Peptide Purity Testing and How to Read a Certificate of Analysis

Identity asks whether the molecule in the vial is the molecule on the label. That question is answered by mass spectrometry, which measures the mass-to-charge ratio of ionized molecules and is compared against the mass calculated from the amino acid sequence. Purity asks what fraction of the material is that molecule rather than related substances, and is answered chromatographically. A document that reports only one of the two has not qualified the material.

Because the two measurements are independent, they can disagree in informative ways. A sample can show the correct mass while a chromatogram reveals a substantial second peak, which points to a related impurity that shares neither retention time nor abundance with the target. The reverse also happens: a clean single peak with a mass offset from theory indicates a consistently modified product rather than a mixture.

Reverse-phase HPLC separates components by hydrophobicity on a nonpolar stationary phase, most often C18 silica, using a water and acetonitrile gradient with an acidic modifier. Detection for peptides is usually ultraviolet absorbance near 214 nm, where the amide backbone absorbs, so the signal scales with peptide bonds rather than with any single side chain.

Purity is reported as the integrated area of the target peak divided by the total integrated area of all peaks, expressed as a percentage. Read the impurity pattern as well as the number. Peaks eluting near the target are typically synthesis-related species such as deletion sequences or incompletely deprotected intermediates; peaks far from the target are usually unrelated residues from processing. A shoulder on the main peak suggests an unresolved isomer rather than a pure single species.

Method conditions decide whether two purity figures can be compared at all. A short, steep gradient can co-elute impurities with the target and report a flattering number, while a longer gradient resolves them and reports a lower and more honest one. Regulatory guidance on demonstrating that an analytical procedure is fit for its purpose, including specificity and its ability to distinguish the target from related substances, is set out in ICH Q2(R2) and in FDA guidance on analytical procedures and methods validation.

Electrospray ionization and matrix-assisted laser desorption ionization are the two soft ionization techniques that made intact peptide and protein mass measurement routine; their development was recognized by the 2002 Nobel Prize in Chemistry. A COA reports a theoretical monoisotopic or average mass derived from the sequence and an observed mass from the instrument, and identity is confirmed when the two agree within the instrument's stated tolerance.

Interpreting the observed value requires knowing the charge state. Electrospray spectra show protonated species, so a peptide of several thousand daltons often appears as a doubly or triply charged ion at roughly one half or one third of the neutral mass. Sodium and potassium adducts sit approximately 22 and 38 mass units above the protonated species. A value near half the expected mass is normally a charge state, not a different compound.

Characteristic offsets are diagnostic rather than random: a deficit near 18 mass units suggests loss of water, an excess near 16 suggests oxidation at a sulfur-containing residue, a deficit equal to a single residue mass suggests a deletion sequence from incomplete coupling during solid-phase synthesis, and a near doubling of mass suggests a disulfide-linked dimer. Reference spectra and peptide mass spectral libraries maintained by NIST are the standard external comparison point for this kind of interpretation.

Lyophilized peptide mass is not pure peptide. Residual water, measured by Karl Fischer titration or loss on drying, and counterion content, commonly acetate or trifluoroacetate, both contribute to the weighed mass in the vial. Net peptide content reports the fraction that is actually peptide once water and counterion are accounted for, and it is the figure to use when calculating a molar concentration.

Where relevant, a bacterial endotoxin result by the LAL method and a sterility statement appear as well. Appearance should read as a white to off-white lyophilized powder or cake; a collapsed or discolored cake is a handling and stability flag independent of any number on the page. General chapters covering water determination, bacterial endotoxins and packaging and storage requirements are published in the United States Pharmacopeia and National Formulary.

Match the lot number on the COA to the number printed on the vial. Read the HPLC purity value and look at the shape of the chromatogram and its stated method. Reconcile the observed mass against the theoretical mass, accounting for charge state and adducts. Check water and net peptide content before calculating any concentration. Confirm the issuing laboratory, the analysis date and that the laboratory is independent of the seller.

If any one of those five checks fails, the material is not qualified for an experiment whose result you intend to trust. All products referenced on this site are supplied for in-vitro laboratory research use only. They are not drugs, foods, cosmetics or medical devices, and they are not for human or veterinary consumption or administration.