What Does Purity % on a COA Mean?

Area-Percent Purity and HPLC Peak Integration

Reverse-phase HPLC (RP-HPLC) separates peptides on a hydrophobic C18 column and measures the area under each peak at 214 nm. Purity is calculated as: (target peak area) / (sum of all peak areas) × 100%. This assumes equal extinction coefficients—reasonable for structurally similar compounds but potentially biased if impurities have vastly different absorption. Modern software applies response-factor corrections when extinction coefficients are known. Typical RP-HPLC achieves purity values of 85–99% for laboratory-synthesized peptides.In vitro · PMID 33443297Review · PMID 35460196

The purity percentage reflects the molar distribution at the analysis time under the specified mobile phase composition. Acetate counter-ions and trifluoroacetate (TFA) influence peptide retention but do not absorb at 214 nm in most cases. Thus, RP-HPLC purity accounts for the intact peptide form but neglects non-peptide salt background. This is why RP-HPLC purity may appear lower than LC-MS purity for the same sample: mass spectrometry detects only ionizable species, ignoring inorganic salts and residual solvents.Review · PMID 27645457Review · PMID 36526726

Composition of Peptide Impurity Profiles

The non-purity fraction (e.g., 5% in a 95% pure sample) typically comprises synthesis-related byproducts. Truncated peptides—sequences missing one or more amino acids from either terminus—are common during solid-phase peptide synthesis (SPPS) if the deprotection or coupling steps are incomplete. Diketopiperazine (DKP) cyclization is a major byproduct when the first dipeptide residue forms a six-membered ring, competing with chain elongation. Racemized amino acids (D-enantiomers instead of L-enantiomers) occur during peptide bond formation, especially for amino acids with bulky side chains. These impurities have retention times close to the target peptide, complicating separation.In vitro · PMID 33443297In vitro · PMID 34690628

Residual synthesis chemistry contributes acetonitrile, trifluoroacetic acid (TFA), water, and protecting group fragments that remain after cleavage and workup. Storage-related degradation includes oxidation of methionine to methionine sulfoxide, deamidation of asparagine or glutamine (conversion to aspartate or glutamate with backbone rearrangement), and hydrolysis of peptide bonds under unfavorable pH or temperature conditions. The counter-ion state affects molar mass: a peptide may be isolated as the acetate salt, TFA salt, or hydrochloride salt, each with different molecular weights and peak integration. A COA ideally specifies the counter-ion form; if not stated, assume the salt form used in the synthesis chemistry.In vitro · PMID 29978532In vitro · PMID 31039402

Standards and Method Comparability

Purity acceptance criteria depend on the application. Research-grade peptides typically meet ≥90% purity; analytical standards require ≥95–99%. For biochemical studies, 90–95% purity is sufficient if impurities do not interfere with the assay. Quantitative assays require higher purity (≥97%) to minimize confounding. The threshold depends on whether impurities (e.g., truncated peptides) could produce a measurable signal in the assay.In vitro · PMID 33443297Review · PMID 35460196

Purity percentages from different sources are not directly comparable unless analytical methods are identical. A 95% result from Lab A on a C18 column may differ from a 95% result from Lab B on a C8 column. Column type, flow rate, temperature, and detection wavelength all affect peak resolution. Two sources differing by 3–5% purity may reflect analytical method differences rather than actual product quality differences. Request detailed method documentation before comparing results.Review · PMID 27645457In vitro · PMID 34783086

LC-MS as Complementary Identity and Purity Confirmation

Liquid chromatography-mass spectrometry (LC-MS) couples RP-HPLC separation with electrospray ionization (ESI) and mass detection. Each eluting peak is ionized (protonated via ESI), and its mass-to-charge ratio (m/z) is measured. For a 3000 Da peptide, a singly protonated ion (M+H)+ appears at m/z ~3001. Doubly protonated peptides (M+2H)2+ appear at m/z ~1500. This mass information provides unambiguous identity confirmation: two compounds with identical retention times but different masses are distinguished as separate species. Purity by LC-MS is reported as percentage by peak area (similar to RP-HPLC) or by total ion current (all ion signals summed).In vitro · PMID 38968868In vitro · PMID 34783086

LC-MS/MS (tandem mass spectrometry) fragments peptide ions using collision-induced dissociation (CID), revealing a sequence-specific fragmentation pattern that confirms identity. For short peptides like GHK (3 amino acids), LC-MS/MS rapidly identifies the compound and purity in a single analysis. A key advantage of LC-MS over RP-HPLC-UV is selectivity: non-peptide impurities such as inorganic salts, water, acetonitrile, and TFA do not ionize efficiently and are invisible to the mass detector. Thus, LC-MS purity often exceeds RP-HPLC purity for identical samples because salts and residual solvents contribute to the RP-HPLC denominator but not to the LC-MS result. For peptides with weak UV chromophores (few aromatic residues), LC-MS/MS is particularly valuable for confirming identity and purity.Review · PMID 36526726In vitro · PMID 35988299

Interpreting Purity and Storage Stability

The purity percentage on a COA reflects the target peptide abundance at a single moment: the analysis date, under the specified conditions (solvent, pH, temperature). This value does not predict future purity after purchase or during storage. Proper storage (frozen at -20°C or below, sealed, dark, desiccated) minimizes degradation, and purity remains stable for months to years. Improper storage—room temperature, light-exposed, humid, or repeated freeze-thaw cycles—promotes hydrolysis, oxidation, and aggregation, decreasing the measured purity over time. If a peptide is stored improperly, a re-analysis weeks or months later will reveal lower purity than the original COA. This is why the COA includes an analysis date and, ideally, recommended storage conditions and shelf-life estimates.In vitro · PMID 33443297In vitro · PMID 31039402

When evaluating purity discrepancies between samples, batches, or sources, request full method details and consider whether the difference is due to analytical method variation, actual storage degradation, or sample handling differences. A 5-point purity difference (95% vs. 90%) could reflect different RP-HPLC gradient programs, or it could indicate genuine degradation of one sample. If mission-critical, request that both samples be analyzed by the same reference laboratory using identical methods. Ideally, a COA specifies the identity of major impurities; if impurities are not named, assume they are below the method's detection limit and individually represent <0.5% each. For research applications, 90–95% purity is sufficient; for analytical standards or reference materials, ≥98% is standard.In vitro · PMID 29978532In vitro · PMID 34690628

The studies at a glance

Peptide purity at different synthesis and purification stages, measured by RP-HPLC

Frequently asked questions