CHAPTER 5: Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS): Principles and Practice in the Biogeosciences

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) provides chemical information on the outermost molecular layers of sample surfaces without solvent extraction. In TOF-SIMS, a pulsed beam of high-energy ions (primary ions) is scanned over a selected analysis area on the sample. During the primary ion bombardment, neutral and charged particles (secondary ions) are released from the outermost molecular layers of the sample surface. Analysis of the secondary ions in a TOF analyser yields a mass spectral data set that allows the retrospective production of(1) ion images showing the spatial signal intensity distribution from selected ions over the analysis area; (2) mass spectra from the total analysis area; or (3) mass spectra from user-defined regions of interest inside the analysis area. In the so-called static SIMS regime, the primary ions are provided in very short pulses and the analysis is completed before the incoming primary ions damage a significant fraction of the surface. Static TOF-SIMS is therefore capable of providing molecularly specific secondary ions, and thus mass spectra with detailed organic information, which is not possible with other (‘dynamic’) SIMS techniques. In this chapter, we describe the principles of static TOF-SIMS instrumentation and data evaluation, review a number of relevant applications, and discuss the potential of this technique in the biogeosciences, with a focus on organic biomarker applications.