Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS)
What is Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS)
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.
Fundamental Principles of Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS)
- a mass spectrum that surveys all atomic masses over a range of 0-10,000 amu,
- the rastered beam produces maps of any mass of interest on a sub-micron scale, and
- depth profiles are produced by removal of surface layers by sputtering under the ion beam.
Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) Instrumentation - How Does It Work?
- 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.
- Elemental/Molecular Surveys;
- Elemental/Molecular Maps; and
- Depth Profiles.
- 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
Strengths and Limitations of Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS)?
- 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.
User's Guide - Sample Collection and Preparation
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.
Data Collection, Results and Presentation
The following literature can be used to further explore Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS)
- 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.
- 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.
For more information about Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) follow the links below.
Teaching Activities and Resources
Teaching activities, labs, and resources pertaining to Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS).