Solid-phase peptide synthesis builds a chain one residue at a time on a resin support, and every coupling step is a reaction with a yield below 100%. The arithmetic is unforgiving: a 30-residue peptide at 99% per-step efficiency finishes at roughly 74% crude purity before any purification. What survives alongside the target is not random debris. It is a predictable family of near-relatives, and knowing the family makes a chromatogram far easier to read.
Deletion sequences
When a coupling step fails on some fraction of chains, those chains continue growing without that residue. The product is a peptide one amino acid short, identical everywhere else. Deletion sequences are the signature impurity of SPPS, and because they differ from the target by a single residue they differ only slightly in hydrophobicity — so they elute close to the main peak, often as a shoulder rather than a resolved peak.
By mass spectrometry they are unmistakable: the mass is lower than calculated by exactly the residue mass of the missing amino acid. This is one of the clearest cases where chromatography and mass spectrometry answer different halves of the same question.
Truncated sequences
A chain that stops elongating entirely produces a shorter peptide. Capping steps, in which unreacted amines are acetylated so they cannot continue, are used specifically to convert would-be deletion sequences into truncated ones — because a truncated peptide is usually much easier to separate than a peptide missing one internal residue. Truncations typically elute well away from the target.
Incomplete deprotection
Side-chain protecting groups must all come off during cleavage. Residues that keep one add mass and change hydrophobicity, usually increasing retention. Bulky protecting groups on arginine are a recurring culprit. On a mass spectrum these appear as the target mass plus the mass of the retained group.
Oxidation
Methionine oxidises to the sulfoxide readily, adding 16 Da and shifting retention earlier because the oxidised residue is more polar. Cysteine and tryptophan have their own oxidation pathways. Any sequence containing methionine or cysteine should be assessed with oxidation products in mind, and the shift is small enough that a compressed gradient can bury it.
Racemisation and diastereomers
Activation chemistry can epimerise a residue, producing a diastereomer with the same mass and, often, a very similar retention time. This is the impurity class that mass spectrometry cannot see at all, because the mass is unchanged. Only chromatographic separation reveals it, and only if the method is good enough to resolve it.
Scavenger adducts and residual reagents
Cleavage cocktails include scavengers to trap reactive cations. Some of those can form adducts with the peptide. Others simply persist as residual material that may or may not absorb UV light — and anything that does not absorb is absent from a purity calculation while remaining present on a balance.
What this means when you read a trace
The impurities that matter most are the ones that resemble the target most closely, which means they cluster around the main peak rather than sitting cleanly apart from it. That has three practical consequences.
First, gradient slope changes the answer: a shallow gradient resolves near-eluting deletion sequences and oxidation products, a steep one merges them into the main peak and returns a higher purity figure from identical material. Second, peak shape carries information — a broad or asymmetric main peak may be several species travelling together. Third, chromatography and mass spectrometry are not redundant: one sees diastereomers the other cannot, and the other identifies mass shifts the first cannot interpret.
Each product in our catalogue is analysed by an independent laboratory using reverse-phase HPLC and the result is published on its product page, so this reading can be done before an order rather than after one.
Research use only. All products supplied by Battle Born Peptides are laboratory reference materials for in-vitro research and analytical use by qualified professionals. They are not drugs, foods, dietary supplements, cosmetics or medical devices; they are not approved by the FDA or any other regulator for use in humans or animals; and they are not intended to diagnose, treat, cure, mitigate or prevent any disease, or to affect the structure or any function of the body of humans or animals. Nothing in this article is preparation, handling or dosing guidance. See our full research-use terms.