How purity is actually tested

These get collapsed into one number constantly, and the collapse is where most buyers get misled. Purity asks what proportion of the sample is the main thing present. Identity asks whether that thing is the molecule you actually wanted. A vial can score 99% pure and be the wrong peptide entirely, because a near-identical sequence separates just as cleanly.

Any certificate that reports a percentage without an identity test has answered half the question, and it is the less important half.

High-performance liquid chromatography separates the components of a sample by how they partition between a liquid mobile phase and a solid stationary phase. Different molecules travel through the column at different rates and elute at different times.

For peptides the detector is usually ultraviolet absorbance around 214 nanometres, which is where the peptide bond absorbs. The instrument then reports the area under each peak as a percentage of the total. That is where the familiar figure comes from: a purity of 98% means the main peak accounted for 98% of the total absorbance at that wavelength.

It is a relative measure, not an absolute one, and several things follow from that.

• 01It does not identify anything. A peak is a peak. It could be your peptide, a closely related sequence, or a synthesis byproduct that happens to elute at a similar time.
• 02It does not measure water or salt content. A lyophilised powder commonly contains counterions, residual moisture, and synthesis residuals. High purity by area percentage can coexist with a much lower peptide content by mass.
• 03It does not detect anything invisible at the detector wavelength. Material without a chromophore passes through unmeasured.
• 04It does not tell you about the vial in your hand, only about the sample that was submitted.

Mass spectrometry measures the mass-to-charge ratio of the molecules in a sample. A peptide has a calculated theoretical mass based on its sequence, so the observed mass can be compared against the expected one. If they match within instrument tolerance, the sample is consistent with the intended sequence.

This is the test that actually confirms what you have. When a certificate reports a mass and the expected mass alongside it, someone did the confirming. When it reports only a percentage, nobody did.

Electrospray ionisation and MALDI-TOF are the two methods you will see named. For confirming a known sequence, either is adequate. What matters is that the expected value is stated, so the comparison is visible rather than asserted.

• 01The peptide name and the sequence or molecular formula, so the expected mass can be derived.
• 02A lot or batch number that matches the vial you were sent.
• 03An HPLC chromatogram — the actual trace, not a summary sentence — with the purity calculation.
• 04Conditions: column type, mobile phase, gradient, and detection wavelength, so the result is reproducible.
• 05Mass spectrometry results with both observed and theoretical mass.
• 06Peptide content or net peptide, distinguished from purity.
• 07The name of the testing laboratory and a date of analysis.

Certificates missing the chromatogram or the theoretical mass are telling you which tests were skipped. The chromatogram in particular is hard to fabricate convincingly and easy to check for basic problems.

You are looking for a single dominant, symmetrical peak rising cleanly from a flat baseline. Things that should prompt questions:

• 01Shoulders on the main peak, which suggest a co-eluting impurity that did not separate.
• 02A rising or drifting baseline, which makes the area percentage unreliable.
• 03Many small peaks throughout, which is normal for crude synthesis material but not for a purified product.
• 04A trace that looks digitally generated — perfectly smooth, with no noise anywhere.
• 05An axis with no scale, or no retention time labelled.

Third-party purity and identity testing is a routine analytical service and most contract laboratories publish price lists. Cost scales with the number of samples and the methods requested, and a combined HPLC plus mass spectrometry run on a single sample is a modest expense relative to the value of a batch.

That asymmetry is the useful part. If a supplier cannot produce a certificate for a lot, it is rarely because testing is prohibitively expensive.

There is no home equivalent of HPLC. Test kits sold for the purpose generally use colorimetric or precipitation reactions, which can indicate that some peptide is present but cannot quantify purity, cannot distinguish related sequences, and cannot tell you the concentration.

A positive result from a kit means the sample reacted. It does not mean the sample is what the label says, and it certainly does not produce a purity figure. Treat kit results as a weak screening signal, not verification.

The certificate establishes a chain only if you close the loop. Take the lot number from the vial, find it on the certificate, confirm the laboratory named actually exists and is independent of the supplier, and check that the document includes the identity test rather than only a purity percentage.

One limit is worth stating plainly: even a flawless certificate describes the sample the laboratory received. It does not prove the contents of your specific vial. For blends or for compounds where a supplier has an incentive to substitute, the only complete verification is testing the material you have.

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