Common peptide impurities and where they come from

The impurities in a synthetic research peptide are not random. They come from a small number of well-understood failure modes in solid-phase synthesis, cleavage and storage, and each has a characteristic mass and chromatographic signature. Knowing them lets you read a COA's minor peaks and mass-spec adducts with understanding rather than anxiety. This note catalogues the common ones, where they come from, and how they are controlled. All Alphex products are for laboratory and in-vitro research only.

Synthesis-related impurities

• Deletion sequences. A coupling step fails to add a residue to some chains, and the next residue couples anyway, giving a peptide one residue short. Mass: target minus that residue's mass. HPLC: usually elutes near the target, slightly earlier if a hydrophobic residue is missing. The most common impurity class; the reason capping steps and double couplings exist. Proline-rich sequences such as BPC-157's Pro-Pro-Pro are prone to it.
• Truncated sequences. Chain growth stops entirely part-way (the N-terminus is capped, deliberately or by side reaction), leaving a shorter peptide. Mass: much lower. HPLC: often much earlier. Usually removed by purification.
• Insertion sequences. A residue couples twice. Mass: target plus that residue. Less common.
• Racemisation. A residue's stereocentre inverts during activation, especially histidine, cysteine and serine, giving a diastereomer. Mass: identical. HPLC: often a shoulder or a nearby peak; the reason a reference standard matters. See reference standards in peptide HPLC.
• Aspartimide formation and related rearrangements. At Asp-Gly and Asp-Ser motifs, side reactions during synthesis give isomers of the same or nearly the same mass with altered retention.

Cleavage and work-up impurities

• Residual protecting groups. Incomplete removal at cleavage leaves tert-butyl (+56 Da), Boc (+100), Pbf on arginine (+252), trityl (+242) or others. HPLC: later-eluting (more hydrophobic). Corrected by longer cleavage; removed by purification.
• Scavenger adducts. Reactive species released at cleavage can alkylate tryptophan, methionine or cysteine; scavengers in the cleavage cocktail prevent it, and traces of scavenger-derived adducts can remain.
• Counter-ion. Not an impurity in the peptide sense, but TFA or acetate is part of the solid mass. See acetate versus TFA.

Degradation impurities (arise in storage or solution)

• Oxidation. Methionine to sulfoxide, tryptophan to oxindole or kynurenine, cysteine to disulfide or sulfonic acid, and histidine under photo-oxidation. Mass: +16 per oxygen (+32 for sulfone). HPLC: usually slightly earlier.
• Deamidation. Asn or Gln to Asp/isoAsp or Glu. Mass: +0.98. HPLC: slightly earlier, sometimes two peaks (Asp and isoAsp). Accelerated by heat and alkaline pH.
• Hydrolysis. Backbone cleavage at labile bonds (Asp-Pro especially), giving two fragments. Mass: fragments. HPLC: earlier peaks.
• Pyroglutamate formation. N-terminal Gln or Glu cyclises, mass -17 or -18. Common in stored solutions of peptides with those N-termini.
• Aggregates. Not a chemical impurity but a physical one; may appear as broad late peaks, or not appear at all if they do not elute.

Storage-related routes are covered in deamidation and oxidation in stored peptides.

How they are controlled

Synthesis quality (double coupling, capping, optimised cleavage) sets the crude profile; preparative HPLC removes most of what remains; the acceptance threshold decides what is shipped. Alphex's threshold of 99+% area purity at 220 nm means the sum of every peak above is under 1% of the peptide fraction. See 98% versus 99+% and how Alphex tests every batch.

Reading them on a COA

Small peaks clustered close to the main peak: deletions, diastereomers, deamidated and oxidised forms, all expected in trace amounts. A late-eluting peak: probably a residual protecting group; check the mass spectrum for +56 or +100. Early peaks: truncations or hydrolysis fragments. A single perfect peak with no minor species on a large peptide: ask how it was integrated. See how to read a chromatogram.

Sources and further reading

• ICH Quality Guidelines (Q1A stability, Q3A/Q3B impurities, Q6 specifications): the reference framework for purity, impurity and stability testing

Alphex research peptides are supplied for laboratory and in-vitro research by qualified researchers only. They are not authorised for human or veterinary use.

Research use only. Alphex products are supplied exclusively for laboratory and in-vitro research by qualified researchers. They are not authorised for human or veterinary use, and nothing in this note is advice on any such use.

Published 5 March 2026, last reviewed 16 August 2026. Alphex reviews research notes when the testing method, the catalogue or UK guidance changes.

Alphex Research Team

Alphex Limited, UK research peptide supplier

The Alphex research team writes and maintains these notes for UK laboratories buying research peptides. Every batch Alphex lists is verified by reverse-phase HPLC with a batch-matched Certificate of Analysis. Content covers laboratory and in-vitro research only.