Lifetimes and Decomposition Rates of Explosive Gas Ions in Air at Ambient Pressure
R2-A.2

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Project Description

Overview and Significance

Specific aims

1. Determine quantitatively the kinetics of decomposition, lifetimes and thermochemical values for gas ions of explosives in air at ambient pressure from 50 to 200°C.

2. Explore the pathways for decomposition and discover relationships between the features of molecular structure and products of decomposition.

3. Establish an improved understanding of the performance of existing trace explosive detectors (ETDs) and establish foundations for innovation with a next generation ETD.

4. Form a first-ever library of thermochemical parameters of gas ions of explosives in air at ambient pressure as a start to filling a void in the knowledge of explosives.

Research challenges

1. The ionization chemistry of explosives is more complex than substances evaluated previously by NMSU’s kinetic methods. New methods and technologies are needed to study explosive ion chemistry.

2. Impurities in explosives and chemistry are unfavorable for simple kinetic technology and the concept of a kinetic ion mobility spectrometer (IMS) was upgraded with an advanced inlet to pre-fractionate impurities from the authentic explosives.

3. Prior flaws in the kinetic IMS with thermal gradients needed to be removed.

4. An instrument with dual shutter operation was needed to isolate specific ion swarms from neutral sample vapors and other ion swarms for accurate measurements.

Background

Explosive trace detectors (ETDs) based on ion mobility spectrometry (IMS) are found in airports worldwide with over ~30,000 instruments used to detect residues of explosive particles on hand bags or luggage. These particles are collected by sampling surfaces of purses, laptop computers and other items and are vaporized for gas phase analysis in a drift tube. The measurements using mobility drift tubes can be categorized into two related yet distinctive steps:

1. The formation of gas ions through ion-molecule reactions in purified air at ambient pressure in an ion source or reaction region.

2. The characterization of ions derived from explosives using mobility measurements in drift regions at low electric fields (<1 to 5 Td).

Historically, current ETDs were designed to detect conventional explosives (TNT, RDX, PETN) and arose in response to devastating attacks in the 1970s and 1980s; notably on Boeing 747s, such as Air India Flight 183 in 1986 and Pan Am Flight 103 in 1988. Design features and operating parameters for current ETDs were made for these substances principally. The pressure to place technology into operation meant that key understandings were undetermined before deployment and not considered subsequently as the ETDs seemed to function well-enough. An expansion of the list of explosive threat materials and the lack of coherent understanding of ion chemistry and behavior and their sometimes anomalous behaviors motivated these studies. The long term goal has been to measure new properties of explosives, ion lifetime and kinetics of decomposition and to anticipate a next generation of ETDs based on these discoveries.

Response in ETD arises in the ion source or reaction region through gas phase chemical reactions, which can be described as displacement reactions, where a small polar adduct(s) is replaced on an ion cluster with the analyte neutrals “M” as shown in Equation 1:

M     +     (H2O)nO2      ———>     M(H2O)n-1O2     +     H2O (1)

                                 Analyte         reactant ion                            product ion

These reactions have low or no activation barriers and all collisions between M and (H2O)nO2 are favorable. The ion source and entire drift tube in ETDs are commonly elevated to 120 to 160°C to prevent condensation of explosives following vaporization in the thermal desorption inlet. Although the adduct ion M(H2O)n-1O2 is at thermal energy, fragmentation is thought negligible.

Explosive trace detectors (ETDs) based on ion mobility spectrometry (IMS) are found in airports worldwide with over ~30,000 instruments used to detect residues of explosive particles on hand bags or luggage.
Phase 2 Year 2 Annual Report
Project Leader
  • Gary A. Eiceman
    Professor
    New Mexico State University
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Faculty and Staff Currently Involved in Project
  • John Stone
    Consultant
    Queens University
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  • Avi Cagan
    Research Professor
    New Mexico State University
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Students Currently Involved in Project
  • Maneeshin Rajapakse
    New Mexico State University
  • Peter Fowler
    New Mexico State University
  • Alexandra Juarez
    New Mexico State University