HPLC and mass spectrometry are the two pillars of research peptide characterization. They answer different questions, and a COA that presents only one has answered only half of what matters. This article explains what each method measures, what each cannot, and how the two combine into a verifiable identity-and-purity claim.
What is HPLC?
High-performance liquid chromatography is a separation technique. A sample is injected onto a column packed with a stationary phase and pushed through with a flowing solvent. Components of the sample interact with the stationary phase to different degrees and elute at different times. A detector — usually UV at 214 nm for peptides — records the signal, producing a chromatogram.
What is mass spectrometry?
Mass spectrometry ionizes molecules and measures their mass-to-charge ratio. For peptides, electrospray ionization (ESI) or matrix-assisted laser desorption ionization (MALDI) are standard. The output is a mass spectrum showing the observed molecular ion, which is compared to the theoretical mass calculated from the sequence.
Strengths of each method
HPLC excels at quantifying relative abundance. It separates closely related species and produces a defensible purity number. It is comparatively poor at identification: two molecules with similar retention times can look identical on a chromatogram.
Mass spectrometry excels at identification. It can distinguish two molecules that elute at the same time if their masses differ. It is comparatively poor at quantification of low-abundance impurities without dedicated method development.
Why both methods matter
Together, HPLC and MS answer both of the questions that matter. HPLC says how much is present. MS says what it is. A COA that reports only HPLC purity has confirmed how much of something is in the vial without confirming what. A COA that reports only MS has confirmed what the molecule is without confirming how much.
Reading the combined report
A complete COA shows the HPLC chromatogram with the main peak and minor peaks integrated, the purity percentage and conditions, the mass spectrum with the observed and theoretical mass, and the mass error. The values should be internally consistent — a chromatographically pure sample whose mass does not match the target is a problem that needs investigation, not a number to ignore.
Common mistakes in interpretation
Three mistakes recur: trusting purity without identity, trusting identity without purity, and reading a chromatogram without checking detection wavelength and column conditions. Each is the kind of small assumption that turns a defensible measurement into an unreliable one.
