How the Number Is Generated

Most peptide purity figures reported by suppliers come from reverse-phase high performance liquid chromatography. A typical method uses a C18 column and a water and acetonitrile gradient, with trifluoroacetic acid added as an ion-pairing agent to improve peak shape. Detection is usually by ultraviolet absorbance at 214 nm, which responds to the peptide backbone, or at 280 nm where aromatic residues such as tryptophan and tyrosine absorb. The chromatogram is then integrated and the areas of the individual peaks are summed.

Purity is calculated by area normalisation. The main peak area is divided by the total area of all detected peaks and expressed as a percentage. This is an important distinction: the result is a relative figure describing the proportion of ultraviolet-detectable material, not a mass fraction of the vial contents.

What the Figure Does Not Count

Because area normalisation only considers what the detector sees, material that is invisible at the chosen wavelength is excluded from the calculation entirely. Water, the residual trifluoroacetic acid counterion, inorganic salts and residual solvents from synthesis and lyophilisation carry no useful ultraviolet chromophore and do not appear as peaks. They are therefore absent from the denominator.

A sample can be reported at 98 per cent purity by area while the peptide itself accounts for a considerably smaller share of the total mass in the vial. The full mass balance is broadly purity plus water plus counterion plus residual solvents. Karl Fischer titration is the standard method for measuring water content directly, and it is a separate determination from chromatography.

Where Impurities Can Be Missed

Some species are difficult to separate under routine gradient conditions, and several of them can co-elute with the target peak.

• Truncation and deletion sequences, which differ from the target by one or a few residues and often behave similarly on a C18 column.
• Diastereomers arising from racemisation at one or more residues, which may share retention time with the correct sequence.
• Oxidised forms, particularly where methionine is present, which may elute close to the parent peak.
• Peaks that fall below an integration threshold, or that are broad and therefore assigned low area.

Numerical settings matter as much as the chemistry. The integration threshold, peak width and baseline selection all change the reported area of small peaks, and a stricter or looser integration policy shifts the final percentage accordingly. Two laboratories working to accepted standards can legitimately report different figures for the same lot, and neither need be in error.

Identity and Purity Are Separate Questions

Mass spectrometry confirms molecular identity: it establishes that the expected mass is present. It does not confirm purity, and a mass spectrum that matches the theoretical value says nothing about the proportion of closely related material in the sample. The two analyses answer different questions and should be read side by side, together with the water determination, rather than treated as interchangeable evidence.

Questions Worth Asking a Supplier

When a certificate of analysis is provided for research purposes, the following details determine how much weight a purity figure can carry.

• The analytical method, column type and dimensions.
• The gradient programme, including ramp rate and run time.
• The detection wavelength or wavelengths used.
• The integration criteria, threshold and peak width settings.
• The data handling package and the reporting convention applied.
• Separate water content determination and the method used for it.

A supplier that can state these parameters is providing a figure that can be interpreted. A figure without method detail cannot be compared meaningfully with another laboratory’s result.

Purity by area normalisation is a comparative statement about ultraviolet-visible material. It is not a claim about total mass, and it should always be read alongside the method conditions that produced it.

Research use only. Not for human or veterinary consumption, diagnostic or therapeutic use.