No peptide synthesis is perfectly efficient, so impurities are a fact of life. The difference between a reliable supplier and an unreliable one is not whether impurities form — it is how rigorously they are prevented, removed and characterized. Here is how impurity control actually works.
Where impurities come from
- Deletion sequences: an incomplete coupling leaves a chain missing one residue — the most common impurity.
- Truncation: chains that stop growing entirely.
- Side reactions: oxidation, aspartimide formation, racemization and incomplete deprotection.
- Cleavage artefacts: adducts formed during resin cleavage and deprotection.
Preventing impurities at the source
The cheapest impurity is the one never formed. Optimised coupling chemistry, double couplings for difficult positions, appropriate reagents, and in-line monitoring all push couplings toward completion — the rationale behind automated synthesis platforms with controlled cycles. Sequence-specific risks (such as aspartimide-prone motifs) are managed with tailored conditions.
Key point: impurity control is designed in during synthesis, then verified during purification — not bolted on at the end.
Removing what remains
Impurities that do form are separated by preparative HPLC, where closely-related deletions are the hardest to resolve and drive the purity-versus-yield trade-off behind purity grades.
Characterizing and specifying
What you cannot measure you cannot control. Orthogonal analytical sciences — HPLC for purity, mass spectrometry to identify each impurity — let you set and defend a specification, which becomes critical as material moves toward GMP grade and regulatory filings.