A section-by-section guide to peptide COAs: mass-spec identity, HPLC purity, net content, lot numbers and the red flags of recycled certificates, walked through on real published Northbridge lots.
A certificate of analysis is the only document that connects the powder in a vial to measured data. Read well, it tells you what the material is, how much of what you can see on a chromatogram is the target peptide, and how much peptide the vial actually holds. Read carelessly, it is a reassuring page with a large percentage on it. This guide goes through a peptide COA one section at a time, using real certificates from our own COA library as worked examples, so every number discussed can be checked against the published document.
A COA reports the results of specific tests on a specific sample. For peptides, the core attributes are the same ones pharmacopeial monographs are built around: identification, purity (related substances) and content, or assay [1]. What matters as much as the results is who produced them. An in-house certificate is issued by the seller, who has an obvious interest in the outcome. An independent third-party certificate comes from an analytical laboratory that received a sample, ran the tests and signed the report under its own name. A useful COA names that lab, gives a way to contact it, and carries a report or accession number the lab can look up.
The header should identify the testing laboratory, the client who submitted the sample, the product and labelled size, a lot or batch number, and at least one date. Two dates are better than one: the date the sample was received and the date results were reported. The lot number is the most important field on the page, because it is the only thing that ties the certificate to a particular production run rather than to a product name in general.
Identity is usually confirmed by mass spectrometry, often coupled to the HPLC as LC-MS. The instrument measures mass-to-charge ratio (m/z), and the result is compared with the mass calculated from the peptide's sequence. With electrospray ionization, a large peptide picks up several protons and appears as a family of multiply charged ions rather than a single peak [3]. Each ion's m/z is the neutral mass plus n protons, divided by n, so several charge states that all back-calculate to the same mass are strong evidence of one intact molecule. A good certificate either prints expected and observed mass side by side or shows a labelled spectrum from which you can do the arithmetic.
Purity is almost always measured by reversed-phase HPLC with UV detection, which separates the target peptide from closely related species largely by hydrophobicity [2]. The chromatogram plots detector response against retention time. The reported purity is the area of the main peak as a percentage of the total integrated peak area. Small peaks either side of the main one are impurities, and in synthetic peptides these are typically deletion or insertion sequences from incomplete coupling or deprotection steps, oxidized or racemized forms, protection-group adducts and degradation products [4].
HPLC purity is a relative number. It tells you what fraction of the UV-absorbing material that eluted is the main peak. It does not tell you how much peptide is in the vial, and it cannot see water, most salts or counter-ions. An impurity that co-elutes with the main peak is counted as product, which is one reason mass confirmation and purity belong on the same certificate. A figure above 99% is meaningful; it is not a statement that the vial contains nothing else.
This is the most commonly misunderstood part of a COA. A lyophilized peptide is a solid that contains the peptide itself, counter-ions and some residual moisture. Because many residues carry charge, synthetic peptides exist as salts, commonly trifluoroacetate from synthesis and purification, or acetate after exchange [4][5]. The counter-ion affects the peptide's physicochemical properties and adds weight that is not peptide [5]. Gross weight is everything in the vial; net peptide content is the part that is actually peptide. Compendial specifications list purity and content as separate attributes for this reason [1]. A vial can be 99.5% pure by HPLC and still hold more or less peptide than the label, so if a certificate reports content, read it separately from purity.
Appearance is a simple visual check, usually recorded as a white lyophilized powder. It is not sophisticated, but a result that does not match the material in hand is worth asking about. The method line tells you how the numbers were produced: the separation technique, the detector and whether mass spectrometry was coupled to it. A certificate with results but no method is harder to interpret and harder to trust.
Here is the certificate published for our Tesamorelin 10mg, lot TSM10001, as it appears in the COA library. (Tesamorelin is also the active ingredient of an approved prescription drug, Egrifta; the research material described here is not that product.)
Two things are worth noticing. First, the certificate reports net content as a mass, 12.01 mg in a vial labelled 10 mg, rather than as a percentage of powder weight. That is the more directly useful form: it is a measurement of the peptide present, reported separately from its purity. Second, identity is reported as a conclusion with the spectrum as evidence, not as a printed expected-versus-observed table. If you want that comparison written out, calculate the theoretical mass from the sequence and check it against the labelled ions, as above.
A certificate describes one sample of one lot at one fill. Our RT-3 is sold in two sizes, and each has its own lot and its own certificate: lot RT10001 (10mg) reported 99.87% HPLC purity, and lot RT30001 (30mg) reported 99.71%, both tested in August 2026. The figures differ because they are different samples, which is exactly what real testing produces. Across the lots currently published in our library, reported HPLC purity ranges from 99.01% to 99.95%. None reads 100%, and none should.
Identity, purity and content say nothing about bacterial endotoxin or microbial contamination. Endotoxin is measured separately, typically with a limulus amebocyte lysate (LAL) assay, and it matters most for cell-based and animal work. In one in-vitro study, endotoxin levels found in some commercially available recombinant proteins were enough to activate sensitive human dendritic cells, and the authors recommended screening reagents before use in LPS-sensitive assays [6]. Sterility testing applies to material that is labelled sterile. The Northbridge certificates described in this guide report identity, purity and net content; they do not report endotoxin or sterility, so labs running endotoxin-sensitive experiments should plan their own testing.
Every Northbridge batch is independently tested by a third-party laboratory, and certificates are published by lot in the COA library at northbridgerl.com/coa. Choose the product, find the lot that matches the label on your vial, and open the certificate PDF. Products do not ship with a printed COA; the online library is where certificates live. If your lot is not listed, or the certificate does not match what you expected, contact us and we will look into it.
Note: All Northbridge Research Labs products are sold strictly for laboratory research use only. They are not for human or veterinary use, and nothing in this guide describes use outside the laboratory.
What an HPLC purity figure measures and what it leaves out, how net peptide content differs from purity, and which impurities, from deletion sequences to racemized residues and TFA counter-ions, matter for research results.
Why independent laboratory testing matters for research compounds, what the common methods show, where in-house testing creates conflicts of interest, what ISO/IEC 17025 accreditation actually means, and how to verify a testing lab yourself.
Research Use Only: The information in this article is for educational and research purposes only. All products mentioned are intended for laboratory research use only and are not approved for human or veterinary use.