Trace chemical sensing of explosives

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.They can be blended into TNT in a variety of ratiosto make the formulations listed in Table 2.3.They can also be formulated inwax or plasticizer.The use of plasticizer is preferred because less dilution of theexplosive occurs.(In the world of performance, TNT, with detonation velocityof 6900 m/s is considered a dilutant of HMX, detonation velocity of 9100 m/s.)In the detonation process, temperatures reach 3000 to 4000 K so that residue,if any, consists of unreacted explosive and, possibly, carbon if the explosive isWHAT TO DETECT? 39TABLE 2.3 TNT-Based Explosive FormulationsIngredient Added to TNT Name % TNTAluminum (Al) Tritonal 20 80Ammonium nitrate (AN) Amatol 20 60RDX Comp B2 40RDX + 1% wax Comp B 39Comp B(+18%Al) Torpex-2 12HMX Octol 25 30PETN Pentolite 50Barium nitrate Baratol 24greatly underoxidized.Presumably any unreacted explosive recovered is materialthat has been thrown out from the perimeter of the charge and not experiencedthe detonation front.The amount of unreacted explosive residue depends on theefficiency of the initiation.Studies focusing on the decomposition products ofexplosives are not studying detonation products; they are looking for products ofthermal decomposition.These may form during long-term or high-temperaturestorage.These products can be produced in laboratory studies by heating theexplosive to 200 to 400�%C.For detection purposes, thermal decomposition prod-ucts may be important, if they are sufficiently volatile.One thermal decompositionpathway accessible to most explosives is homolytic cleavage of the nitro group:"O NO2 �! O" + NO2"C NO2 �! C" + NO2 (2.3)"N NO2 �! N" + NO2The energy required for this process depends on the exact linkage.In general,the C NO2 linkage found in nitroarenes is quite strong.Other decompositionpaths, which require lower energies, dominate.For nitrate esters, the homolyticcleavage of the nitro group is a fairly low energy pathway.Furthermore, theevolved nitro group can accelerate the decomposition of the nitrate ester; thus,nitrate esters such as nitrocellulose, are stored with stabilizers, which functionto bind evolved NO2.Regardless of whether evolution of NO2 is a dominantor minor thermal decomposition pathway, nitro-containing explosives can bedetected by the use of chemiluminescence.In fact, chemiluminescence is thedetection scheme used in the thermal energy analyzer (TEA), a detector used inconjunction with gas chromatographic analysis by forensic explosive laboratories.It is highly specific and sensitive to nitro-containing explosives.The difficulty isimmediately obvious what if the explosive does not contain a nitro group?There are four types of explosives that do not contain nitro groups: inorganicoxidizers (in combination with any fuel), peroxide explosives, primary explo-sives, and newly developed high-nitrogen explosives.Chapter 3 discusses boththe inorganic oxidizers principally nitrate or chlorate-based and peroxide explo-sives TATP and HMTD.We have previously reported on the thermal stabilities40 WHAT TO DETECT?TABLE 2.4 DefinitionsStable Does not readily decompose at elevated temperatures or over timeSensitive Easily initiated to decomposition or detonation by flame, impact,friction, or sparkVolatile Transitions from solid to gas at a relatively low temperature; it can bereturned to its original state upon coolingand decomposition pathways of these materials [1,2].Detection of the inorganicoxidizers is challenging since they have extremely low volatility, though if thecation is ammonium or urea, there may be trace amounts of ammonia or ureapresent.TATP is readily detectable because of its high volatility 0.05 torr atroom temperature [3], about 10,000 times the vapor pressure of TNT, whichcan be detected by most sniffing instrumentation, including canines.Unfortu-nately, there is a tendency to equate volatility, stability, and sensitivity (seeTable 2.4).Pure TATP is thermally stable; it does not decompose any faster thanPETN.However, TATP is highly volatile, readily subliming to the atmosphere atroom temperature if its container is open.For the terrorist, TATP and HMTD offer easy sources of primary explosives.Consulting the do-it-yourself literature, it can be seen that there are two othercommonly recommended primary explosives lead azide Pb(N3)2 and mercuryfulminate Hg(ONC)2, but these are difficult to prepare cleanly.The synthesisof diazodinitrophenol (DDNP) (Fig.2.5), common in commercial detonators, isreported in such publications, but apparently is rarely attempted by clandestinechemists.Typically, the brisance of a primary is less than TNT, but the efficacyis the fact that a shock wave can result from a relatively mild insult.Synthesis of new chemical explosives continues.The trend has been towardcompounds with high positive heats of formation, high nitrogen content, and/orlarge ring strain (Figs.2.6 and 2.7).However, synthesis of these requires sophis-ticated chemical expertise and, generally, is not attractive to the clandestinechemist.For the next decade we can expect that the challenge will remain todetect nitrated (X NO2) materials and composite materials.As detection limitsO NNO2NNO2Figure 2.5 Primary explosive DDNP.REFERENCES 41N NNN NNNTetrazolidine[1,2,4]TriazolidineFigure 2.6 Nitrogen rings are basis of new explosive compounds.N O2N NO2H2NOXNN NN NN NO OH2NX = O, SOn (n=0,1,2), N=N, N(O)-N,N(NO2)CH2N(NO2)NNO2NONNO2 O2N N+NNNNNNHNNO2NNHN NH2 H2NNO2NNO2 NO2N NO2N NNO2 N NNNFigure 2.7 Examples of fused ring systems.become lower and lower, the significance of minute levels compared to back-ground or other environmental exposition will require careful consideration.REFERENCES1.Oxley, J.C., J.L.Smith, and H.Chen.Decomposition of multi-peroxidic com-pound: Triacetone triperoxides (TATP).Propellants, Explosives Pyrotechnics 27,209 216 (2002).2.Oxley, J.C., J.L.Smith, H.Chen, and E.Cioffi.Decomposition of multi-peroxidiccompounds: Part II: Hexamethylene triperoxide diamine (HMTD).Thermochem.Acta388(1 2), 215 225 (2002).3.Oxley, J.C., J.L.Smith, J.Moran, and K.Shinde.Determination of the vapor densityof triacetone triperoxide (TATP) using a gas chromatography headspace technique.Propellants, Explosives, Protechnics 30(2), 127 130 (2005).CHAPTER 3DANGEROUS INNOVATIONSKIRK YEAGER3.1 INTRODUCTIONThe use of explosives by terrorists is not a new innovation.Examples of groupsattempting to violently address political grievances range from the GunpowderPlot to blow up the British Houses of Parliament in 1605 to the attack on theUSS Cole in Aden Harbor in October 2000.Explosives have been throughoutthe last half-century, and remain to this day, the primary tool of terrorists [ Pobierz całość w formacie PDF ]

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