If a peptide is 98% purity, what is the other 2%?

Short answer: It’s mostly tiny amounts of synthesis-related by-products and peptide variants—not fillers.

On a COA, “98% purity” typically means the main peak is 98% of the HPLC area; the remaining ~2% are minor components detected by the method.

What can be in the ~2%?

• Truncated/deletion sequences (n−1, n−2): Peptides missing one or more residues from incomplete couplings.
• Sequence variants/epimers: Rare isomerization or miscouplings that create closely related sequences or D/L epimers.
• Modified forms: Slightly oxidized (e.g., Met(O)), deamidated (Asn→Asp/isoAsp), or other minor chemical modifications from handling/storage.
• Aggregates/dimers: Small amounts of peptide that associate non-covalently or form trace dimers.
• Trace process residues: Minute remnants from synthesis/cleavage/purification (usually below reporting limits and screened by QC).

Note: HPLC area % ≠ weight %. Water, counter-ions (e.g., TFA/acetate), and residual volatile solvents don’t always show up in the HPLC purity number but can affect net weight and salt form.

Does 98% purity affect research use?

For most research applications, 98% is considered high purity.

Key takeaways

• The “other 2%” isn’t “free amino acids”; it’s closely related peptide species and trace process by-products.
• COAs report both identity (e.g., MS) and purity (e.g., HPLC). Always consult the COA for the exact method and lot-specific data.

RUO reminder: All products are for laboratory research use only and not for human or veterinary consumption.