Peptide Purity and Certificates of Analysis: What the Data Means

A certificate of analysis, commonly abbreviated CoA, is a laboratory document reporting analytical test results for a specific batch or lot of material. For research peptides, a CoA typically reports two categories of information: purity, usually via high performance liquid chromatography, and identity confirmation, usually via mass spectrometry. Together these answer two separate questions: how much of the material in the vial is the intended peptide versus other substances, and does the molecular weight of the material match the expected peptide structure.

High performance liquid chromatography works by forcing a dissolved sample through a column packed with material that interacts differently with different molecules, causing them to separate and exit the column, or elute, at different times. A detector, typically measuring UV absorbance, records a signal as each separated component exits, producing a chromatogram: a plot of detector signal against elution time.

Each distinct peak in the chromatogram represents a chemically distinct component of the sample. Purity by HPLC is calculated as the area under the main peak, corresponding to the intended peptide, divided by the total area under all peaks in the chromatogram, expressed as a percentage.

Mass spectrometry ionizes a sample and measures the mass-to-charge ratio of the resulting ions, producing a spectrum with peaks corresponding to the molecular weight of the species present. For a peptide, the observed mass is compared against the theoretical mass calculated from its known amino acid sequence.

Most peptide purity analysis uses reversed phase high performance liquid chromatography. The stationary phase is a silica particle bonded with hydrophobic C18 alkyl chains; the mobile phase is a gradient that starts mostly aqueous and shifts toward acetonitrile. Peptides partition between the two phases according to surface hydrophobicity, so more polar species elute early and more hydrophobic species elute later.

An acid modifier, typically trifluoroacetic acid at around 0.1 percent, is added to suppress silanol interactions and keep peak shape sharp. Detection is usually ultraviolet absorbance at 214 nanometers, which reads the peptide bond itself, rather than 280 nanometers, which reads only aromatic side chains and would miss sequences lacking tryptophan and tyrosine.

Because detection is by absorbance at the peptide bond, area percent responds approximately to the number of peptide bonds present. A short truncation impurity therefore contributes less peak area per mole than the full length product, which is one structural reason area percent purity should be read as an estimate rather than an exact molar ratio.

Chromatographic purity describes the ratio of peptide related species detected by the method. Several categories of contaminant fall outside it entirely.

Counterion and residual water are not chromophoric peptide species and do not appear in the trace, yet they contribute to vial mass. Residual solvents from synthesis and purification require a separate gas chromatography method. Bacterial endotoxin requires a limulus amebocyte lysate or recombinant factor C assay. Bioburden and sterility are microbiological tests. Heavy metals require elemental analysis. None of these are covered by an HPLC purity figure, and a certificate that reports only purity is silent on all of them.

Aggregation is a further blind spot. Reversed phase conditions are denaturing and can dissociate aggregates that exist in the vial, so size exclusion chromatography is the appropriate method when aggregation state is the question.

A certificate of analysis is only as meaningful as its traceability. Useful documents identify the testing laboratory, the batch or lot number, the analysis date, the instrument method including column, gradient and detection wavelength, and include the raw chromatogram and mass spectrum rather than a summary number alone.

Weak documents share recognizable patterns: no lot number, no method details, a chromatogram image reused across products, a stated purity that does not match the visible integration, or a mass spectrum whose reported mass does not correspond to the sequence given. Third party testing, where an independent laboratory analyzes the batch, addresses the obvious conflict of interest in self reported data, and matching the lot number on the document to the lot number on the physical vial is the step most often skipped.

A CoA reflects the specific sample tested at the time of testing, from a specific manufacturing batch. It does not verify sterility, endotoxin levels, or that the vial a researcher receives is actually from the tested batch rather than a different one. Sterility and endotoxin testing are separate analytical processes, typically reported independently if performed at all, and researchers should not assume a purity and identity CoA implies these additional properties.

A CoA also cannot confirm storage history after leaving the testing laboratory. Degradation that occurs due to improper storage, temperature excursions, or extended time after reconstitution is not reflected in a CoA generated before those events occurred.