Every research peptide on a lab bench started as a sequence of amino acids built up one residue at a time on a solid support. That process, solid-phase peptide synthesis (SPPS), is worth understanding on its own terms — not because researchers need to run it themselves, but because the way a peptide is made directly explains the impurity profile a Certificate of Analysis reports later. This guide covers the core mechanism, the two dominant chemistries, and why the process is inherently imperfect at scale.
Peptides can in principle be built in solution, coupling one amino acid to the next in a flask. In practice, SPPS is the standard approach for both small research-scale batches and multi-kilogram production, because it was designed to solve solution synthesis's core problem: after every coupling step, solution chemistry requires isolating and purifying an intermediate before the next step can begin, which is slow and lossy over a chain of many residues. Robert Bruce Merrifield introduced solid-phase synthesis in the early 1960s specifically to remove that bottleneck, work that was later recognized with the 1984 Nobel Prize in Chemistry.
The core of SPPS is a repeated two-step cycle, carried out on amino acids anchored to an insoluble polymer resin bead:
Because the growing peptide chain stays attached to an insoluble resin throughout, excess reagents and byproducts from each step can simply be washed away by filtration, without ever isolating the intermediate. That is the entire advantage of the solid-phase approach: synthesis becomes a repeated wash-and-react cycle instead of a repeated purify-and-isolate cycle. Once the full sequence is assembled, the finished peptide is cleaved off the resin and its permanent side-chain protecting groups are removed, typically in the same step.
The temporary group that masks the growing chain's N-terminus between coupling steps has to come off cleanly, on demand, without damaging the rest of the molecule. Two chemistries dominate:
Fmoc chemistry's base-labile/acid-stable design gives it a substantially better safety and handling profile than Boc chemistry's reliance on hydrogen fluoride, which is why Fmoc-based SPPS is now the more common approach for research- and commercial-scale peptide manufacturing, including automated synthesizers.
No coupling step runs at 100% efficiency. Even a highly optimized cycle leaves a small fraction of chains that failed to react at that step, and that fraction compounds over the length of the sequence — a 30-residue peptide requires roughly 30 coupling cycles, each with its own incremental chance of a miss. Two impurity types trace directly back to this:
This is why the HPLC purity figure on a Certificate of Analysis exists in the first place: it is measuring how much of the finished batch is the full, correct-length target sequence versus these synthesis-related near-neighbors, which is also why deletion and truncation impurities are often the hardest peaks to separate from the main peak — they can be extremely similar in both mass and chromatographic behavior to the target peptide.
Every batch we sell is tested by an independent third-party laboratory after synthesis, and the resulting purity, identity and impurity data is published per lot on our Analysis / COA page. For a walkthrough of how to read those figures, see How to Read a Peptide COA.