How Peptide Purity Is Verified: HPLC, Mass Spectrometry and What a COA Actually Shows

Why a single 'purity' number is not enough

Synthetic peptides are produced through iterative coupling cycles, most commonly solid-phase peptide synthesis (SPPS), and every coupling step carries some probability of an incomplete reaction, an unwanted side reaction, or a deletion of a residue. The result is that a crude synthesis product is essentially never a single molecular species — it is a mixture of the target sequence together with truncated, deleted, oxidised or otherwise modified related substances. Purification and analysis exist to characterise and reduce that mixture to a documented, reproducible specification.

A single 'purity' percentage on a label is a shorthand for a much richer analytical picture. In a rigorous laboratory setting that number should be traceable to a specific chromatographic method, a specific detection wavelength, and an integration convention for what counts as an impurity peak versus baseline noise. Two different analytical methods, applied to the identical vial, can legitimately report different purity values because they are measuring different things — this is normal and expected, not a sign of fraud, provided the method is disclosed.

Reversed-phase HPLC: separating by hydrophobicity, not identity

Reversed-phase high-performance liquid chromatography (RP-HPLC) is the workhorse technique for peptide purity assessment. A sample is dissolved and injected onto a column packed with hydrophobic silica particles (commonly C18-bonded silica), and an increasing gradient of organic solvent — typically acetonitrile with a small amount of trifluoroacetic acid or formic acid as an ion-pairing agent — is pumped through the column. Peptides partition between the mobile phase and the stationary phase according to their surface hydrophobicity, so more hydrophilic species elute earlier and more hydrophobic species elute later.

A UV detector, usually set at 214 nm to capture the peptide backbone amide bond absorbance (rather than 280 nm, which depends on the presence of aromatic residues), records absorbance over time as a chromatogram. The area under the main peak, expressed as a percentage of the total integrated area of all resolved peaks, is the conventional RP-HPLC purity figure. Critically, RP-HPLC separates by physicochemical behaviour, not by molecular identity — two different impurities that happen to co-elute with the main peak will be invisible to this method, which is why HPLC purity is a necessary but not sufficient piece of evidence and is always paired with an identity-confirming technique.

Method parameters that a laboratory should record or request from a supplier include the column chemistry and dimensions, gradient profile, flow rate, detection wavelength, injection volume, and the mobile-phase modifier used. Without these details, a bare percentage figure cannot be meaningfully compared between suppliers or reproduced independently.

Mass spectrometry: confirming identity, not abundance

Where HPLC answers 'how much of a single hydrophobicity class is present', mass spectrometry answers 'what is the molecular mass of the species I am looking at, and does it match the expected sequence'. Electrospray ionisation (ESI-MS) and matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) are the two methods most commonly applied to synthetic peptides.

In ESI-MS, the peptide is ionised in solution, typically acquiring multiple charges, and a mass spectrometer measures the mass-to-charge ratio (m/z) of each charge state. Deconvolution software reconstructs the neutral monoisotopic or average molecular mass from the observed charge-state series. That measured mass is then compared against the theoretical mass calculated from the intended amino-acid sequence. A match within a small tolerance (commonly a fraction of a Dalton to a few Daltons depending on instrument resolution) confirms that the dominant species present has the correct molecular formula — though mass alone cannot distinguish sequence isomers or stereoisomers that share an identical mass.

MALDI-TOF works differently: the sample is co-crystallised with a UV-absorbing matrix, ionised by a laser pulse, and ions are separated by time-of-flight down a vacuum tube, with lighter ions arriving at the detector sooner. It is comparatively tolerant of minor buffer contaminants and gives a rapid mass read-out, though typically at lower mass accuracy and resolution than a well-tuned ESI system.

For labelling comparison, mass spectrometry is the technique that most directly supports an identity claim — that the vial contains the intended peptide sequence rather than a different molecule of similar chromatographic behaviour. It is not, by itself, a quantitative purity measurement in the way RP-HPLC peak-area integration is.

Reading a certificate of analysis

A certificate of analysis (COA) is the document that should accompany a specific lot of research material and report the results of the analytical testing performed on that lot, distinguished from generic marketing claims about a product line. A COA that is genuinely useful to a laboratory typically includes: the lot or batch number, matched to the number printed on the vial; the analytical methods used, ideally including at least one chromatographic purity method and one mass-confirmation method; the numerical results with pass/fail criteria against a stated specification; the date of analysis; and, ideally, an appended chromatogram or spectrum image rather than only a summary number.

• Lot-specific identifiers that tie the document to the physical vial, not a generic template.
• Named analytical methods (e.g., 'RP-HPLC, C18, 214 nm' rather than 'HPLC tested').
• A mass-spectrometry result with the observed versus theoretical mass, not purity alone.
• Appended raw chromatogram/spectrum traces, which allow independent visual inspection of peak resolution and baseline.
• A clear statement of the labelling of the material, consistent with its intended research-only status.

Where a peptide's receptor pharmacology has been characterised in the literature — for example the GLP-1 receptor work summarised in our overview of [semaglutide](/research/semaglutide) — a COA that also reports the mass-spectrometry-confirmed identity gives a laboratory confidence that the material used to generate a result matches the sequence discussed in that literature.

Common gaps and how to interrogate them

A frequent gap in supplier documentation is reporting only a single aggregate purity number without the underlying method, or presenting a COA template rather than a lot-specific report. Another is quantifying purity only by HPLC peak area without a corresponding mass-confirmation step, leaving open the possibility that a co-eluting impurity of similar hydrophobicity is being counted within the 'main peak'. A third is omitting counter-ion content or residual trifluoroacetic acid/acetate from synthesis and purification, which affects the interpretation of net peptide content versus gross vial mass — two figures that are often conflated but are not the same measurement.

Laboratories evaluating research peptides such as those characterised in our overviews of [BPC-157](/research/bpc-157) and [semaglutide](/research/semaglutide) should request lot-specific COAs before relying on a vial for any experiment, and should treat aggregate marketing claims of '99% pure' as a starting point for further questions rather than as a substitute for the underlying chromatogram and mass spectrum.