How Peptides Are Made: Synthesis, Purity, and Why Quality Varies

Most people who use peptides never think about how the molecule in the vial got there. But the manufacturing process explains almost everything that matters about quality: where impurities come from, why two products with the same name can be very different, and why a Certificate of Analysis is worth reading carefully. This guide walks through how peptides are made, the predictable ways things go wrong, and how quality is actually measured.

Two ways to make a peptide

There are two main routes to manufacturing a peptide, and which one is used depends mostly on size.

Chemical synthesis is the workhorse for small and medium peptides. The dominant method is solid-phase peptide synthesis (SPPS), introduced by Robert Bruce Merrifield in a 1963 paper and recognized with the 1984 Nobel Prize in Chemistry. The idea is elegant: the first amino acid is anchored to an insoluble polymer bead (the “solid phase”), and the peptide chain is built one residue at a time. Because the growing chain stays bolted to the bead, excess reagents and byproducts can simply be washed away by filtration after each step. The cycle repeats — remove the protecting group from the chain’s free end, couple the next protected amino acid, wash, repeat — until the sequence is complete. The finished peptide is then cleaved off the bead and its side-chain protecting groups are removed.

Two chemistries dominate. The original Boc approach used repeated strong acid and a harsh final cleavage with hydrogen fluoride. The modern standard is Fmoc, which is “orthogonal”: a mild base removes the chain’s end-group protection while acid-sensitive side-chain protections stay intact until a final acid cleavage. Fmoc is gentler and handles longer sequences better, which is why most synthetic peptides today are made this way.

Recombinant (biological) production is used for larger peptides and proteins, such as insulin. Here the gene encoding the peptide is inserted into a host organism — often E. coli or yeast — which expresses it, frequently as a fusion protein: the target peptide is attached to a carrier protein that improves expression and folding. The carrier is then enzymatically cleaved off, and the peptide is folded and purified. Some regulated therapeutic peptides use hybrid routes, combining a recombinant precursor with chemical modification.

Where impurities come from

This is the part that matters most for quality, and the key insight is that impurities in synthetic peptides are predictable byproducts of the chemistry, not random contamination. Because the chain is assembled one step at a time, every step is a chance for something to go slightly wrong.

Sequence errors are the most intuitive. If a coupling step fails to finish, you can get truncated chains (assembly stopped early), deletion sequences (an internal residue is missing), or insertion sequences (an extra residue slipped in). A common manufacturing trick is “capping”: after each coupling, any chains that failed to react are chemically blocked so they stop growing, which turns hard-to-separate deletions into shorter truncations that purification can remove more easily.

Side reactions are subtler. The best known is aspartimide formation, an internal cyclization at aspartic acid residues. It is primarily base-catalyzed — driven by the piperidine used in Fmoc deprotection — though acidic conditions contribute too. It is strongly sequence-dependent: the Asp-Gly motif is the worst offender, with several other following residues (including Ala, Ser, Thr, Cys, Arg, Asp, and Asn) also prone to it. Aspartimide formation spawns a family of byproducts, including rearranged peptides and racemized (mirror-image, D-form) versions, some of which are difficult to separate from the target. Other side reactions include broader racemization, oxidation (for example at methionine), deamidation, incomplete deprotection, and the formation of dimers and adducts.

Non-peptide residues round out the picture. SPPS peptides are usually isolated as trifluoroacetate (TFA) salts, and leftover TFA counterion can skew bioassays and the actual peptide content of a vial unless it is exchanged. Depending on the route and how well it is controlled, products can also carry residual solvents and reagents, heavy metals, bacterial endotoxin, or microbial contamination — concerns that loom larger for recombinant and poorly controlled material.

How purity and identity are measured

Once a peptide is made, it has to be purified and checked. Purification usually relies on reversed-phase HPLC (high-performance liquid chromatography), sometimes alongside ion-exchange chromatography, with UV detection near 215 nm to catch the peptide bond itself.

Two analytical methods then answer two different questions:

HPLC tells you how pure the material is. It separates the contents into peaks, and purity is reported as the target peak’s area versus the total. It is a relative, separation-based number — which is why a credible result shows the actual chromatogram, not just a percentage.
Mass spectrometry (LC-MS or ESI-MS) tells you what it is. By measuring molecular weight, it confirms whether the molecule’s mass matches the expected sequence. Electrospray ionization often produces multiply-charged ions — a peak at roughly half the expected mass-to-charge ratio is a normal feature of the technique, not an error.

