Laboratory quality
Purity
The fraction of a sample that is the intended molecule, what impurities typically look like in peptides, and why "pure" is a relative term.
- Last updated:
- 2026-07-22
Overview
Purity is the fraction of a sample that is the intended peptide, expressed as a percentage. It is method-dependent and always relative to the analytical technique that produced the number.
In peptide synthesis, common impurities include truncated sequences (the synthesis stopped early), deletion sequences (one amino acid was skipped), oxidised species (methionine or tryptophan oxidised during handling), and process residues (from the synthesis chemistry).
Why it matters
Two vials of the same peptide can differ substantially in the identity and abundance of the non-peptide material inside them. Two "99% pure" vials tested by different methods are not necessarily comparable.
Key concepts
Related versus unrelated impurities
Related impurities are close variants of the target peptide (deletions, oxidations). Unrelated impurities are process chemicals (residual solvents, scavengers, counter-ion excess). Both matter but for different reasons.
Threshold vs. characterisation
">98% purity" tells you the impurities are below 2%, in total, by that method. It does not tell you what those impurities are. Full characterisation identifies each impurity peak.
Purity is not the same as identity
A sample can be 99% pure by area percent and still be the wrong peptide entirely — 99% pure something-else.
Common misconceptions
- "Higher purity is always better." More purity is generally better, but purity numbers are only meaningful with a specified method.
- "Free of impurities" is a marketing phrase, not a scientific claim. Every real sample contains something else at some level.
- Purity does not imply sterility or endotoxin freedom. A highly pure peptide can be contaminated with bacteria or endotoxin.
Practical interpretation
When a CoA reports purity, read it as "X% of the material captured by this method is the expected peptide." That is useful, but it is not the whole quality story.
Limitations
The distribution of impurities matters, not just the total. A single 1.5% impurity peak of unknown identity is different from ten 0.15% peaks of characterised related peptides.