Exploring the peptide fragmentation mechanisms under atmospheric pressure photoionization using tunable VUV synchrotron radiation

- APPI in-source fragmentation mechanism of Substance P was investigated.
- Toluene exhibits a low adiabatic ionization energy due to vibrational excitation.
- Substance P ions originate from pneumatic nebulization then from charge reduction.
- c-Type fragments originate from ECD-like process on the doubly charged species.
- b/y-Type fragments originate from decomposition of neutrals followed by proton transfer.