How to Read a GLP-1 Peptide Certificate of Analysis, Field by Field

Insight · reviewed 2026-10-19 · by Pepti GLP-1 editorial

What a research-use COA is, and what it is not

A certificate of analysis for a research-use peptide is a one-page summary of the tests a manufacturer ran on a single lot before release. For a GLP-1-family raw material manufactured by Pepti, Guangzhou, the record lists the product, a lot alias, the nominal vial strength, the CAS number, the manufactured and test dates, a retest date, an identity result from mass spectrometry, a purity result from reversed-phase HPLC, and a conclusion line that reads "Meets specification".

Three things it is not. It is not a regulatory certificate: no agency issued it. It is not a full monograph test set: it usually omits water, counter-ion, residual solvent and content assays. And it is not a product-level document: it describes one lot, tested on one date.

Pepti publishes its batch records at peptioem.com/coa, 76 entries as of 2026-10-19, in one layout. Once you can read one, you can place any of them against the published purity band for its compound on the quality and batch records page.

The header block: product, lot alias, strength, CAS

The header carries four identifying fields, and each has a cross-check that takes a minute and saves a dispute later.

Product name. Match it to the compound you ordered and to the carton and vial labels. A research-use record uses the non-proprietary name, not a brand name.

Lot alias. A Pepti alias such as PEP260819-RT20 is structured: the digits after the prefix read as the manufactured date in year-month-day order, and the suffix names the compound and vial strength. The alias on the record must match the outer carton and inner labels character for character, and the date encoded in it must match the manufactured field lower down. A record whose alias is not on the physical goods is not the record for those goods.

Strength. The strength field (20 mg, 30 mg, 10 mg) is the nominal fill per vial: a label claim, not a measured value.

CAS number. Cross-check it against a public database before filing the record. The identifiers below are from PubChem and the NCATS GSRS; the formula and molecular weight are needed again at the mass-spectrometry line.

One caveat: a CAS search in PubChem does not return a hit for every newer peptide, because CAS numbers are attached as synonyms over time. If the CAS returns nothing, search by name or CID, or look up the UNII in GSRS, and note the route in your log. A CAS that resolves to a different molecule should stop the receipt.

The three dates: manufactured, tested, retest

Below the header are three dates. Read them in order and check that the order makes sense.

Manufactured date is the date the lot was completed, in practice the end of lyophilization and filling. It agrees with the date encoded in the alias, so PEP260819-RT20 was manufactured 2026-08-19.

Test date is when the release analyses were run. It must come after the manufactured date; on the worked example below it is 2026-08-22. A test date that precedes manufacture, or sits a long way after it unexplained, is a question to raise before accepting the goods.

Retest date is the most misread of the three. It is a re-examination trigger: the date after which the manufacturer expects the material to be tested again before the original result is relied on. It is not a statement that purity will hold until that date, and not an expiry in the sense used for finished products. When it passes, re-run identity and purity or ask for a re-examination record, and note which you did.

Commonly used storage conditions for lyophilized peptides are −20 °C for long-term storage and 2–8 °C for short periods. If the record states a condition, the retest date assumes it, and your storage log should show the material was held that way from receipt.

Appearance and other physical fields

Appearance is a visual check recorded at release: for these compounds, white to off-white lyophilized powder. It is a simple field but a meaningful one, because discoloration, a collapsed cake or visible particulates each point to a specific problem: oxidation, moisture ingress during freeze-drying, or foreign matter.

Solubility in water, recorded as a clear, colorless solution, may also appear. Its absence from a research-use record means the manufacturer did not include it in the release set for this format, not that the material failed it. If your specification needs it, ask before ordering, and inspect the vials yourself on receipt regardless.

Identity by mass spectrometry

The identity line reports the mass observed by electrospray mass spectrometry against the theoretical mass calculated from the molecular formula: for retatrutide C221H342N46O68 and about 4731 g/mol, for tirzepatide C225H348N48O68 and about 4813 g/mol, for semaglutide C187H291N45O59 and about 4114 g/mol. The record should print the theoretical value it used so you can check it against the public one.

