TOF-SIMS analysis of polymers

When solid polymers are irradiated with heavy ions, atomic and molecular particles are ejected from the uppermost layers of the surface. A technique to determine the mass spectrum of the charged fraction of these particles is time-of-flight secondary-ion-mass spectrometry, TOF-SIMS. The present article describes, how the mass spectra measured with polymers are generally structured and under which conditions the various types of secondary ions like cationized oligomers, fragment ions and “fingerprint” ions are observable. The mechanisms leading to formation and ejection of the ions are not well understood. At bombarding energies of 10 keV, they are mainly based on atomic collision processes, at 100 MeV on the electronic excitation of the solid in the vicinity of the nuclear track. Processes, which are capable to desorb large organic molecules, seem not to work with oligomers of similar mass. Mass spectrometry of “real world” polymers, i.e. thick samples, depends mostly on the low-mass fingerprint spectrum, which can be produced by keV MeV SIMS. Modern TOF-SIMS instruments are equipped with a pulsed ion gun and an energy focussing ion mirror. They provide high mass resolution ( m Δm ⋍ 10000 ) and high transmission (20–50%). Examples of applications are given, like the determination of mean molecular weights or investigations of radiation induced modifications of polymers.