Definition
The proportion of peptide material in a sample that is the correct target sequence, usually measured by HPLC.
Peptide purity is the percentage of the peptide material in a sample that is the intended sequence, as distinct from synthesis by-products. Solid-phase synthesis produces truncated sequences, deletion sequences, incompletely deprotected species and oxidised forms; purification removes most of them. Purity is measured by reversed-phase HPLC as the main peak area relative to the total integrated area at 214–220 nm. Research-grade peptides are commonly specified at ≥98%. Purity is not the same as peptide content: a 99%-pure sample can still contain 20% or more non-peptide mass in the form of counter-ions and water. Purity also says nothing about identity, which requires mass spectrometry.