How to Read a Certificate of Analysis for Peptides: HPLC and Mass Spec Explained

A Certificate of Analysis (COA) is the primary document that tells a laboratory what is actually inside a vial. For research peptides it answers two independent questions: is this the correct molecule (identity), and how much of the material is that molecule rather than something else (purity). Identity is established by mass spectrometry; purity is established by high-performance liquid chromatography. A COA that reports only one of the two is incomplete. Every ENOS Labs USA lot is third-party tested in an independent analytical laboratory, and the COA for each lot is issued by that lab rather than by us.

Start at the header block. A usable COA names the compound and its scientific name, a unique lot or batch number, the nominal quantity per vial, the date of analysis, and the name of the testing laboratory. The lot number is the single most important field: it is what lets you tie an experiment, a vial in the freezer, and a purity figure together six months later. If the lot number on the COA does not match the number printed on the vial label, the document does not describe your material and should not be used to qualify it.

Next, read the HPLC section. Reverse-phase HPLC separates the components of a sample by hydrophobicity, and the detector — usually UV absorbance at 214 nm, where the peptide backbone absorbs — produces a chromatogram of peaks over time. The large peak is the target peptide. Purity is reported as the area of that main peak divided by the total area of all peaks, expressed as a percentage. A result reported as 99.1% by HPLC at 214 nm means the main peak accounts for 99.1% of the integrated UV signal.

The peaks that are not the main peak matter as much as the number itself. Small peaks eluting close to the target are typically closely related impurities from synthesis — deletion sequences missing a residue, incompletely deprotected intermediates, or diastereomers. Peaks well away from the target are usually unrelated species such as residual scavengers or solvents. Broad, drifting baselines or a main peak with a visible shoulder suggest partial degradation or the presence of an isomer that the gradient did not fully resolve. This is why a chromatogram image on the COA is more informative than a bare purity percentage: two lots reporting 98% can have very different impurity profiles.

Also check the HPLC method conditions, usually printed under the chromatogram: column chemistry (commonly C18), the mobile-phase gradient (typically water and acetonitrile with 0.1% trifluoroacetic acid), flow rate, detection wavelength, and run time. Purity figures are only comparable across lots or suppliers when the methods are comparable. A short, steep gradient can co-elute impurities with the target and inflate apparent purity, while a longer gradient resolves them and reports a lower — more honest — number.

Now move to the mass spectrometry section, which confirms identity. For peptides, electrospray ionization (ESI-MS) or MALDI-TOF is used to measure the mass-to-charge ratio of ionized molecules. The COA reports a theoretical (calculated) molecular weight derived from the amino acid sequence and an observed (found) mass from the instrument. Verification means the two agree within the instrument's stated tolerance — typically within about 1 Da for ESI on a peptide of a few thousand daltons, and tighter on a high-resolution instrument.

Interpreting the observed mass requires knowing which form is reported. ESI spectra show protonated species: [M+H]+ is one mass unit above the neutral monoisotopic mass, and larger peptides frequently appear as multiply charged ions such as [M+2H]2+ at roughly half the m/z, or [M+3H]3+ at roughly a third. A reported m/z near half the expected mass is normally a doubly charged ion, not a fragment or the wrong compound. Sodium and potassium adducts appear at roughly +22 and +38 Da relative to the protonated species and are common in samples handled with glass or saline.

A mass that is off by a characteristic amount is diagnostic rather than random. A deficit of about 18 Da suggests loss of water, often cyclization or anhydride formation. An excess of about 16 Da suggests oxidation, most commonly at a methionine residue. A deficit matching the residue mass of a single amino acid indicates a deletion sequence from incomplete coupling during synthesis. A doubling of the expected mass, minus two hydrogens, indicates a disulfide-linked dimer. When the observed mass does not reconcile with the theoretical mass by one of these known modifications, treat the identity as unconfirmed and request re-analysis.

Several supporting fields round out a complete COA. Water content, measured by Karl Fischer titration or loss on drying, matters because lyophilized peptide mass includes residual water — a high water figure means less peptide per vial than the label mass implies. Counterion content, usually acetate or trifluoroacetate, likewise contributes to the total weighed mass; TFA in particular can affect cell-based assays and is worth checking if you plan cell work. Peptide content or net peptide assay reports the fraction of the vial mass that is peptide after water and counterion are accounted for, and it is the figure to use when calculating a precise molar concentration. Where relevant, bacterial endotoxin (LAL) and sterility results appear as well. Appearance should read as a white to off-white lyophilized powder or cake; a discolored or collapsed cake is a handling and stability flag independent of any number on the page.

Finally, confirm independence and traceability. An informative COA identifies the analytical laboratory that ran the tests, is dated, and is signed or electronically issued by that laboratory. Third-party testing means the lab has no commercial interest in the result — the value of the document comes from that separation. ENOS Labs USA operates out of the Phoenix metropolitan area in Gilbert, Arizona, and each lot we release ships with its own third-party COA tied to that lot number, so the purity and identity data you receive describes the exact vial in your hand rather than a representative batch.

In practice, a five-minute review covers it: match the lot number to the vial, read the HPLC purity percentage and look at the shape of the chromatogram, reconcile the observed mass against the theoretical mass accounting for charge state and adducts, check water and net peptide content before calculating concentrations, and confirm the issuing laboratory and date. If any one of those five checks fails, the material is not qualified for use in an experiment whose results you intend to trust.

All materials described are for in-vitro laboratory research only and are not for human or veterinary use.