Over the past decade, we at the National Institute of Standards and Technology have been working closely with the Department of Homeland Security to stop the threat of terrorist-based attacks in the form of explosives or explosive-based devices. Our program encompasses many different aspects of this threat, from development of measurement standards for trace explosives detection at airports, to the development and application of new metrology for the characterization these explosives. Here we present, to our knowledge, the first investigation into the application of surface analytical techniques, such as Secondary Ion Mass Spectrometry (SIMS) and X-Ray Photoelectron Spectroscopy (XPS) for the characterization and differentiation of plastic explosives. This particular work is focused on the characterization of composition C-4 explosives from several different regions.

Unlike traditional analytical techniques such as GCMS and/or LCMS, these powerful surface analytical tools allow for the simultaneous and direct characterization of all the components in C-4 (explosive components, additives, binders, contaminants etc.), as opposed to a partial analysis of extracted portions. Furthermore, the characterization of the explosive samples with ToF-SIMS and XPS will enable rapid identification of both organic and inorganic constituents as well as their characteristic isotopic abundances with excellent sensitivity. Most importantly, these techniques are well-suited for direct analysis of small explosive particulates collected directly in the field, and are already employed for homeland security applications that effect national policy.

As the sizes of nanostructures decrease, the surface-to-volume ratios increase immensely such that the smallest nanoparticles are theoretically seen by their surroundings as only the chemistry exposed on the surface and not the bulk of the nanoparticle interiors. We believe that the ability to track the surface chemistry of nanoparticles vs. size and vs. environmental exposures will show dramatically altered surface chemistry, and thus altered chemical reactivity of the nanoparticles. This concept has widely been suspected and/or believed to be true within the nanoparticle field, but experimentalists have yet to devise a standard approach at measuring what the surface chemistry is and to what extent that surface chemistry can be altered in real-world conditions. Since we are interested only in the surfaces of the nanoparticles and not necessarily the bulk, we have worked to develop a new paradigm for monitoring the surface chemistry of engineered nanoparticles with secondary ion mass spectrometry (SIMS). When kept in a static mode analysis, SIMS can be highly surface-sensitive to just a few nm, where other commonly employed techniques for measuring nanoparticle chemistries either cannot separate bulk vs. surface information, or only arrive at surface information indirectly. We will present progress made towards reaching our aims, including nanoparticle preparation considerations, sensitivity of SIMS to monitoring changes in the nanoparticle surfaces, and projections into the future of this methodology for such a purpose.