Peptide testing and verification, method by method

Identity and purity are separate questions

Almost every misunderstanding in this field starts by collapsing two questions into one.

• Identity: is the substance in this sample the molecule named on the label — the right sequence, the right terminal chemistry, the right modifications and therefore the right mass?
• Purity: of the material present, what proportion is the named molecule, and what else is detectable alongside it?

A sample can be 99% pure and completely mislabelled: a pure substance that is not the substance claimed. It can equally be correctly identified and unremarkably impure. Purity without identity is a statement about homogeneity, not about what the material is. Identity without purity says nothing about how much of the sample is the thing you identified. A report that answers only one has answered half a question — we look at the practical consequences in identity versus purity.

HPLC: what the percentage actually is

High-performance liquid chromatography separates a mixture by pushing it, dissolved in a moving liquid, through a densely packed column. For peptides this is almost always reverse-phase HPLC: a non-polar stationary phase, a water/acetonitrile gradient with an acidic modifier, and a detector watching the column outlet. Components that interact more strongly with the stationary phase move more slowly, so each emerges at a characteristic retention time.

The detector is usually ultraviolet absorbance. Peptides are commonly monitored near 214–220 nm, where the peptide bond itself absorbs, and sometimes at 280 nm, where tryptophan and tyrosine absorb. Software integrates the area under each peak, and the reported figure is normally:

purity (%) = area of main peak ÷ total integrated area × 100

Read that formula carefully, because three important things follow from it. It is a proportion of detected signal, not of mass. It depends on which detection wavelength was used. And it depends on what the operator integrated — where the baseline was drawn, which small peaks were included, and whether the solvent front was excluded.

Mass spectrometry: confirming the molecular weight

Mass spectrometry ionises molecules and sorts the ions by mass-to-charge ratio. Coupled to a liquid chromatograph — LC-MS — it gives a mass measurement for the material under each chromatographic peak, which is what makes it the standard identity test for synthetic peptides.

Electrospray ionisation, the usual technique for peptides, tends to produce multiply charged ions. A peptide of mass M may appear as [M+H]⁺, [M+2H]²⁺ and [M+3H]³⁺, each at a different m/z. A competent report either states the observed and expected values explicitly or shows a spectrum from which they can be read. “Mass confirmed” with no numbers is an assertion.

Tandem mass spectrometry (MS/MS) goes further: it fragments the peptide and reads the pattern of fragments, which can confirm the order of residues rather than only the total. It is comparatively rare on commercial certificates, and its absence is worth noting when a sequence claim is the whole point.

Other tests that legitimately appear

These matter because a certificate showing only HPLC purity has answered one question out of several. That is normal and not in itself a criticism — but it does mean the other questions remain open, rather than answered favourably.

The limitations you should assume are present

Every analytical method has a boundary. These are the ones that most often invalidate a confident-sounding claim:

• Co-elution. Two substances can leave the column at the same time and be integrated as a single peak. A single sharp peak is evidence of chromatographic homogeneity under those conditions — not proof of a single compound.
• Detector blindness. A UV detector reports only what absorbs at the chosen wavelength. Inorganic salts, many counter-ions and various process residues are effectively invisible, so they cannot appear in the percentage.
• Isomers and isobars. Mass spectrometry cannot distinguish species of identical mass. D- and L-amino acid substitutions, leucine versus isoleucine, and scrambled disulfide isomers can all give the correct mass.
• No reference standard. Retention time only identifies a compound when it is compared with an authentic standard under identical conditions. Without one, retention time is a fingerprint with nothing to match it against.
• Sample, not batch. A result describes the vial that reached the laboratory. It extends to the batch only if sampling was representative and the chain of custody holds — the subject of batch traceability.
• One injection, one moment. Chromatography is not a permanent property of a material. Results drift with age, temperature and moisture exposure, which is why the date on the report is part of the result.
• Integration is a judgement. Baseline placement and peak-inclusion decisions can move a reported purity figure by a meaningful amount without anything being falsified.

Why method details decide whether a number means anything

Analytical guidance — for example the ICH validation framework and the Eurachem guidance on fitness for purpose — treats a result as inseparable from the procedure that produced it. That principle translates directly into what a certificate has to show.

None of this requires expertise in chromatography. It requires the habit of asking what a number is a number of.

What “third-party tested” does and does not establish

“Third-party” means the testing was performed by a laboratory independent of the party making the claim. In its strong form it involves a named laboratory, formal accreditation such as ISO/IEC 17025 with the relevant test inside the scope of that accreditation, and a report issued in the laboratory’s own name that identifies the sample it received.

In its weak form the phrase means very little. It can describe an unnamed laboratory, a test outside any accredited scope, a report on a sample the client chose and submitted with no documented link to a production batch, or an image with no verifiable provenance at all. Accreditation also applies to specific methods, so a laboratory can be genuinely accredited and still have run your test outside its accredited scope.

The honest reading of a third-party claim is therefore conditional: it is meaningful exactly to the extent that the laboratory, the method, the sample and the dates are all identified. Next, read a certificate line by line in how to read a COA.