Cleavage, purification and why purity varies
What happens between the synthesiser and the vial, and why two batches of the same compound can differ.
Synthesis produces a mixture, not a product. The step that turns one into the other is purification, and it is where most of the cost and most of the quality difference between suppliers actually sits.
What comes off the resin
Cleavage releases the peptide from the solid support and removes the side-chain protecting groups, typically using trifluoroacetic acid with scavengers added to mop up the reactive fragments that come loose in the process.
The crude material contains the target sequence plus a predictable set of companions: chains missing a residue, chains where a coupling went wrong, fragments, and residues of the cleavage chemistry. Crude purity for a moderately long peptide might be anywhere from 50% to 80%.
Preparative HPLC
Purification uses the same separation principle as analytical HPLC, scaled up. The crude mixture is run through a column, the components separate in time, and the fraction containing the target peak is collected while everything else is discarded.
The trade-off is simple and unavoidable: collect a narrow window around the peak and you get high purity with low yield; collect a wider window and you get more material of lower purity. Where a manufacturer sets that window is a commercial decision as much as a technical one.
Why batches vary
Several things shift between runs. Coupling efficiency is affected by the sequence itself — some sequences are prone to folding on the resin and coupling poorly. Reagent quality varies. Column performance degrades over time. The operator’s judgement on fraction collection varies.
None of this is irregularity; it is the normal variation of a chemical process. It is precisely why a certificate is issued per batch rather than per product, and why a certificate with no lot number is worthless.
Desalting and final lyophilisation
After purification the peptide is in a solvent system containing acid. A desalting or ion-exchange step exchanges the counterion where required — often trifluoroacetate to acetate — and the solution is then lyophilised to a dry powder, filled and sealed.
What this means when you compare suppliers
Two vials labelled identically can represent genuinely different materials: different crude quality, different purification windows, different counterions, different residual solvent levels. The label tells you almost nothing. The batch certificate tells you nearly everything.