Purity

Peptide purity refers to the proportion of the product of interest relative to all chemical species present in the final batch, expressed as area percent (HPLC-UV) or weight percent. It is the central quality indicator for any research use: insufficient purity introduces contaminating species that can have their own biological activity, confound experimental results, and generate non-reproducible artifacts.

Standard analytical methods rely on reverse-phase high-performance liquid chromatography (RP-HPLC) coupled with UV detection (usually 220 nm for the amide bond, 280 nm for tyrosine/tryptophan). Elution is performed on a C18 column with acetonitrile/water + 0.1 % TFA or formic acid gradient. HPLC purity is calculated as the ratio of the main peak area to the sum of all detected peak areas. Mass spectrometry (MS, typically ESI-MS or MALDI-TOF) confirms the identity of this main peak and identifies impurities by their mass. Complementary techniques (amino acid analysis, 1D/2D NMR, circular dichroism) round out the dossier for critical batches.

Purity thresholds depend on the application. In preclinical research, the convention accepts ≥ 95 % for standard in vitro assays and ≥ 98 % for precision quantitative assays (binding, enzymology, animal pharmacokinetics). In pharmaceutical development, regulatory authorities (FDA, EMA) require ≥ 98 % purity with exhaustive characterization of each impurity > 0.1 %, identification, and, if applicable, toxicological qualification. The European Pharmacopoeia (Ph. Eur.) and the United States Pharmacopeia (USP) detail methods and criteria for marketed peptides.

Typical impurities of an SPPS peptide include: truncated peptides (incomplete coupling of an amino acid, N-1, N-2 sequences…), deleted peptides (internal skip), peptides with oxidized amino acids (Met sulfoxide), racemized peptides (D-aa in place of L-aa), residual protection adducts, deamidated peptides (Asn→Asp/iso-Asp), incorrect disulfide dimers, residual salts (TFA, acetate, ammonium), residual solvents (acetonitrile, DMF), and residual water.

Interpreting a Certificate of Analysis (COA) requires checking several points: HPLC purity ≥ 95 %, molecular mass confirmed by MS with tolerance < 0.1 Da for monoisotopic mass, net peptide content (corrects gross content for counter-ions and water), profile of main impurities, endotoxin test result (LAL) if used in cell or animal studies, bioburden, and visual aspect. A serious COA mentions the analytical institution, issue date, batch number, quality officer's signature, and provides raw chromatograms and spectra.

For a research peptide user, comparing several COAs from different suppliers while demanding raw supporting documents (HPLC chromatograms, MS spectra, impurity analytics) remains the best defense against under-specified batches sold at bottom prices.