Peptide impurities are whatever sits in a sample besides the target chain, and knowing their types explains what the remaining percentage on a purity result can contain.

A certificate might report a purity of 98.5%. That number answers one question well: how much of the peptide-related material the method detected is the intended sequence. So the next question is what makes up the other 1.5%.

This article sorts the possibilities into families, using the European Medicines Agency’s 2025 guideline on synthetic peptides and the FDA’s 2021 guidance on synthetic peptide generics. It does not repeat how HPLC works; our HPLC testing guide covers the method.

Quick answer: Peptide impurities are the chains and other materials in a sample that are not the target sequence. Most come from synthesis, such as chains missing a residue or carrying an extra one. Others form later, as the peptide oxidises or deamidates. HPLC purity counts the peptide-related ones only. Water, salt and residual solvents need their own tests, and mass spectrometry helps name what each peak is.

Where Peptide Impurities Come From

Regulators group impurities by origin. Some arise during synthesis, because no coupling or deprotection step is perfect. Others form later, as the molecule degrades during processing or storage. A third group has nothing to do with the peptide chain at all, such as residual solvents, reagents and metals.

The FDA guidance lists that third group explicitly and points to the ICH guidelines on residual solvents and elemental impurities. Those materials matter, yet a peptide purity figure is not designed to capture them.

Synthesis-Related Impurities

The EMA guideline describes the main peptide-related impurities that synthesis can leave behind:

• Deletion sequences. Peptides with one or several amino acids missing, typically from an incomplete coupling or deprotection step.
• Truncated sequences. Chains that stopped growing early; when a process caps failed chains by acetylation, they become acetylated fragments.
• Insertion sequences. Chains carrying an extra residue, because an amino acid coupled twice in one step.
• Incomplete deprotection. Chains that still carry a side-chain protecting group after cleavage.
• Diastereomers. Chains in which one residue flipped its chirality, which the guideline attributes to epimerisation.

Each type behaves differently in analysis. A deletion or insertion changes the mass by one residue, so mass spectrometry can usually identify it. A diastereomer, by contrast, has exactly the same mass as the target, so chromatography has to do the work of separating it.

Why Long Chains Carry More Peptide Impurities

Solid-phase synthesis builds a chain one residue at a time. Every cycle has a small failure rate, and those failures add up. So a 40-residue peptide passes through far more chances for a deletion than a 5-residue one.

Think of it as a running tally. If each coupling left a tiny share of chains behind, the target fraction would shrink a little at every step. That is why purification matters so much for longer sequences. It is also why two batches of the same compound can show different impurity profiles, even from the same maker.

Many deletion products sit very close to the target. They differ by one residue out of dozens, so they behave almost the same on a column. As a result, they are the impurities most likely to hide under or beside the main peak.

Degradation Products

Some impurities appear after synthesis. The EMA guideline lists oxidation of cysteine and methionine residues, deamidation and hydrolysis among the degradation pathways that stability studies should consider. These products can grow over time, which is one reason sealed lyophilised powder stays dry, sealed and out of direct light.

Stored that way, sealed powder is stable at room temperature; cold storage is possible but never required. After reconstitution, however, refrigerate the solution at 2–8°C, since most degradation routes run faster in water.

Reading Peptide Impurities by Mass

Mass spectrometry adds names to the peaks. It weighs each species, so the gap between an impurity and the target hints at what changed. A 2014 review of related impurities in peptide medicines, by D’Hondt and colleagues, sets out the common shifts.

• Deletion. The mass falls by the residue mass of the missing amino acid, which is its free mass minus one water. A missing glycine, for example, removes about 57 Da.
• Insertion. The mass rises by one residue mass, the mirror image of a deletion.
• Oxidation. Methionine sulfoxide adds about 16 Da, one oxygen atom.
• Deamidation. Asparagine or glutamine turning into an acid adds about 1 Da.
• Diastereomer. No change at all, which is exactly why mass alone cannot find it.

A one-dalton shift is small, so it needs a good instrument and careful reading. Even so, these patterns let an analyst move from “an unknown peak” to “probably the oxidised form”. That step matters, because the identity of an impurity says more than its size.

What HPLC Purity Leaves Out

An HPLC purity figure compares peptide peaks with each other. Therefore it does not count material that produces no peak under the method, and two common components fall into that gap:

• Water. Lyophilised peptides hold some residual moisture. The EMA guideline lists water content as a separate specification test.
• Counter-ions. Most peptides sit as salts, usually acetate and sometimes trifluoroacetate. The guideline lists residual trifluoroacetate as its own test, too.

The guideline also expresses assay limits on a counter-ion-free, anhydrous basis. In other words, a sample can show high HPLC purity and still contain a meaningful share of water and salt by weight. That gap is the net content idea, and it is a separate question from purity.

Peptide Impurities on a Real Certificate

A worked example makes this concrete. Our published certificate for BPC-157, batch BC10/2026-0310, comes from Janoshik and is dated 19 March 2026. It reports a purity of 99.764% for a 10 mg vial. So peptide impurities make up the remaining 0.236% of the detected peptide material.

The same report gives a content figure of 9.72 mg. That is a different measurement. It describes how much peptide the vial holds, not how clean the peptide is. Reading the two together tells you far more than either one alone.

Note also what the certificate does not do. It does not list each impurity by name, and it does not report residual solvents. That is normal for this kind of report, but it is worth knowing. You can open the full report, and check it on Janoshik’s own site, from our lab results page. Our certificate of analysis guide explains how to read the purity line against the other results.

