What a ≥99% purity figure actually describes, how related impurities and residual solvents are reported, and why purity alone never tells the full analytical story.
Purity is a relative measurement
An HPLC purity figure is the main peak’s share of total detected peak area at a given wavelength. It excludes water, counter-ions and anything that does not absorb or elute under the method used.
What ≥99% actually measures
A purity figure on a peptide certificate is normally an HPLC peak-area percentage: the area of the main product peak divided by the total area of all peaks detected at a given wavelength, usually 214 nm where the peptide bond absorbs. If the main peak accounts for 99.2% of the detected area, purity is reported as 99.2%.
That is a relative measurement of chromatographically detectable species. It is an excellent indicator of synthesis quality, and it is not a statement that 99% of the vial’s mass is peptide.
What the figure does not include
- Water content — lyophilised peptides are hygroscopic and typically retain a few percent residual moisture.
- Counter-ions — most peptides are supplied as acetate or trifluoroacetate salts, which add mass.
- Residual solvents from synthesis and purification, reported separately when tested.
- Anything that does not absorb at the detection wavelength or does not elute under the method used.
Related impurities and why they exist
The impurities in a well-made peptide are usually close relatives of the target: deletion sequences where a coupling step was incomplete, truncated chains, oxidised methionine or tryptophan residues, or diastereomers formed by racemisation during coupling. These elute near the main peak, which is exactly why a good analytical method uses a shallow gradient — a lazy method makes a mediocre peptide look clean.
This is also why identity testing matters alongside purity. A single sharp peak proves homogeneity; only mass spectrometry proves the homogeneous thing is the sequence you ordered.
Reading purity sensibly
Treat purity as one figure among several. A defensible certificate shows the purity value, the method and gradient used, the observed mass against the theoretical mass, the appearance, and the batch and release date. Where net peptide content matters to your calculations — quantitative binding work, for example — ask whether amino acid analysis or nitrogen determination is available for the batch, because that is the measurement that answers it.
References
- European Pharmacopoeia — general chapter on peptide identification and related substances, EDQM.
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