Generally, ionization of most chemical species needs photons with 10 eV or higher energy, requiring access to synchrotron facilities or high power lasers in the laboratory. The use of latter can result in extensive fragmentation and prevent proper mass analysis. A custom-built PI-TOFMS uses frequency-tripled light from a Nd:YAG laser to irradiate a mixture of Xe:Ar in the PI-chamber of the instrument.(63) Ionization of this inert gas mixture produces tripled light at 118 nm than can ionize the analyte. While this set up has been used to ionize species that have large ionization potentials such as iodine oxides I x O y , PI of atmospherically relevant species is a challenging task in the laboratory and is harder to achieve in field studies. .