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Time-of-Flight Secondary Ion Mass Spectrometry

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.

SIMS
Figure A. (a) A simplified schematic of the collision cascade, showing the desorption of atoms and molecules as a result of “primary” ion bombardment. (b) The time-of-flight mass analyzer separates ions based on mass, determined by its time taken to reach the detector which is a function of its mass due to conservation of energy.

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).

SIMS
Figure B. (a) ToF-SIMS secondary ion image of a fingerprint imaged at t = 1, 24, 48, 72, and 96 h. (b) Line scan of the fingerprints showing the intensity of palmitic acid as a function of distance from the edge of the fingerprint. The extent of diffusion can be used to tell how old the fingerprint is.

SIMS
Figure C. ToF-SIMS secondary ion image of ammonium nitrate explosive particles on a surface, showing the particle size and their distribution (a) before and (b) after thermal treatment at 300 °C for 9 s. By determining the percent of particles lost and changes in the particle size as a function of different temperatures, the optimum temperature for its detection in an ETD could be determined.

Secondary Ion Mass Spec
Figure D. Visualization of the desorption electrospray ionization (DESI) sputtered flux using ToF-SIMS. In this technique, a high velocity stream of liquid originates from the probe to desorb molecules on the sample surface and into the inlet of the MS. (a) Optical micrograph and schematic of the DESI probe in relation to the inlet of the ambient MS. (b) Secondary ion image showing the distribution of the marker molecules sputtered from the surface, illustrating the angular dispersion of the material relative to the inlet (blue circle). Using this method, the optimum DESI probe angle could be determined to maximize signal.

 

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).

Secondary Ion Mass Spec
Figure E. (a) Optical micrograph of the automotive paint multilayer cross-section, and (b) the corresponding ToF-SIMS secondary ion image showing the different layers. (c) The molecular depth profile of the paint layer using the Ar2000 cluster source, showing the intensity of the unique molecules in the clearcoat, basecoat, primer, e-coat, and the substrate as a function of time.

Created August 21, 2017, Updated November 15, 2019