A peptide isn’t harvested but built, amino acid by amino acid. Once you know that process, you also understand where the impurities come from that you later see in the chromatogram.
Solid-phase synthesis
The chain grows on a solid support: a resin bead with the first amino acid attached to it. After that, the same cycle repeats. The protecting group comes off, the next amino acid is coupled, and everything that didn’t react is washed away. For a peptide of thirty amino acids, you go through that cycle thirty times.
The advantage is that you can wash after every step without losing the product, since it’s attached to the resin. Only at the end is the chain cleaved off.
Where it goes wrong
No coupling is ever one hundred percent complete. If an amino acid is skipped somewhere, you get a chain missing one link: a deletion peptide. It resembles the target molecule so closely that it ends up right next to it in the HPLC, which makes separation difficult.
Variants also arise where a protecting group has stayed on, and with longer chains the chance of degradation during synthesis increases. The longer the peptide, the more of these relatives you ultimately have to remove.
Purification
After cleavage from the resin, the crude mixture goes through preparative HPLC. It is separated in the same way as during analysis, only now to collect the main fraction instead of measuring it. This is the step that decides whether you end up at 95% or at 99%, and it’s also where yield is lost: stricter separation means less output.
Lyophilization
The collected fraction is frozen and dried under vacuum, with the ice turning directly into vapor. What remains is a fluffy white lyophilizate that can be shipped without refrigeration and stays stable for years at 2–6 °C. Only when you add solvent does the shelf-life clock start again.