How to Read a Peptide COA: A Field-by-Field Guide

What is a peptide COA?

A certificate of analysis is a testing laboratory's report on a sample from one manufacturing lot. Pharmaceutical quality control evaluates peptides with a series of complementary tests for identity, purity and strength¹. A research-grade COA is a shorter version: a header tying the report to a lot, then identity, purity and, on more complete certificates, net peptide content.

A third-party COA comes from a laboratory independent of the seller; an in-house COA comes from the vendor. Either can be accurate, but only a third-party report can be checked with someone who has no stake in the sale. This guide uses a real certificate from the Peptides Only COA library as its worked example: ipamorelin 10 mg, lot VNA7TB, reported on 12 May 2026 by Freedom Diagnostics Testing.

The header: lot number, dates and testing laboratory

The lot (or batch) number connects the paper to the vial, and it should match the vial label character for character. A certificate without one could describe any vial, which means it verifies none of them.

The example header shows the lot, received and reported dates (11 and 12 May 2026), an accession number and search code assigned by the lab, and the lab's website and email. A client field names whoever submitted the sample, often a manufacturer or distributor, and a closing statement says the results are specific to the sample provided, which is exactly why the lot match matters. Check the header for:

• Lot number: identical to the one printed on the vial.
• Testing laboratory: named and contactable.
• Dates: received and reported dates that fit the lot.
• Report identifier: an accession number the lab can look up.
• Product and label claim: the compound and the stated amount per vial.

Identity: how to read the mass spectrum

Mass spectrometry weighs ions rather than molecules and reports every signal as a mass-to-charge ratio (m/z). Peptides are normally ionized by electrospray, which produces intact ions carrying one or more extra protons; a hallmark of electrospray spectra is a family of multiply charged ions from the same molecule².

Three labels cover most certificates. [M+H]+ is the intact molecule (M) plus one proton, appearing at the molecular weight plus about 1. [M+2H]2+ carries two protons and two charges, appearing near (M + 2) divided by 2. [M+Na]+ is a sodium adduct about 22 Da above [M+H]+. Larger peptides tend toward higher charge states: in a published analysis of melanotan II vials, identity was confirmed by fragmenting the doubly charged [M+2H]2+ ion at m/z 513³.

On the example certificate, ipamorelin (molecular weight about 711.9) shows [M+2H]2+ at m/z 356.6, [M+H]+ at m/z 712.2 and [M+Na]+ at m/z 734.2, each within a few tenths of the predicted value; the small signal at 713.2 is the same ion carrying a naturally occurring carbon-13 atom. To check a certificate yourself, take the molecular weight from the product specification and add 1 for [M+H]+, or add 2 and halve it for [M+2H]2+. Agreement within about 1 Da is normal, since labs may quote monoisotopic or average mass.

Larger gaps mean something. A shift of 16 suggests an added oxygen, and a shortfall equal to one amino acid residue suggests a deletion sequence; both are recognized synthesis-related impurities⁴. Identity testing also has a blind spot: isomers and epimers have the same mass, so a matching spectrum shows the formula fits, not that every residue has the right configuration^1,5.

Purity: what the HPLC chromatogram shows

High-performance liquid chromatography (HPLC), most often in reversed-phase mode, is the standard method for separating and analyzing peptides⁶. Components separate on a column by hydrophobicity and a UV detector records each as a peak, with retention time in minutes on the x-axis and detector response (usually mAU) on the y-axis. Purity is normally reported as area percent: the main peak's area divided by the total area of all integrated peaks.

The example chromatogram shows one tall, narrow peak labeled ipamorelin at roughly 6.5 minutes on a flat baseline across the rest of the run, reported as 99.90%. Smaller peaks, where present, are usually related impurities. In synthetic peptides these include deletion and insertion sequences, incompletely deprotected side chains, oxidized residues, diastereomers and degradation products such as pyroglutamate and succinimide forms, and such impurities can influence early functional studies enough to produce erroneous conclusions⁴.

