When sourcing polypeptides, the most critical factor is purity, which should typically be ≥98% for research or cosmetic grade and even higher for pharmaceutical applications. You must verify the amino acid sequence accuracy through Mass Spectrometry (MS) and ensure High-Performance Liquid Chromatography (HPLC) reports are provided for every batch. Additionally, pay attention to the salt content (e.g., TFA, Acetate, or Hydrochloride) as the counter-ion can significantly affect the solubility and biological activity of the peptide in your final formulation.