A pure protein is one that is free from any quantifiable amounts of impurities. Any purity determination is only as reliable as the analytical methods used, and factors such as the structural properties of the protein itself, the amount of protein available, the nature of potential contaminants in the sample, and the accuracy of the estimate required should always be considered when selecting the method of analysis.

Peptide content is not an indication of peptide purity; these are two measurements. Purity is determined by HPLC and indicates the presence/absence of contaminating peptides with undesired sequences. Net peptide content only gives information on the percent of total peptide versus total non-peptide components independently of the presence of multiple peptides. Net peptide content is accurately found by performing amino acid analysis or UV spectrophotometry. This information is important when calculating concentrations of peptide during sensitive experiments.