What does HPLC measure?
High-performance liquid chromatography pushes a dissolved sample through a column packed with a separation medium. Different components travel at different speeds and emerge — elute — at different times, producing a chromatogram of peaks. The target compound forms one peak; related impurities form others.
Purity is calculated from the area of the target peak relative to the total area of all peaks. Reported as a percentage, this is the figure that appears as the purity result on a Certificate of Analysis.
What does mass spectrometry confirm?
Mass spectrometry ionizes the sample and measures the mass-to-charge ratio of the resulting ions, yielding the molecular weight of the material. That measured weight is compared against the theoretical weight calculated from the compound's known structure.
When the measured and expected masses agree, the material's identity is confirmed: it is the labeled compound, not a substitute or a mislabeled material of similar appearance.
How do HPLC and mass spectrometry differ?
The two methods answer different questions and are read differently. The table summarizes the distinction.
| HPLC | Mass spectrometry | |
|---|---|---|
| Question answered | How pure is the sample? | Is it the right compound? |
| Property measured | Relative amount of each component | Molecular weight of the material |
| Typical readout | Purity as a percentage (peak area) | Measured mass vs. expected mass |
| What it detects | Related impurities and their proportion | Identity / molecular-weight mismatch |
Why are both methods needed?
A sample can be highly pure yet not be the labeled compound, and the right compound can still carry impurities. Purity without identity, or identity without purity, leaves one of those gaps open.
Running both closes the gap: HPLC establishes that the sample is predominantly one component, and MS establishes that the component is the intended one. This is why a complete Certificate of Analysis reports both an HPLC purity result and an MS identity confirmation.