Time of Flight Secondary Ion Mass Spectrometry

TOF-SIMS is a mass spectrometry technique in which a solid sample is bombarded by ions, which then release fragments of the surface molecules. The material ejected from the sample typically is from only the first few atomic layers. The depth of penetration relies on the initial energy (KeV) imparted to the primary ions. A general diagram of this effect is presented in Figure 1.

The impact of the primary ions with the sample surface leads to fragmentation and ejection of the molecules on the immediate surface. As with most mass spectrometry techniques, molecules fragment result in a characteristic pattern which can be used for identification. The precise amount of fragmentation depends on the KeV of the primary ions used.

If a low enough KeV is employed, it is possible to eject molecules with extremely low amounts of fragmentation, even up to 10,000 Atomic Mass Units (amu). These fragments are then examined in the time of flight mass spectrometer. The KeV can also be tuned to lead to ejection of particular ions in a sample at a certain depth, and can thus additionally be employed for depth profiling experiments. These experiments usually identify low molecular weight ions or individual elements, since large ions tend to be unable to penetrate the surface from a depth of multiple nanometers.

Ion Impact and Ejection from Sample Surface.

Figure 1. Diagram of Ion Impact and Ejection from Sample Surface.


Why Should One Use TOF-SIMS and Why is It Important?

  • Identification of the molecules present on a sample’s surface, surface chemistry identification
  • Detection limits are normally from ppm to ppb
  • High mass resolution (minimum two decimal place mass accuracy)
  • Variable depth can be used for penetrate angstroms to multiple nanometers deep


Sample Preparation and Introduction

Samples are generally analyzed as received. Sample modification or preparation tends to leave behind detectable residues, which are not ideal when performing an analysis of the surface chemistry of the sample. The sample is normally gently cleaned with nitrogen instantly before analysis in order to remove contaminants, even though it is not a required step. It is possible to place small particulate samples on indium foil, which produces a conductive surface in order to highlight the particles being analyzed. Areas studied can be as small as 0.2 μm, the size of the probe, and as large as 200 mm.

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Applications of TOF-SIMS: Understanding Sample Slipping

4-(5-Dodecyl) benzenesulfonate.

Figure 2. 4-(5-Dodecyl) benzenesulfonate.

TOF-SIMS excels at identifying surface chemistry as compared to bulk chemistry, and can thus provide insight regarding the surface layer and how slip properties arise in a material. Figures 3-4 show acquired data in negative and positive ion modes respectively for a sample that was displaying good slip characteristics. The sample was a polydimethylsiloxane.

The spectrum displays that the surface layer on the good sample is composed primarily of hydrocarbons including aromatic components. In Figure 4, the negative ion mode spectrum displays signals consistent with a surfactant. A common surfactant that is consistent with the fragmentation patterns observed is 4-(5-Dodecyl) benzenesulfonate (Figure 2). Analysis of a control sample with poor slip properties failed to show these hydrocarbon components. Additional analysis of this sample could comprise of surface mapping of the sample using TOF-SIMS.

Surface mapping shows the location of a specific chemical component (ion distribution). An example would be searching the spectra for 127 and 265 in order to identify the existence of 4-(5-Dodecyl) benzenesulfonate across the whole surface of the sample. This can be carried out for multiple ions, and thus multiple compounds on a single sample.

Time of Flight Mass Spectrum, Positive Ion Mode.

Figure 3. Time of Flight Mass Spectrum, Positive Ion Mode.

Time of Flight Mass Spectrum, Negative Ion Mode.

Figure 4. Time of Flight Mass Spectrum, Negative Ion Mode.

Jordi Labs

This information has been sourced, reviewed and adapted from materials provided by Jordi Labs.

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