Variable energy photoionization device and method for mass spectrometry

A mass spectrometer using a variable energy photoionization device for ionizing and/or cleaving molecules is disclosed. The device permits ionizing photon wavelengths to be selected from a range of wavelengths allowing the ionizing photon energies to be tuned so as to ionize molecules without excessive fragmentation or to cleave molecules in a controlled manner by breaking only certain molecular bonds. Selection of the wavelengths is afforded by the choice of a plasma-forming gas combined with windowlessly radiating the ionizing photons from a plasma chamber. A method of mass spectrometry featuring selected ionizing photon wavelengths is also disclosed.