How purity is measured and what a certificate of analysis shows

Reversed-phase HPLC

RP-HPLC separates components of a sample by hydrophobicity on a C18 column under a gradient of increasing organic solvent. Peptides elute as discrete peaks and are detected by ultraviolet absorbance, typically at 214 nm where the amide bond itself absorbs.

The reported purity figure is a peak-area percentage: the area of the main peak divided by the total area of all peaks. It is a relative measure. It says the target sequence accounts for that share of UV-absorbing material, and it says nothing about water content, counter-ion mass, or any impurity that fails to absorb at the detection wavelength.

Mass spectrometry

HPLC establishes homogeneity; mass spectrometry establishes identity. Electrospray ionisation produces a series of multiply charged ions from which the neutral molecular mass is deconvoluted, and that observed mass is compared with the theoretical mass of the specified sequence.

A mass within a few daltons of theory confirms the sequence length, terminal modifications and any bridges. A mass sixteen daltons high suggests oxidation; two daltons low can indicate an unexpected disulfide bond. Purity without identity is incomplete evidence, which is why both appear on a complete certificate.

Content, endotoxin and residuals

Measured content — sometimes called assay — reports how much target peptide is actually in the vial, independent of the nominal fill weight. This is the number quantitative work should use.

Bacterial endotoxin is quantified by the limulus amebocyte lysate assay and reported in endotoxin units per milligram; it matters in any cell-culture context because endotoxin activates innate immune signalling at picogram levels and will confound cytokine readouts. Residual solvent and residual TFA figures reflect the synthesis and purification history.

Reading a certificate critically

Check that the lot number on the certificate matches the lot printed on the vial. Confirm the analysis date is for that lot and not carried over from a previous one. Look for the chromatogram itself, not only the summary figure — peak shape and baseline tell you things a percentage cannot. Independent third party certificates carry more weight than in house ones because the analysing laboratory has no interest in the result.

The limits of a purity percentage

Chromatographic purity is a ratio of peak areas at a single detection wavelength, almost always 214 nanometres where the amide bond absorbs. Anything that does not absorb there is invisible to the measurement: inorganic salts, residual solvent, counter-ion and water all pass unrecorded. A 99 percent figure therefore describes the peptide-related composition of the sample, not the composition of the vial.

This is why a complete certificate pairs purity with measured content. Purity says the material is overwhelmingly one species; content says how much of that species is present. Either alone leaves a quantitative gap, and the two together are what make a lot usable for work that reports concentrations.

Detection wavelength also determines what counts as an impurity. A related substance that co-elutes with the main peak is not resolved and contributes to the reported figure; one that elutes in the void volume may be integrated or excluded depending on the method's parameters. Purity figures from different laboratories are comparable only when the method is.

Matching the grade to the experiment

Not every application needs the highest available purity, and the useful question is which impurities would confound this particular readout. For a binding assay, a closely related deletion sequence at one percent is usually irrelevant. For a cell-culture experiment reading cytokine output, endotoxin at levels well below the purity method's detection is decisive, and the purity figure says nothing about it.

That mismatch is the most common documentation error in practice: selecting on the headline purity number while ignoring the assay-relevant specification. Endotoxin, residual TFA and residual solvent each dominate a different class of experiment, and each appears on a complete certificate.

Third-party analysis carries more evidentiary weight than in-house analysis for a simple structural reason: the analysing laboratory has no commercial interest in the result. Lot-specific third-party certificates, matched by lot number to the vial in hand, are the standard worth holding suppliers to.

Which specification governs which experiment