Purity assessment is central to research peptide quality control. Two analytical methods dominate purity characterization in this field: high-performance liquid chromatography (HPLC) and mass spectrometry (MS). Understanding the capabilities and limitations of each method is essential for interpreting COA data accurately.
HPLC: Separation-Based Purity
HPLC separates the components of a sample based on their differential affinity for a stationary phase (the column packing material) versus a mobile phase (the solvent flowing through the column). Components with higher affinity for the stationary phase elute later; those with higher affinity for the mobile phase elute earlier.
A UV detector monitoring absorbance at 214nm or 280nm measures the concentration of each eluting component. Purity is calculated as the area of the target peptide peak divided by the total area of all detected peaks, expressed as a percentage.
HPLC purity reflects relative purity — the proportion of the total UV-absorbing material that corresponds to the target compound. It is the standard method for research peptide purity assessment and is the basis for the ≥98% purity threshold used in quality specifications.
Limitations of HPLC
HPLC cannot definitively identify what the detected compounds are — only that they are present and absorb UV light. A sample could contain impurities that co-elute with the target compound (overlapping peaks), which would falsely elevate the apparent purity. Additionally, compounds that do not absorb UV light at the detection wavelength will not be detected, creating potential blind spots.
Mass Spectrometry: Identity and Structural Confirmation
Mass spectrometry measures the mass-to-charge ratio (m/z) of ionized molecules. For peptides, electrospray ionization (ESI) is typically used to generate multiply charged ions, which are then separated by the mass analyzer.
MS provides definitive molecular weight confirmation — if the measured mass matches the theoretical mass of the target compound, identity is confirmed. It can also detect specific impurities, degradation products, or sequence errors that HPLC cannot distinguish.
HPLC-MS: The Gold Standard
The combination of HPLC separation with mass spectrometric detection (HPLC-MS or LC-MS) provides both purity quantification and identity confirmation in a single analysis. This approach can distinguish between co-eluting compounds that would be invisible to UV detection alone and is considered the most comprehensive single analytical method for peptide characterization.
What Researchers Should Look For
A COA incorporating both HPLC purity (≥98%) and mass spectrometric identity confirmation provides the strongest quality assurance. FTIR, while valuable for identity confirmation, is less sensitive than MS for detecting minor structural variants or sequence errors. When evaluating vendor COAs, the combination of HPLC and either MS or FTIR identity confirmation represents current best practice in research peptide quality documentation.