Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS)

David W. Mogk, Montana State University

Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) is a surface-sensitive analytical method that uses a pulsed ion beam (Cs or microfocused Ga) to remove molecules from the very outermost surface of the sample. The particles are removed from atomic monolayers on the surface (secondary ions). These particles are then accelerated into a "flight tube" and their mass is determined by measuring the exact time at which they reach the detector (i.e. time-of-flight). Three operational modes are available using ToF-SIMS: surface spectroscopy, surface imaging and depth profiling. Analytical capabilities of ToF-SIMS include:

• Mass resolution of 0.00x amu. Particles particles with the same nominal mass (e.g. Si and C₂H₄, both with amu = 28 ) are easily distinguished from one another because as Mr. Einstein predicted there is a slight mass shift as atoms enter a bound state.
• Mass range of 0-10,000 amu; ions (positive or negative), isotopes, and molecular compounds (including polymers, organic compounds, and up to ~amino acids) can be detected.
• Trace element detection limits in the ppm range.
• Sub-micron imaging to map any mass number of interest.
• Depth profiling capabilities; sequential sputtering of surfaces allow analysis of the chemical stratigraphy on material surfaces (typical sputtering rates are ~100 A/minute).
• Retrospective analysis. Every pixel of a ToF-SIMS map represents a full mass spectrum. This allows an analyst to retrospectively produce maps for any mass of interest, and to interrogate regions of interest (ROI) for their chemical composition via computer processing after the dataset has been instrumentally acquired.

ToF-SIMS uses a focused, pulsed particle beam (typically Cs or Ga) to dislodge chemical species on a materials surface. Particles produced closer to the site of impact tend to be dissociated ions (positive or negative). Secondary particles generated farther from the impact site tend to be molecular compounds, typically fragments of much larger organic macromolecules. The particles are then accelerated into a flight path on their way towards a detector. Because it is possible to measure the "time-of-flight" of the particles from the time of impact to detector on a scale of nano-seconds, it is possible to produce a mass resolution as fine as 0.00X atomic mass units (i.e. one part in a thousand of the mass of a proton). Under typical operating conditions, the results of ToF-SIMS analysis include:

1. a mass spectrum that surveys all atomic masses over a range of 0-10,000 amu,
2. the rastered beam produces maps of any mass of interest on a sub-micron scale, and
3. depth profiles are produced by removal of surface layers by sputtering under the ion beam.

ToF-SIMS is also referred to as "static" SIMS because a low primary ion current is used to "tickle" the sample surface to liberate ions, molecules and molecular clusters for analysis. In contrast, "dynamic" SIMS is the method of choice for quantitative analysis because a higher primary ion current results in a faster sputtering rate and produces a much higher ion yield. Thus, dynamic SIMS creates better counting statistics for trace elements. Organic compounds are effectively destroyed by "dynamic" SIMS, and no diagnostic information is obtained.

ToF-SIMS instruments typically include the following components:

• An ultrahigh vacuum system, which is needed to increase the mean free path of ions liberated in the flight path;
• A particle gun, that typically uses a Ga or Cs source;
• The flight path, which is either circular in design, using electrostatic analyzers to direct the particle beam (see Charles Evans TRIFT design), or linear using a reflecting mirror (see the "reflectron" design of Cameca's IonTOF system); and
• The mass detector system.

ToF-SIMS instruments are also equipped with a powerful computer and software for system control and analysis. One of the key features of the ToF-SIMS software is the ability to perform "retrospective" analysis, that is, every molecule from the sample detected by the system can be stored by the computer as a function of the mass and its point of origin. This allows the user to obtain chemical maps or spectra of specific regions not previously defined after the original data has been collected.

ToF-SIMS is widely used in material science disciplines in studies of materials such as polymers, pharmaceuticals, semi-conductors. The three main modes of data acquisition include:

• Elemental/Molecular Surveys;
• Elemental/Molecular Maps; and
• Depth Profiles.

In principle, ToF-SIMS is applicable to any surface-mediated reaction such as: catalysis, sorption, redox, and dissolution/precipation reactions. Only recently has ToF-SIMS been applied to geologic materials. Some examples include:

• Organic Films on mineral grain boundaries
• Identification of organic biomarkers in the rock record
• Characterization of organic macromolecures in coal deposits
• Analysis of metals precipitated from magmatic fluids in seafloor hydrothermal systems
• Analysis of interplanetary dust particles

• Surveys of all masses on material surfaces; these may include single ions (positive or negative), individual isotopes, and molecular compounds;
• Elemental and chemical mapping on a sub-micron scale;
• High mass resolution, to distinguish species of similar nominal mass (mass resolution is at least 0.00x amu);
• High sensitivity for trace elements or compounds, on the order of ppm to ppb for most species;
• Surface analysis of insulating and conducting samples;
• Depth profiling (in the near surface environment, on the order of individual atomic layers to 10s of nanometers);
• Non-destructive analysis;
• Retrospective analysis, for post-data acquisition analysis and interpretation of stored images and spectra.

