Peptides, small or large, can efficiently be synthesized by solid phase synthesis.
From the method and the chemicals used it must be possible to predicts which impurities we can expect.
The method is described by the inventor Merrifield. Many years later he got a Nobel Prize for it.
https://www.nobelprize.org/uploads/2018/06/merrifield-lecture.pdf for those interested in the chemical details.
On youtube there are several films how it is done and explainers.
https://www.youtube.com/watch?v=jVXUfC2pLh4
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