How Regulators Set Limits

For authorised medicines, regulators set numerical thresholds. Citing the European Pharmacopoeia, the EMA guideline states that peptide-related impurities should be reported above 0.1%, identified above 0.5% and qualified above 1.0%.

The FDA guidance, written for generic versions of five listed peptides, goes further. It asks applicants to identify each peptide-related impurity at 0.10% or more. It also expects any new impurity not found in the reference product to stay at or below 0.5%. Notably, it explains this partly through immunogenicity: an altered sequence could contain a new T-cell epitope.

A 2024 study by Roberts and colleagues examined exactly that question for salmon calcitonin impurities, using computer prediction and cell-based assays. These frameworks govern medicines, not research materials, but they show why the small remaining percentage attracts serious attention.

The Documentation Gap in Research Material

Research peptides sit outside those medicine rules, so documentation varies widely. A 2015 study by Verbeke and colleagues shows why that matters. The group ordered a set of synthetic research peptides at a stated purity of at least 95%. In their own analysis, only 44% met it.

Moreover, the supplier certificates often disagreed with the in-house results. One sample held a different main structure altogether. Most of the impurities they found were chains missing one or more amino acids, the deletion family described above. The lesson is not that every certificate is wrong. Rather, a certificate is only as useful as the batch link and the laboratory behind it.

Handling That Limits Peptide Impurities

Storage cannot remove synthesis impurities, but it can slow new degradation products. First, keep sealed powder dry, dark and at room temperature. Next, avoid moving it between a fridge and warm air, since condensation brings water to the cake.

After reconstitution, the clock runs faster. Store the solution at 2–8°C and never freeze it. Work out the concentration in simple terms: 10 mg dissolved in 2 mL gives 5 mg/mL. Then write that figure, the date and the batch on the vial. Finally, log the batch number against the certificate, so any later question has an answer.

A Checklist for Peptide Impurities

1. Batch match. Check that the certificate’s batch code matches the vial in your hand.
2. Named laboratory. Look for an independent laboratory you can contact or verify online.
3. Identity. Look for a mass result beside the purity figure, since purity alone cannot confirm the sequence.
4. Purity and content. Read them as two separate numbers, and note which one the document reports.
5. Test date. Note when the testing happened, and how the material has been stored since.
6. Storage claims. Expect powder and solution to carry different instructions.
7. Your own records. Label reconstituted vials and log every batch you open.

Where SHLabz Fits

SHLabz supplies its range as lyophilised research vials, and every batch goes to an independent laboratory for testing. Certificates appear by batch on the lab results page as they are issued.

Sealed powder does not require cold chain. Even so, every UAE order travels with cold-chain delivery, because it removes a variable.

The Short Version

In short, peptide impurities fall into synthesis by-products, such as deletion, truncated and insertion sequences, and degradation products such as oxidised or deamidated forms. Diastereomers share the target’s mass, so only separation reveals them.

Meanwhile, HPLC purity leaves out water and counter-ions entirely. For medicines, regulators report, identify and qualify impurities at fixed thresholds.

Frequently Asked Questions

What is in the remaining percentage of an HPLC purity result? Mostly peptide-related impurities, such as deletion, truncated or insertion sequences, diastereomers and degradation products. The exact mix depends on the sequence and process.

Does HPLC purity include water and salt? No. Water and counter-ions such as acetate or trifluoroacetate fall outside the purity figure, which is why specifications test them separately.

What is a deletion sequence? It is a version of the target peptide with one or more residues missing. It usually comes from a coupling or deprotection step that did not go to completion.

Can mass spectrometry find every impurity? No. Diastereomers have the same mass as the target, so they need chromatographic separation to detect.

What thresholds do regulators use for peptide impurities? For medicines in Europe, the EMA cites reporting above 0.1%, identification above 0.5% and qualification above 1.0%. The FDA’s generic peptide guidance uses 0.10% for identification.

Do peptide impurities increase during storage? Degradation products can, especially in solution. Synthesis impurities do not grow, but oxidised or deamidated forms can, so dry and dark storage matters.

Is 99% purity the same as 99% peptide by weight? No. Purity compares peptide peaks with each other. Water and counter-ions add weight without adding peaks, so net content is a separate figure.

References

• Guideline on the Development and Manufacture of Synthetic Peptides — European Medicines Agency, 2025. Impurity types and thresholds, as well as water and counter-ion tests.
• ANDAs for Certain Highly Purified Synthetic Peptide Drug Products That Refer to Listed Drugs of rDNA Origin — US Food and Drug Administration, 2021. Identification and new-impurity limits, together with the immunogenicity rationale.
• Assessing the immunogenicity risk of salmon calcitonin peptide impurities using in silico and in vitro methods — Roberts et al., Frontiers in Pharmacology, 2024. Impurity types and T-cell epitope assessment.
• Related impurities in peptide medicines — D’Hondt et al., Journal of Pharmaceutical and Biomedical Analysis, 2014. Mass shifts for deletion, oxidation and deamidation.
• Quality evaluation of synthetic quorum sensing peptides used in R&D — Verbeke et al., Journal of Pharmaceutical Analysis, 2015. Purity of research-grade peptides against their certificates.

Regulation in this area moves quickly. This article offers general information rather than legal advice, so anyone making a procurement decision with legal consequences should confirm the current position independently.

Finally, the catalogue lists every compound with its format and batch documents.

This article is provided for general research and educational purposes only. All SHLabz products are sold strictly for laboratory research use and are not intended for human or veterinary consumption, diagnosis, treatment, or prevention of any disease. Nothing in this article constitutes medical advice. Always consult a qualified professional and comply with applicable local regulations regarding the handling and use of research compounds.