Guide
Peptide impurities: what they are and how they are detected
Impurities in a synthetic peptide are mostly other peptides: chains missing a residue, cut short, still carrying a protecting group, or altered by oxidation or deamidation. HPLC purity measures how much of the chromatogram is the target peptide, and mass spectrometry identifies what the other peaks are. Water, counterions, endotoxin and metals are separate, non-peptide impurities measured by their own methods.
Peptide-related impurities from synthesis
- Deletion sequences: a residue missed when one coupling step did not go to completion, leaving a chain one amino acid short somewhere in the middle.
- Truncated sequences: chains that stopped growing early, usually because a capping step terminated them.
- Incompletely deprotected species: chains still carrying a side-chain protecting group that the final cleavage did not remove.
- Diastereomers: a residue whose configuration changed from L to D during coupling (racemisation).
- Insertion sequences: a residue coupled twice.
Because these differ from the target by one residue or one group, they often elute close to it on HPLC, and resolving them is what a well-developed purity method is for.
Degradation products
- Oxidation, chiefly of methionine and, to a lesser extent, tryptophan and cysteine, adding 16 Da per oxygen.
- Deamidation of asparagine and glutamine, converting them to aspartic and glutamic acid and adding about 1 Da.
- Aggregates and dimers, more relevant for longer and more hydrophobic chains.
These are the pathways that moisture and temperature accelerate, which is why sealed, dry, cold storage matters.
Non-peptide impurities
- Water and counterions: part of every lyophilized peptide, accounted for by net peptide content rather than purity.
- Residual solvents from synthesis and purification.
- Endotoxin: bacterial cell-wall material, measured by the LAL test.
- Elemental impurities (heavy metals), measured by ICP-MS.
Which test finds which impurity
Certificates in this catalogue report identity, HPLC purity and net peptide content, and where the panel includes them, heavy metals by ICP-MS, endotoxin, sterility and a fentanyl screen — each against its own specification.
Common questions
What are the most common impurities in synthetic peptides?
Other peptides produced during synthesis: deletion sequences missing a residue, truncated chains, and chains that retain a protecting group, followed by oxidation and deamidation products.
Does 99% HPLC purity mean 99% of the powder is peptide?
No. HPLC purity is the share of the peptide material that is the target sequence. Water and counterions are not seen by that measurement; net peptide content accounts for them.
How are peptide impurities identified?
HPLC separates them and measures how much of each is present; mass spectrometry identifies them by the mass difference from the target peptide.
References
- Related impurities in peptide medicines — D'Hondt M et al., Journal of Pharmaceutical and Biomedical Analysis, 2014
- Aspects of complexity in quality and safety assessment of peptide therapeutics and peptide-related impurities. A regulatory perspective — Colalto C, Regulatory Toxicology and Pharmacology, 2024
- Advances in Fmoc solid-phase peptide synthesis — Behrendt R et al., Journal of Peptide Science, 2016
- ICH Q6B: Specifications — Test Procedures and Acceptance Criteria for Biotechnological/Biological Products — International Council for Harmonisation (ICH)
From the catalog
Compounds referenced on this page
For laboratory research use only. Not for human or veterinary consumption.