Other orthogonal checks include amino acid analysis, peptide mapping, NMR, and measurement of counterion, water, and residual-solvent content. These are the same principles covered in our guide to reading a COA: purity without identity is hollow, because a sample can be 99% pure and still be 99% of the wrong molecule.

What “good” quality control looks like

For pharmaceutical peptides, there is a published benchmark. USP General Chapter ⟨1503⟩, “Quality Attributes of Synthetic Peptide Drug Substances” (United States Pharmacopeia, 2021), lays out expectations for manufacturing methods, raw materials, assay and content, impurities and related compounds, microbial contamination, and bacterial endotoxins. The common analytical toolkit it describes — HPLC/UPLC, LC-MS and LC-MS/MS, amino acid analysis, NMR, and peptide mapping — maps directly onto the impurities described above. A companion chapter, ⟨1504⟩, covers the starting materials used in chemical synthesis. By USP convention these chapters (numbered above 1000) are informational guidance rather than mandatory requirements, but they represent the standard regulated manufacturers are held to.

Why gray-market quality varies so much

None of these standards apply to peptides sold as “research chemicals.” That is the heart of the problem, and there is direct evidence for it.

In a 2008 study, researchers profiled the same peptide (obestatin) from five different manufacturers. One product turned out to be a completely different peptide, and roughly two-thirds of the rest were of insufficient quality for reliable laboratory use — purity below 95% or individual impurities above 1%. That is what variable quality looks like in practice: not subtle differences, but mislabeled products and material that would fail any serious specification.

The “research use only” label does not fix this. The US Department of Defense’s Operation Supplement Safety program notes that for peptide-hormone products, “neither the purity nor the potency… can be ensured,” and that “for research purposes only” labeling is used even as products are marketed to consumers and athletes. The label is a legal and marketing shield, not a quality guarantee — a point we cover more fully in Research Use Only, Explained.

Regulators have flagged related problems with the compounded and gray-market GLP-1 drugs that now dominate this space. The FDA has raised concerns that some products use salt forms (such as semaglutide sodium or acetate) that are different active ingredients than the approved base, that these products get no premarket review for safety or quality, and that adverse events are likely underreported. The agency has separately warned about dosing errors with compounded injectable semaglutide — reports of doses 5 to 20 times intended, with serious adverse events — and has launched a “Green List” of compliant foreign API manufacturers under an import alert, the implication being that much imported material does not meet that bar. The FDA has also issued warning letters to research-peptide marketers selling unapproved tirzepatide without a prescription, and has proposed excluding semaglutide, tirzepatide, and liraglutide from the list of substances that outsourcing facilities may compound from bulk.

Anti-doping rules add another layer worth stating precisely. As of 1 January 2026, markers of semaglutide and tirzepatide are on WADA’s Monitoring Program — watched, but not prohibited and carrying no sanction. By contrast, growth hormone secretagogues such as GHRP-2, hexarelin, ipamorelin, and CJC-1295 are prohibited under Section S2 of the Prohibited List. BPC-157 is not named explicitly but is generally treated as covered by the list’s catch-all provisions for non-approved substances. We go deeper on this in Peptides and Anti-Doping.

How this ties back to the COA

Everything above is why a meaningful Certificate of Analysis matters. The COA exists precisely to detect the truncation, deletion, aspartimide, racemization, and counterion problems that synthesis inevitably produces. A useful one lets you confirm five things: that the identity matches the label, that HPLC purity is shown with the actual chromatogram, that mass spectrometry confirms the expected mass, that a batch or lot number ties the document to the vial in your hand, and that a named, identifiable lab ran the tests — ideally an independent third party, because a self-reported COA can simply be fabricated. The obestatin study is the reason to care about that last point: labels and certificates can be wrong, which is the whole argument for independent verification. See our notes on independent testing and the running ledger for how this plays out across real products.

Bottom line

Peptides are made either by building a chain one amino acid at a time on a solid support (chemical synthesis, the route for most peptides) or by growing them in engineered cells (recombinant production, used for larger molecules). The chemistry produces a predictable set of impurities — truncations, deletions, aspartimide and racemization byproducts, and residual counterion and reagents — which is exactly what purification and testing are designed to catch. Pharmaceutical manufacturing is held to published standards like USP ⟨1503⟩. Gray-market “research” peptides are not, and the evidence shows their quality genuinely varies — sometimes to the point of being a different compound entirely. That gap is why a real, lab-backed COA, not a marketing claim, is the only objective window you have into what is actually in a vial.