Multiply charged ions

Peptides of this size pick up several protons and appear in an electrospray spectrum as a series of multiply charged ions, commonly z = 3, z = 4 and z = 5 for a molecule near 4731 g/mol, at m/z values equal to the neutral mass plus the attached protons, divided by the charge. The software deconvolutes that series to a neutral mass, which is what the record reports as observed. If raw m/z values are printed instead, check that at least two charge states deconvolute to the same mass.

What agreement means

On a well-calibrated instrument the observed and theoretical masses should agree to within a fraction of a dalton once you compare like with like: monoisotopic with monoisotopic, or average with average. Mixing the two is a common source of apparent mismatch. A difference of a whole dalton or more is not calibration noise and needs an explanation. Known shifts are diagnostic: +16 Da is oxidation of a methionine or tryptophan side chain, +1 Da is deamidation, and −18 Da is loss of water.

The isobaric limitation

Mass spectrometry confirms that the molecule has the right mass. It does not confirm the sequence, and it cannot distinguish species that share a mass: a D-amino-acid epimer, a sequence with two residues swapped, or a peptide with the fatty diacid on the wrong lysine all give the same observed mass. Those species are separated, if at all, by the chromatography, which is why identity and purity are read together.

Purity by RP-HPLC

The purity line is an area-percent result: the main-peak area, from a reversed-phase run with low-wavelength UV detection, divided by the total area of all peaks. Pepti records show results such as 99.26% for retatrutide lot PEP260819-RT20, 99.7% for tirzepatide lot PEP260722-TR30 and 99.6% for semaglutide lot PEP260718-SM20.

What the published bands tell you

Across the public library the HPLC results for each compound fall inside a band. The bands describe what the process has delivered; they are not the specification. The specification is the acceptance limit printed on the record, and a tirzepatide lot at 98.4% and another at 99.7% both meet it as long as both are above that limit. The difference is worth logging, but it is a difference within specification, not a pass next to a borderline case.

Impurity lines

Some records add "largest single impurity" and "total impurities". The first is the area-percent of the biggest peak other than the main peak; the second is the sum of all non-main peaks, what remains once the main peak is taken out. They show how the impurity burden is distributed: many small peaks and one peak close to the main one behave differently in a method transfer.

Two things the purity line does not tell you. It does not identify the impurities; naming a peak as deamidated or oxidized needs mass spectrometry on the eluting peak, which a routine release record does not include. And it says nothing about what the detector cannot see: water, counter-ion, salts and residual solvents are invisible at the peptide wavelength and are not part of the area-percent.

Fields often missing, and when to ask for them

Five fields that appear on fuller specifications are usually absent from a research-use record, and each has a reason you might ask for it.

• Water content (Karl Fischer). Lyophilized peptides are hygroscopic. Ask when you need to weigh material accurately as an analytical standard.
• Counter-ion or TFA content. Peptides purified by reversed-phase chromatography are usually isolated as trifluoroacetate salts, and TFA can be a substantial fraction of the powder mass. Ask when you need an accurate gross-to-peptide weight conversion.
• Peptide (net) content. The share of the gross powder that is actually peptide, after water and counter-ion are subtracted, usually measured by amino acid analysis or a quantitative UV assay. It is the figure people often assume "purity" means, and it is not on a routine research-use record. The independent Janoshik Analytical report #210105 shows the distinction: for a retatrutide sample labeled 20 mg it reports 19.52 mg of content and 99.837% purity, analyzed 2026-10-07, verifiable at verify.janoshik.com. Two measurements, two different questions; the result is sample-specific, not a claim about other lots.
• Residual solvents. Acetonitrile and other solvents left from purification and lyophilization. Ask when solvent carry-over would interfere with your analytical method.
• Bioburden and bacterial endotoxin. Specifications for other product formats, not part of a lyophilized research-use raw-material record. If a record for this format claims them, ask how and where they were measured.

A worked read-through: PEP260819-RT20

Open the record for PEP260819-RT20 and read it top to bottom with the goods in front of you.

1. Product and lot alias. Retatrutide, PEP260819-RT20. Confirm the alias on the outer carton, inner carton and a sample of vial labels. Decoded, the digits read 2026-08-19 and the suffix reads retatrutide, 20 mg.
2. Strength. 20 mg nominal per vial: the label claim, not a content measurement.
3. CAS. 2381089-83-2. Confirm against PubChem CID 171934787 or UNII NOP2Y096GV, by name or CID if the CAS search returns nothing.
4. Manufactured date. 2026-08-19, which agrees with the alias.
5. Test date. 2026-08-22, after manufacture, as it should be.
6. Retest date. Put the printed date in your calendar as a re-examination trigger. If it falls inside your planned holding period, raise it before accepting.
7. Appearance. White to off-white lyophilized powder. Hold a vial against a white surface, then a dark one, and confirm.
8. Identity. The observed mass should be reported as consistent with the theoretical mass for C221H342N46O68, about 4731 g/mol. Check the formula against the identifier table and note the mass convention used.
9. Purity. 99.26% by RP-HPLC. Against the public retatrutide band of 98.72%–99.55% it sits inside, toward the upper end, and above the acceptance limit printed on the record.
10. Conclusion. "Meets specification": the manufacturer's statement that every tested field met the acceptance limit on the record. It applies to the listed tests only.
11. Research-use line. The record closes with its statement of purpose: For research use only. Not for human or veterinary use. Copy it into your file so the document's scope travels with it.

What to write in the incoming-goods log

Record the lot alias exactly as printed, the strength, the CAS and the route that confirmed it, the three dates, the purity figure and method, the identity result with formula and theoretical mass, the conclusion line, the record URL, the date and initials of your check, the storage location and condition, and any discrepancy with the action taken. If you ran your own HPLC on receipt, write the result beside the supplier figure with your column and gradient, so a difference can be traced to method rather than material. The retatrutide page lists the strengths with public records.

Common misreadings

Treating HPLC purity as net content. A purity of 99.26% means the main peak accounts for that share of the chromatographic area. It does not mean a vial labeled 20 mg contains 20 mg of peptide, because water, counter-ion and salts are not in the chromatogram. The Janoshik example above, 19.52 mg of content against a 20 mg label at 99.837% purity, is the clearest public illustration.

Treating a representative COA as the shipped lot. A record for PEP260722-TR30 describes PEP260722-TR30. It is not evidence about the lot in your carton unless the alias matches. Ask for the lot-specific record before dispatch, not after.

Ignoring the retest date. Filing the record and forgetting the date is the most common failure. A lot past its retest date has not failed, but its release result no longer stands on its own until a re-examination is done.

Reading the published band as a specification. The tirzepatide band of 98.4%–99.7% describes 9 records; it is not the limit on any one of them. A lot at the lower end of the band is a passing lot. The tirzepatide and semaglutide pages show their bands the same way.

Reading mass agreement as sequence confirmation. Mass agreement confirms composition. Sequence is a separate claim that needs a separate method.

Sources and verification

• Pepti public COA library — 76 batch records as of 2026-10-19; purity bands quoted per compound
• Record PEP260819-RT20, retatrutide 20 mg — worked example: 99.26% by HPLC, manufactured 2026-08-19, tested 2026-08-22
• Record PEP260722-TR30, tirzepatide 30 mg — 99.7% by HPLC, manufactured 2026-07-22, tested 2026-07-29
• Record PEP260718-SM20, semaglutide 20 mg — 99.6% by HPLC, manufactured 2026-07-18, tested 2026-07-28
• PubChem CID 171934787, retatrutide — formula and computed molecular weight used for the identity cross-check
• NCATS GSRS, UNII NOP2Y096GV — substance record with sequence; alternative route when a CAS search returns nothing
• Janoshik Analytical report #210105 — content versus purity example; sample-specific result analyzed 2026-10-07

Related pages

• Quality and batch records
• Retatrutide raw material
• Tirzepatide raw material
• Semaglutide raw material

This article explains documents and procedures; it is not regulatory or medical advice.