Research Notes

Reading Impurity Peaks: What Deletion Sequences Reveal in a Peptide HPLC Chromatogram

August 12, 2026 · Peak Labs Quality & Verification · COA, HPLC, Peptide Education, Quality & Handling

Abstract still life of a laboratory glass vial beside a printed chromatogram curve in warm gold and ivory tones

Educational information for a laboratory audience. Not medical advice, not a recommendation for human use. Peak Labs products are for laboratory research use only.

A single purity percentage on a certificate of analysis tells a researcher very little about what is actually sitting next to the main peak in the chromatogram. Two batches can report an identical HPLC area percent purity figure and still differ meaningfully in the identity of the small remaining fraction. For laboratories that depend on consistent, well-characterized material, learning to read the impurity profile itself, not just the summary number, is a practical skill worth developing.

This article looks at what chromatogram peaks represent, how deletion sequences and truncated byproducts arise during synthesis, what other impurity classes commonly appear, and how to translate that information into better questions when evaluating a certificate of analysis.

What a Chromatogram Peak Represents

In reversed-phase HPLC, a peptide sample is carried through a column packed with a stationary phase, and different molecular species separate based on their interaction with that phase as a mobile phase gradient changes. Each resulting peak corresponds to a distinct chemical species eluting at a characteristic retention time. The height and area of a peak are proportional, under controlled conditions, to the relative amount of that species in the injected sample.

Retention Time and Peak Area

The main peak, ideally the largest and most symmetric, corresponds to the target sequence. Area percent purity is calculated by dividing the area of that main peak by the total area of all detected peaks. This is a useful summary figure, but it treats every non-main peak as equivalent, when in practice those peaks can represent very different kinds of impurities with different implications for downstream analytical work. A detailed comparison of how HPLC and mass spectrometry data are used together to confirm both purity and identity is covered in HPLC vs Mass Spectrometry: Verifying Peptide Purity and Identity.

Deletion Sequences: How They Form

Deletion sequences, sometimes called truncated sequences, are peptide chains that are missing one or more amino acid residues from the intended sequence. They are among the most common impurity classes in synthetic peptides and arise from the mechanics of stepwise chain assembly rather than from degradation after synthesis.

Truncation During Solid-Phase Synthesis

In solid-phase peptide synthesis, each amino acid is coupled one at a time to a growing chain anchored to a solid resin. If a coupling step is incomplete, a fraction of chains fail to add that residue. Standard practice is to cap unreacted chains so they cannot continue growing, which prevents them from becoming a different deletion sequence later in the synthesis, but the capped, truncated chain remains in the final crude product until it is removed by purification. Because deletion sequences are usually shorter than the target peptide and share much of its sequence, they can elute close to the main peak, which is why resolution and gradient conditions matter as much as detection sensitivity. The synthesis route itself also shapes what impurity profile to expect, as discussed in Solid-Phase vs Liquid-Phase Peptide Synthesis.

Other Common Impurity Peaks

Deletion sequences are not the only species that can appear alongside the main peak. Recognizing the broader categories helps a researcher interpret a chromatogram rather than treating every secondary peak as the same problem.

Aggregates, Oxidation Products, and Diastereomers

Aggregates, such as dimers formed through disulfide bonding in cysteine-containing sequences, typically elute differently from the monomer and are more readily distinguished by size-based methods than by reversed-phase retention time alone. Oxidation products, commonly affecting methionine, tryptophan, or cysteine residues, shift retention time slightly due to the added polarity of the oxidized side chain. Diastereomers, arising from racemization at a chiral center during synthesis, can be particularly difficult to separate because they share the same molecular formula and mass as the target sequence, differing only in three-dimensional configuration. Each of these impurity types requires a different analytical strategy to characterize fully, and molecular identity for any given peak can be cross-checked against reference data such as molecular formula and structure listings on PubChem.

Why the Purity Percentage Alone Is Not Enough

A single number cannot distinguish between a batch where the residual fraction is a single, well-characterized deletion sequence eluting far from the main peak, and a batch where that same fraction is spread across several unidentified peaks close to the main peak, some of which may co-elute and go undetected. The first scenario is generally easier for a downstream laboratory to account for; the second introduces more uncertainty. This is one reason area percent purity and mass balance purity, calculated differently, can report different figures for the same material. That distinction is explored further in the article on how to read a peptide certificate of analysis.

Reading an Impurity Profile on a COA

A well-documented certificate of analysis will typically include the chromatogram trace itself, or at minimum a table listing the retention time and relative area of each detected peak above a stated reporting threshold, alongside the analytical method and column conditions used. When reviewing this information, it is worth checking whether impurity peaks are simply listed as unidentified area percent, or whether any have been further characterized by mass spectrometry to confirm whether they correspond to a deletion sequence, an oxidation product, or another known impurity class. Peak Labs maintains documentation practices for the research materials it supplies; general information on certificate of analysis format and interpretation is available on the COA page.

Questions to Ask a Supplier About Impurity Profiles

Before relying on a batch for comparative or quantitative research work, it is reasonable to ask a supplier several specific questions: what analytical method and column were used to generate the reported purity figure, whether the chromatogram or peak table is available alongside the summary percentage, whether any major impurity peaks have been identified by mass spectrometry, and whether the reporting threshold used to define a peak as significant is stated. Suppliers who can answer these questions directly, with documentation to match, are generally easier to build a consistent research relationship with. The full range of research peptides Peak Labs supplies, each with accompanying documentation, can be browsed in the product collection.

Sources and further reading

Research use only. Peak Labs products are supplied strictly for in-vitro laboratory research. They are not medicines or supplements, are not for human or veterinary use, and are not intended to diagnose, treat, cure, or prevent any condition.