Solid-phase peptide synthesis — introduced by Merrifield and refined continuously since — anchors the first amino acid to a solid bead of resin, then adds the rest of the chain one unit at a time, washing away leftover chemicals after each cycle. Modern Fmoc chemistry is the dominant version.
Because each step of adding an amino acid is imperfect, a finished batch contains the target sequence alongside related impurities: chains missing an amino acid, chains cut short, and leftover chemical protecting groups. Characterising exactly that mixture is what purity testing exists to do, and it is why a purity figure is meaningless without the method that produced it.
