A vapor is a fluid that develops (and will always be formed) above a condensed phase such as a solid or liquid. Understanding vapors is critical to fundamental thermodynamics (that is, vapor liquid equilibrium) as well as day to day chemical analyses. Often the vapor above a condensed phase (the headspace) is analyzed to provide insight into the composition of the condensed phase itself. All these are aspects of vapor characterization.
Energetic materials, as distinct from fuels, include explosives, propellants and ignitable liquids. One typically associates the study of energetic materials with forensics, in the detection and prevention of terrorism or crime. Indeed, this is the primary focus of the Group's activity in the area of energetic materials, although many technological spinoffs have resulted from these aspects. Our goal here is to provide the characterization of vapors of energetic materials that form the basis of detection, in the laboratory and in the field.
The detection of energetic materials (primarily explosives and propellants) in the field depends on a sound knowledge base that allows for the certification of instruments. Thus, it is critical for the detection of, for example, a plastic explosive formulation that one understands what components might form in the vapor space (or headspace) above the solid material, and at what concentration. We have developed a highly sensitive sampling method that makes the measurement possible. A number of interesting and useful technical spin offs (in forensics and food safety) have resulted from our work on explosive vapors. Related to the characterization of the vapor space is the measurement of the fundamental quantity of vapor pressure. In any theoretical description of fluid behavior, the first fundamental property that must be measured is the vapor pressure. Unfortunately, the vapor pressure of low volatility materials such as explosives and propellants is extremely difficult to measure. The task is rendered even more challenging by the presence of multiple components and impurities. We have developed a technique to measure this important parameter with very low uncertainty. Our method is an extension of classical gas saturation or (vapor transpiration) metrology. Another in-the-field problem that has become prominent is the remote or standoff detection of the fluid contents of plastic bottles. Especially in a transportation setting, it is critical to know if the bottle being carried by a passenger contains water or something that can be used to generate an explosion or fire. To help address this issue, we measure the permeation of various liquids across polymer barriers. Finally, we have applied our fuel characterization metrology to the study of ignitable liquids that have been used in arson fires.
Explosive Vapor Characterization and Related Work
Vapor detection methods for sampling and detecting energetic compounds that may be components of improvised explosive devices (IEDs) are most attractive because they are sensitive, selective, and afford non-invasive, standoff detection. To develop reliable vapor detection devices for energetic materials, we need to know (1) what and (2) how much is in the vapor phase above the energetic material or IED. We have developed a method that both identifies and quantifies components (even trace components of low volatility) above energetic materials. This method is a modified purge and trap approach that makes use of cryoadsorption on short alumina-coated porous layer open tubular (PLOT) columns. The method is particularly valuable because of its sensitivity and repeatability. One can obtain quantitative uncertainty of 10 % from a sample containing 10 ppb of sample, and a yes/no answer for a sample as low in concentration as 2 ppb.
Nichols, J.E., Harries, M.E., Lovestead, T.M., Bruno, T.J., Analysis of arson fire debris by low temperature dynamic headspace adsorption PLOT columns, J. Chromatogr. A, 1334, 126–138, 2014. (featured, C&EN, pg. 29, July 7, 2014)
Vapor Pressure of Low Volatility Materials
While vapor pressure is one of the most important fundamental thermodynamic properties, the measurement of the vapor pressure of solids and low volatility liquids is extraordinarily difficult. The task is even more difficult when the sample is presented as a mixture, or if significant impurities are present. One of the only methods available for such situations is the gas saturation method, sometimes called the vapor transpiration method. The most troublesome aspect of this technique is that it typically requires long measurement times. We have addressed this issue with the introduction of the concatenated gas saturation instrument, which allows the simultaneous measurement of up to 18 samples. Thus, we can typically measure five different samples in triplicate, plus a standard, known sample, which we include for quality control. Uncertainties as low as 7 % are possible for samples with vapor pressures in the Pa range. In addition to applications in explosives characterization, our work has figured in smog formation modeling and biofuel characterization.
Technical Spin-Offs of Vapor Characterization
The sensitive measurement of vapor headspace is a general analytical problem that has applications well beyond the characterization of the vapors above explosives. We have also applied the metrology to the early detection of food spoilage by sampling protein decomposition products in the vapor space above poultry. Continuing with the theme of sampling protein decomposition, we have also used this as a method to detect illegally buried cadavers (that is, the detection of grave soil).
Bruno, T.J., Nichols, J., Method and apparatus for pyrolysis-PLOT-cryoadsorption headspace sampling and analysis, J. Chromatogr. A, 1286, 192-199, 2013.
Fluid Permeation Through Polymeric Barriers
To enable the development of stand-off analytical metrology to determine the fluid content of bottles (for example, bottles being carried aboard aircraft), some fundamental measurements are required. One of the more important parameters needed is the permeation of polymer barriers to fluids. We have developed a method to measure the permeation through real barriers:
Widegren, J.A., Lovestead, T.M., Bruno, T.J., Trace detection and quantitation of acetone, methyl ethyl ketone, nitrobenzene, sulfuric acid, nitric acid, and hydrazine permeation through PET polymer barriers, NIST-IR 6667, (administratively restricted, ITAR) 2012.