How to read a Certificate of Analysis (COA)

A Certificate of Analysis (COA) is the analytical record that documents what is actually in a research peptide reagent vial. Reading one correctly lets qualified laboratories distinguish verified material from unsupported marketing figures.

HPLC purity expressed as area percent

The headline figure on most peptide COAs is chromatographic purity from reversed-phase high-performance liquid chromatography (RP-HPLC), reported as area percent. The instrument separates the target peptide from related impurities by hydrophobicity, and a UV detector (commonly 214 or 220 nm, where the peptide bond absorbs) records a chromatogram. Purity is the area of the main peak divided by the total integrated area of all peaks, expressed as a percentage. A figure such as 98 percent area means the main peak accounts for 98 percent of the detected absorbance, with the remainder being truncated sequences, deletion or insertion analogues, oxidation products, or scavenger residues. A credible COA shows the actual chromatogram, the column, gradient, flow rate, and detection wavelength, not just a number. Note that area percent is a relative measure within one detection method; it is not the same as absolute mass purity. Reviewers should check the integration baseline and whether co-eluting peaks could be hidden under the main peak.

Mass spectrometry confirms identity

Purity describes how much of a sample is one substance; mass spectrometry (MS) confirms that the substance is the intended peptide. An LC-MS or MALDI-TOF analysis ionises the molecule and measures its mass-to-charge ratio, allowing calculation of the observed monoisotopic or average molecular weight. The COA should list the theoretical molecular weight derived from the amino acid sequence and the observed mass, which should agree within the instrument's tolerance. For multiply charged electrospray spectra, several charge states deconvolve to one parent mass. A matching mass strongly supports correct identity and flags gross errors such as a wrong sequence, missing residue, or unexpected adduct or modification. MS does not by itself quantify purity, and a clean mass can still accompany a contaminated sample, which is why MS and HPLC are reported together. A rigorous COA includes the raw spectrum or deconvoluted mass, the ionisation mode, and the calculated versus found values so a reviewer can verify the arithmetic against the published sequence and formula.

Net peptide content and water content

Lyophilised peptide powder is rarely 100 percent peptide by mass. Net peptide content (peptide content assay) reports the fraction of the dry mass that is actual peptide backbone, with the balance made up of bound water, residual counter-ions such as trifluoroacetate or acetate salts, and other non-peptide material. It is typically determined by amino acid analysis or nitrogen content and is distinct from HPLC purity: a sample can be 98 percent pure by HPLC yet only 80 percent peptide by mass because the rest is salt and water. Water content, often measured by Karl Fischer titration or loss on drying, quantifies residual moisture from lyophilisation and affects stability and accurate mass calculations. Both figures matter when laboratories need to know the true quantity of compound present. A COA that reports purity alone, omitting net peptide and water content, gives an incomplete picture of how much target molecule a vial actually contains for reconstitution and experimental calculations.

Batch identifiers and named third-party labs

Traceability separates a meaningful COA from a generic template. Every legitimate certificate carries a unique batch or lot identifier that ties the document to the specific production run in the vial, plus the compound name, a CAS number where one exists, the test date, and ideally a retest or expiry reference. The lot number lets a laboratory request the matching COA and confirm that the paper describes the material in hand rather than a representative sample. The most important credibility signal is who performed the analysis. A named, independent third-party laboratory (for example, an ISO-accredited contract lab) that issues the report on its own letterhead is far stronger evidence than vendor-internal figures with no traceable source. Vendor-only numbers are not worthless, but they are unverified self-reporting. Qualified researchers should cross-check the lot on the certificate against the vial label, confirm the testing laboratory is identifiable, and treat any COA without a lot number or named analyst as unverifiable.

Related guides

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