When evaluating peptide quality, the most critical factor is Purity, which should ideally be ≥98% for research or cosmetic grade and even higher for pharmaceutical applications. You must request a COA (Certificate of Analysis), HPLC (High-Performance Liquid Chromatography) report, and MS (Mass Spectrometry) data to verify the molecular weight and sequence integrity. Additionally, pay attention to the TFA (Trifluoroacetic acid) content, as high levels can be toxic to cells in certain applications; look for suppliers offering TFA removal services.