Area percent cannot tell you how much peptide is in the vial, or what else is there that the detector cannot see. In one published case, two commercially sourced custom peptides contained 20% and 43% w/w of undeclared mannitol, a UV-transparent compound of the kind LC-based checks are blind to, which quantitative NMR detected and measured⁷. A 99% figure describes the chromatogram, not the full contents of the vial.

Net peptide content vs gross weight

A lyophilized peptide is rarely the peptide alone. Peptides with charged side chains exist as salts paired with counter-ions⁸, and peptides made by solid-phase synthesis, cationic ones in particular, are typically obtained as trifluoroacetate (TFA) salts because trifluoroacetic acid is used in cleavage and purification; exchanging it for a counter-ion such as acetate is a separate step⁹. The powder also holds residual water. Gross weight is everything in the vial; net peptide content is the mass of the peptide itself.

The difference can be large. In a candidate reference material of angiotensin II, the TFA counter-ion alone accounted for nearly 25% of the mass, and the assigned peptide purity was 691 mg/g¹⁰. A mass-balance assignment for synthetic oxytocin, which subtracted related impurities, water, counter-ions, inorganic elements and residual solvents, arrived at 796.5 mg/g¹¹. Content can be measured by HPLC assay against a characterized standard, quantitative NMR or amino acid analysis, so the method should be stated^1,10.

The counter-ion also matters in cell work: TFA at 10 to 100 nM reduced cell numbers in fetal rat osteoblast cultures, and the TFA salts of several peptides produced misleading results compared with their hydrochloride salts¹². The example ipamorelin certificate reports a net content of 13.12 mg against a 10 mg label claim. Net content, not label weight, is the figure that says how much peptide the vial actually holds.

Endotoxin, sterility and heavy metals

HPLC and mass spectrometry cannot see bacterial endotoxin, live microorganisms or trace metals; each needs its own test. Endotoxin is measured with the Limulus amebocyte lysate (LAL) assay, which became established in pharmaceutical quality control as an alternative to the rabbit pyrogen test¹³. It matters in experiments: proinflammatory effects once attributed to a recombinant C-reactive protein preparation were shown to be consistent with bacterial contamination, with 46.6 EU of endotoxin activity per mg against 0.9 EU/mg in purified natural protein¹⁴.

Heavy metals such as lead, arsenic, cadmium and mercury are measured by inductively coupled plasma mass spectrometry (ICP-MS), valued among elemental-impurity techniques for its sensitivity at trace levels¹⁵. Where these tests appear, look for numeric results (EU/mg for endotoxin, µg/g or ppm for metals), the method and the limit applied; a bare "pass" tells you little. The example certificate covers identity, purity and net content; the other tests are separate analyses, reported as separate lines or documents where run.

Red flags on a peptide COA

Independent testing of peptide products bought online shows why these checks matter: melanotan II vials from three web shops, each claimed to hold 10 mg, contained between 4.32 and 8.84 mg³. Most reused or fabricated certificates fail on basics. Watch for:

• No lot number, or a lot number that does not match the vial label.
• No named laboratory, or a lab with no website, address or contact details.
• A purity figure with no chromatogram, or a chromatogram with no axes, retention times or peak labels.
• An identity result of "confirmed" with no spectrum and no m/z values.
• An observed mass that misses the expected ion by more than about 1 Da.
• The same purity, chromatogram or dates on every lot; genuine results vary slightly from lot to lot.
• Signs of editing: mismatched fonts, misaligned fields, text sitting over other text, or dates that contradict the lot.

How to verify a COA with the issuing lab

Find the laboratory independently, by searching its name rather than following a link in the PDF, and check that it is a real analytical lab with contact details. Then use the report identifiers: the example certificate gives an accession number, a search code and the lab's website. Where a lab offers no online lookup, email it the accession number, lot, product and report date and ask it to confirm the result. A genuine laboratory can tell you whether it issued a report.

It is also fair to ask what the lab is accredited for (ISO/IEC 17025 is the general standard for testing-laboratory competence). For Peptides Only lots, the COA library has a lookup for the lot number printed on your vial, and the quality standard page explains what each check is for. If you are comparing vendors, the supplier checklist puts these questions in order.