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Detection of Inorganic Salt Based Home Made Explosives (HME) by Atmospheric Flow Tube – Mass Spectrometry Robert G. Ewing, Blandina R. Valenzuela, David A. Atkinson, and Eric D. Wilcox Freeburg Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b01261 • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018
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Analytical Chemistry
Detection of Inorganic Salt Based Home Made Explosives (HME) by Atmospheric Flow Tube – Mass Spectrometry
Robert G. Ewing*, Blandina R. Valenzuela, David A. Atkinson and Eric D. Wilcox Freeburg Pacific Northwest National Laboratory, 902 Battelle Blvd., Richland, WA 99352, United States
Abstract Using a commercial mass spectrometer interfaced with an atmospheric flow tube (AFT) allowed detection of a variety of inorganic compounds used as oxidizers in homemade explosives at picogram levels. The AFT provides reaction times between 3-5 seconds with flows of 6 L/min, enabling detection levels, after thermal desorption, similar to those previously demonstrated for RDX vapors in the low parts-per-quadrillion range. The thermal desorption of chlorate and perchlorate salts resulted in the production of the corresponding anions which have higher electron affinities than the nitrate reactant ions. A dielectric barrier discharge, used as the ionization source, produced the nitrate reactant ions. In some instances, the molecular salt formed adducts with the nitrate, chlorate, and/or perchlorate anions, giving insight into the original identity of the salt cation. Urea nitrate, guanidine nitrate, and potassium nitrate were also detected as adducts with the nitrate reactant ion. Direct room-temperature vapor detection of urea nitrate and hydrogen peroxide, which have relatively high vapor pressures compared to the other salts in this study, is also demonstrated. Room-temperature vapor detection of chlorate and perchlorate salts is possible by the addition of a dilute acid which converts the salt into a more volatile acidic form. A discussion of the instrumentation, methods used, and the ionization chemistry is provided.
Key words: chlorate, perchlorate, nitrate, detection, hydrogen peroxide, mass spectrometry, ambient ionization, homemade explosives, HME
*Corresponding Author:
[email protected] 1 ACS Paragon Plus Environment
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Introduction Due to the widespread availability of strong oxidizers and the relative simplicity of making a detonable mixture, these chemicals have become key components of homemade explosives (HMEs). HMEs are often made from a simple mixture of a readily-available oxidizer and a fuel, which can be something as simple and easy to obtain as fuel oil, sawdust or sugar. Pyrotechnics are also typically based on chemical oxidizers mixed with fuels.1 Because of the extensive assortment and ubiquity of fuel material, the detection of the oxidizer is a more pragmatic approach for the discovery of HME materials. Ion mobility spectrometry (IMS) has been a cornerstone of explosives detection for many years, with good detection performance for a wide array of organic explosives.2,3 Mass spectrometry (MS) is another viable option, providing a higher resolution separation than IMS, although it has been less widely deployed for the field detection of explosives.4 Since many oxidizer compounds are salts with extremely low vapor pressures, gas phase detection techniques such as IMS or mass spectrometry require high desorption temperatures to release collected sample particles into the vapor phase.5,6 High temperatures, however, are not always an optimal approach for explosives detection, as some materials are thermally labile (e.g. TATP, EGDN, nitroglycerine) and may degrade under high temperature conditions. In some cases, ionization chemistry manipulations such as adduct formation can be used to reduce the thermal fragmentation.7,8 To mitigate the need for higher desorption temperatures to overcome vapor pressure issues with respect to chlorate and perchlorate salts the use of sample acidification has been demonstrated to enhance the vapor concentration by forming the more volatile conjugate acid of the salt anion.9,10 Vapor pressures of most inorganic salt based oxidizers are often exceedingly low, even when compared to very low volatility nitrated organic explosives, such as HMX (2.37x10-17 atm)11 or hexanitrostilbene (6.09x10-21 atm)12. These low vapor pressures make it very difficult to measure directly at room temperature. Thus, room temperature vapor pressures for most inorganic salts need to be estimated from extrapolation of data taken at higher temperatures. Langmuir 13 has succinctly described the process of determining the vapor pressure from low volatility substances, including the derivation of a vapor pressure equation from high temperature data based on the Clausius-Clapeyron equation. Using this method, inorganic salt vapor pressure values can be obtained from published high temperature data (as an example, 300 to 1000 °C range for potassium chloride 5,6,14) and extrapolated to obtain the desired values. By extrapolating one of the data sets 6 down to 25 °C for potassium chloride, as an example case, an estimated vapor pressure value in the 1x10-27 atm range is obtained. Potassium chloride is being used as an example salt, since high temperature vapor pressure data for perchlorate salts to use for extrapolation is unavailable, but the values are expected to be exceedingly low (
