The Study of Positive-Ion Mass Spectrometry of 5-Fluorouracil Using Low-Energy Electron Impact

The aim of the experiment described within this thesis was to generate a molecular beam of 5-fluorouracil and to investigate the fragmentation processes that were induced by low-energy electron impact. The apparatus is contained in a vacuum chamber with three main compartments: the expansion chamber, the collision chamber and the flight-tube. The expansion chamber is where the molecular beam is produced by a resistively heated oven containing 5-fluorouracil powder. The molecular beam enters the collision chamber through a skimmer where it is crossed with a pulsed electron beam. The electron beam has a pulse width of 0.5 ms providing good time-of-flight resolution. Positive ions produced by electron collisions with the molecules are extracted into a reflectron time-of-flight mass spectrometer. The field-free region and the reflector of the time-of-flight mass spectrometer are both located in the flight-tube. The reflector is positioned at the end of the flight-tube and directs the ions back into the field-free region towards the detector. A multichannel scaler triggered synchronously with the electron pulse is used to accumulate time-of-flight spectra. Using computer-controlled data acquisition, mass spectra were measured for electron impact energies from 5 eV - 100 eV in 0.25 eV steps. Ion yield curves for most fragment ions were determined by fitting groups of peaks in the mass spectra with sequences of normalised Gaussians. The appearance energies for these ions were determined by fitting onset functions to the ion yield curves. By comparing the mass spectra and the appearance energies with those of uracil, new information about fragmentation processes is obtained