a blind GLP-1 test

1. Ask the lab for its candidate list, and check that it covers the peptides you suspect.
2. Leave the label claim off the order.
3. Send the sealed vials the quote asks for, plus unopened vials for endotoxin or sterility. See how to send peptides for testing.
4. Add purity and content if the blind test does not include them.
5. In a group test, let the organiser choose the vials (how to run a peptide group test).

The peptide testing price comparison lists labs’ published prices for this test with their check dates.

What HK Analytical does for blind GLP-1 identification

Our blind GLP-1 identification is an LC-MS test, and you do not tell us what the label says. We compare the measured masses with the calculated mass of each candidate. The report names the GLP-1 family peptide in the vial, or a mix, and how much, and lists every candidate it looked for. If you suspect cagrilintide, ask us before you send whether it is on the list, or order a standard identity test for it. We do not offer per-component blend analysis.

Purity (HPLC), endotoxin (LAL) and Sterility (USP <71>, 14 days) can go on the same sample. 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, in a signed record you can verify. Quotes are on request at /quote, and the example report shows a blind panel.