HPLC-UV and LC-MS answer different questions
Purity and identity are not the same property, and the two methods that measure them are not substitutes for one another.
Two methods, two questions
Neither method substitutes for the other. Every PEPTiQX batch carries both.
Every batch we release carries both an HPLC-UV purity result and an LC-MS identity confirmation. People sometimes ask why we pay for both. The short answer is that they answer different questions.
## What HPLC-UV tells you
Reverse-phase HPLC separates components by hydrophobicity. A UV detector at 214 nm sees the peptide bond, which means it responds to essentially any peptide in the sample — your target and its deletion sequences alike. Integrating the peaks gives you a relative purity figure. What it cannot do is tell you which peak is which.
## What LC-MS tells you
Mass spectrometry measures mass-to-charge ratio. Comparing the measured mass against the theoretical monoisotopic mass of your sequence confirms you have the compound you ordered. A single amino acid substitution shifts the mass measurably — this is how sequence errors get caught.
## Why both
HPLC without MS gives you a pure unknown. MS without HPLC gives you a confirmed identity in an uncharacterised mixture. Run together, they tell you that the right molecule is present and that little else is.