• Generally does not produce quantitative analyses (semi-quantitative at best);
• Optical capabilities are typically limited, making it difficult to find grains or specific regions of interest for analysis;
• Charging may be a problem in some samples, although charge compensation routines are generally sufficient to overcome these problems;
• There is commonly an image shift when changing from positive to negative ion data collection mode; this makes it difficult to collect positive and negative ion data on exactly the same spot; and
• Too much data; the benefit of retrospective analysis is also its curse. Every pixel of an image produced by ToF-SIMS also contains a full mass spectrum for that point. Thus, it may take hours, days or weeks to fully analyze a single data set. Consequently, it is extremely important to have a very clear purpose in collecting ToF-SIMS data, and focus on analyzing and interpreting the data that are specifically related to the question at hand.

ToF-SIMS is extremely sensitive to any sample preparation treatments: there is typically a residue related to any pre-treatment of the sample, and there is always "adventitious" (or environmental) contamination in the form of compounds sorbed onto material surfaces from the atmosphere.

In general, we try to analyze samples "as received." Solid materials (e.g. mineral grains) are typically pressed into an Indium foil, which is both malleable and conducting. Any mapping of the sample prior to insertion into the sample chamber will greatly increase the ability to find and identify areas of interest. As a first step in the analytical procedure, we will typically "dust off" the surface with a very light (<1 minute) sputtering interval in an attempt to clean off any sorbed surface contamination.

An example of ToF-SIMS elemental mapping of a garnet amphibolite from the KTB deep borehole, Germany. An anastomosing network of carbon can be seen on grain boundaries and cleavage traces. From Mogk and Mathez (2000).

General References on ToF-SIMS Techniques and Applications

• Benninghoven, A., Chemical Analysis of INorganic and Organic Surfaces and Thin Films by Static Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), 1994, Angewandte Chemie International (in English), vol 33 #10, 1023-1043.
• VanVaeck, L., Adriaens, A., and Gijbels, R., 1999, Static Secondary Ion Mass Spectrometry: (S-SIMS) Part 1. Methodology and Structural Interpretation, Mass Spectrometry Reviews, v. 18, p. 1-47.
• Adriaens, A., VanVaeck, L., and Adams, F., 1999, Static Secondary Ion Mass Spectrometry (S-SIMS) Part 2: Material Science Applications, Mass Spectrometry Reviews, v. 18, p. 48-81.

References on the use of ToF-SIMS to Characterize Earth Materials

• Mathez, E. A., and Mogk, D. W., 1998, Characterization of carbon compounds on a pyroxene surface from a gabbro xenolith in basalt by time-of-flight secondary ion mass spectrometry, Amer. Min., v. 83, p. 918-924.
• Mogk, D. W., and Mathez, E. A., 2000, Carbonaceous films in midcrustal rocks from the KTB borehole, Germany, as characterized by Time-of-Flight, Geochemistry, Geophysics, Geosystems (G3), Amer. Geophys. Union, November 13, 2000 (e-publication).
• Toporski, J., and Steele, A., 2004, Characterization of purified biomarker compounds using time of flight secondary ion mass spectrometry (ToF-SIMS), Organic Chemistry, v.35, #7.
• Xiaoquiang Hou, Deyi Ren, Heling Mao, Jiajin Lei, Kuli Jin, Chu, P.K., Reich, F., and Waynde D. H., 1995, Application of imaging ToF-SIMS to the study of some coal macerals, International Journal of Coal Geology, v. 27 #1, p 23-32.
• Scott, S. D., and Yang, K., 2007, Melt inclusion evidence for magmatic fluids as a source for metals in seafloor hydrothermal systems, Geophysical Research Abstracts, Vol. 9.
• Stephan, T., Jessberger, E. K., Kloeck, W., Rulle, H., Zehnpfenning, J., 1994, ToF-SIMS Analysis of Interplanetary dust, Earth and Planetary Science Letters, v. 128 #3-4, p. 453-467.

Information available through Evans Analytical Group:

• Evans Analytical Group Tutorial - An online tutorial on the use of ToF-SIMS
• SIMS Theory - An online tutorial on the theory behind ToF-SIMS