These notes sit beside the service page for custom peptide synthesis. They explain how sequences are assembled, why purity grades differ, and when a long or difficult peptide needs a different route.
Start with the complete synthesis guide if you are writing a first specification. Use the SPPS and Fmoc versus Boc articles when the question is the chemistry of assembly, not the commercial scope. Use the purity, long-peptide and difficult-sequence notes when the sequence or the HPLC grade is the risk. None of these articles replaces a quotation; they exist so the quotation can be specific.
Catalog peptides are a separate path. If the classical sequence is already listed, confirm termini and CAS on the product page. If it is not listed, treat the request as custom synthesis. All materials described here are for laboratory research only.