Time-of-flight secondary ion mass spectrometry (TOF-SIMS) employs a pulsed primary ion beam and a time-of-flight mass analyzer for the detection of molecular ions with mass-to-charge ratios ranging from m/z 1 to m/z 10,000 in a single spectrum. This technique can resolve patterned features smaller than a micrometer with a molecular specificity unique to MS, allowing the detection and visualization of trace chemicals within very complex surfaces.
ToF-SIMS is an imaging mass spectrometry (MS) technique that allows us to obtain isotopic, elemental, and molecular information from the surface of solid samples. A pulsed, energetic “primary” ion bombards the surface and induces a collision cascade, liberating “secondary” ions that are then sent to a time-of-flight mass analyzer for detection (Figure A). The ion dose needs to be kept low (< 1% of the number of molecules on the surface) to minimize chemical damage (such as molecular fragmentation) to the sample.
The instrument is unique in that it is capable of constructing a large area map showing the distribution of chemistries on the surface. This has allowed the instrument to support a wide variety of critical programs at NIST, such as determining the age of a fingerprint by measuring the extent of diffusion of palmitic acid (Figure B), characterizing the effect of temperature on the desorption rate of explosive particles for the optimization of trace explosive detector (ETD) systems (Figure C), and the visualization of the sputtered flux for the optimization of ambient mass spectrometry systems (Figure D).
Cluster Ion Sources for Molecular Depth Profiling
ToF-SIMS normally employs an ionized bismuth metal as a primary ion source for analysis. This analysis source can be operated in tandem with a second primary ion source made up of a cluster of argon gas atoms to “sputter” away the sample. The result is a repeating cycle of analysis and sputter sources that leads to the acquisition of a molecular depth profile where concentrations of certain molecules can be seen as a function of depth into a film, or 3D images where structures such as thin films or particles can be visualized inside the film. No other technique can produce 3D chemical maps that combines molecular information with nanometer depth resolution, making ToF-SIMS an incredibly powerful technique for the characterization of complex organic samples.
For example, the individual layers in an automotive paint film can be characterized both from the side (its cross section) as well as from the top (depth profile). Knowing the chemical makeup of certain layers in the paint can be beneficial for identifying the make and model of the car that may have been involved in an accident or a crime (Figure E).