How to Read a Peptide Certificate of Analysis

A certificate of analysis (COA) is a lab report on one production lot. It states identity (is it the right molecule), purity (what share of the sample is that molecule) and sometimes peptide content (what share of the powder's weight is peptide). None of it says what a peptide does in a dog, cat or horse.

What does the identity line tell you?

Identity usually comes from mass spectrometry, which weighs molecules. The lab compares the mass it observed with the mass calculated from the peptide's sequence, the same comparison researchers use to name fragments and impurities (Stoppacher et al., 2013, Analytical and Bioanalytical Chemistry).

A match is a good sign, with limits. A paper on building peptide reference standards notes that mirror-image or same-mass amino acids can need extra techniques for full characterization (McCarthy et al., 2023, Pharmaceutical Research).

What does purity mean?

Purity usually comes from high-performance liquid chromatography (HPLC), which separates a sample into its components and reports the target peptide's share of the total peak area, the signal the detector sees. It is a relative figure. It compares the intended peptide with related material and says nothing about how much of the powder is peptide.

Synthesis leaves a mixed product. A review of related impurities in peptide medicines lists amino acids deleted, inserted or flipped to their mirror-image form, and chains with protecting groups left on. It also lists oxidation and dimers or larger assemblies (D'Hondt et al., 2014, Journal of Pharmaceutical and Biomedical Analysis). Truncated chains come from coupling steps that did not finish (Wunderlich et al., 2023, Journal of Peptide Science). Methionine can be oxidized during the final acid step that frees the peptide from its support (Nandhini et al., 2023, ACS Omega). One synthetic C-peptide study material held more than 65 related impurities, about 8% of its mass in total (Li et al., 2018, Analytical and Bioanalytical Chemistry).

Purity versus net peptide content

A sample with few related impurities can still be only about three-quarters peptide by weight. The remainder is water and counterions.

In a reference-material study of the peptide sublancin, related organic impurities made up 0.4% of the sample's mass, yet the peptide was 78.6% of it, because water accounted for 5.46% and trifluoroacetate ion for 15.48% (Chen et al., 2026, Journal of Chromatography B). A study of an angiotensin II reference material put trifluoroacetic acid at nearly a quarter of the mass (Melanson et al., 2018, Analytical and Bioanalytical Chemistry).

Trifluoroacetate is a leftover of the method. Trifluoroacetic acid is the cleavage agent and a purification aid in synthesis, so synthetic peptides are often obtained as trifluoroacetate salts (Erckes et al., 2025, Pharmaceuticals).

Counterions can matter beyond the weight. In a mouse model of autoimmune encephalomyelitis, two commercial preparations of one peptide with similar purity had peptide fractions of 45% and 72%, and the mice showed different average clinical scores. Onset came about five days earlier with the trifluoroacetate form than with the acetate form, though incidence and severity were similar. The authors wrote that peptide fraction matters as much as purity when peptides come from different sources (Boullerne et al., 2014, Journal of Neurochemistry). The abstract gives no group sizes. In test-tube work, salt form (acetate, hydrochloride or trifluoroacetate) changed how five antimicrobial peptides acted against Staphylococcus aureus, with no consistent pattern (Sikora et al., 2018, Amino Acids).

What have outside labs found in peptide products sold online?

A Belgian laboratory bought the ten most frequently encountered falsified peptide drugs from three suspected illegal internet pharmacies. It reported high variation in how much drug each product held and purity from 5% to 75% for cysteine-containing peptides. It also found arsenic and lead: one sample was contaminated with lead, and several samples held concentrations up to ten times the ICH toxicity limit for parenteral drugs, with arsenic confirmed as elevated (Janvier et al., 2018, Talanta).

A 2024 study of semaglutide sold online without a prescription completed test purchases from six illegal online pharmacies and received three vials. Measured purity ran from 7.7% to 14.37% against the 99% claimed on labels, semaglutide content exceeded labeled amounts by 28.56% to 38.69%, and endotoxin was detected in every sample (Ashraf et al., 2024, Journal of Medical Internet Research).

Both audits covered products sold to people by illegal or suspected illegal sellers, and neither examined a product sold for animals. Three vials is also a small sample. Even so, in the semaglutide study the 99% purity on the label sat far from the measured 7.7% to 14.37%, and the Belgian products varied widely in drug amount and purity, which is the case for asking to see the test report.

A checklist for reading any COA

A missing item does not make a report false. Each gap is a question to put to the seller.

• The lot number on the report matches the lot number on the vial.
• A test date, so you can tell a current report from an old one.
• A named method beside each result, such as HPLC for purity or mass spectrometry for identity, instead of a bare "pass".
• The chromatogram or mass spectrum itself, not only a percentage.
• The name of the lab that ran the test, with a way to reach it.
• A report specific to that lot, instead of a generic sheet reused across lots or products.
• Which results appear. If peptide content, water or counterion lines are absent, ask whether they were measured.

Pure Pet Peptide puts a QR code on every vial that opens the certificate of analysis for that lot.

What the research did not show

Not one paper here studied a dog, cat or horse. The work is analytical chemistry, product audits, laboratory mice and test-tube assays.

A COA does not show that a peptide does anything in an animal, or what would suit one. Identity and purity describe the powder, not the animal.

A COA lists only the tests that were run. Arsenic and lead turned up in the Belgian study and endotoxin in the 2024 one because someone looked for them, and a report with no such line says nothing either way.

The water and counterion figures above come from reference materials for single peptides. No paper here gives typical values for any peptide an owner might meet.

Where your veterinarian fits in

Bring the COA, or a photo of the vial's lot number and the QR link, to the appointment. Your vet, or a pharmacist your vet works with, can tell you which lines they would want to see. Whether any peptide product has a place in your animal's care is a veterinary decision, and a COA cannot make it for you.

Write down everything your animal currently receives, with dates, so the report can be read in context.

Where to go from here

The plain-language basics are in what are peptides: a pet owner's guide. Our quality page covers lot reports. For another look at how well labels match what is inside, see probiotics for dogs, which includes a label audit.