Explosives analysis sample types

Microscopy (qv) plays a key role in examining trace evidence owing to the small size of the evidence and a desire to use nondestmctive testing (qv) techniques whenever possible. Polarizing light microscopy (43,44) is a method of choice for crystalline materials. Microscopy and microchemical analysis techniques (45,46) work well on small samples, are relatively nondestmctive, and are fast. Evidence such as sod, minerals, synthetic fibers, explosive debris, foodstuff, cosmetics (qv), and the like, lend themselves to this technique as do comparison microscopy, refractive index, and density comparisons with known specimens. Other microscopic procedures involving infrared, visible, and ultraviolet spectroscopy (qv) also are used to examine many types of trace evidence. 

For onsite analysis, the examination of the vast number of samples necessitates the use of quick, reliable, field portable equipment that can rapidly, quantitatively verify the many chemically different types of ammunition, explosives, and pyrotechnics. The most common suite of analytes to detect is large, consisting of very chemically different compounds and usually occurs at trace levels in complex environmental matrices. This suite encompasses smokeless powders, black powders, and numerous propellant and energetic formulations. Detection should also be sought for common decomposition products of these explosives such as the methylanalines, aminonitrotoluenes, nitrotoluenes, mono- and dinitoroglycerines, and the nitrobenzenes under on-site conditions. 

FIGURE 12.1 The mobility spectra of several types of explosives. (From Hilton et al., Improved analysis of explosives samples with electrospray ionization-high resolution ion mobility spectrometry (ESI-HRIMS), Int. J. Mass Spectrom. 2010,298,64-71. With permission.)... 

Infrared Spectroscopy. The technique of infrared spectroscopy is commonly used in the controlled substance and the trace evidence sections of the crime laboratory. This technique can identify illicit drugs present in unknown samples, the type of fiber found at a crime scene or on a person, the polymer present in a paint chip, or the organic compounds present in explosive residues. The evidence is prepared for analysis in several ways, depending on the type of sample. 

Pressure tends to increase the chemical reactivity of nitromethane as well as the rate of thermal decomposition. It was observed, quite accidentally, that a pressure-induced spontaneous explosion of single crystals of nitromethane at room temperature can occur. Further study revealed that single crystals grown from the liquid with the (111) and either the (001) or the (100) crystal faces perpendicular to the applied load direction in the DAG, if pressed rapidly to over 3 GPa, explode instantaneously accompanied by an audible snapping sound. The normally transparent sample becomes opaque instantly. Visual examination of the residue revealed a dark brown solid which was stable when heated to over 300 C. Subsequent x-ray analysis showed the material to be amorphous. Mass spectral analysis of the residue was inconclusive because no well defined spectra were observed. Because most of the sample is recovered as solid residue after the explosion and is stable to over 300°C, the material may be amorphous carbon. This stress-induced explosion occurs only in protonated nitromethane because similar attempts on the deuterated form did not result in explosion. Shock experiments on oriented pentaerythritol (PETN) crystals have shown similar type behavior [25]. In this case it was suggested that the sensitivity of shock pressures to crystal orientation is the result of the availability of slip planes or system of planes in the crystal to absorb the shock, thereby increasing the threshold to explosion. A similar explanation may be applicable to the nitromethane crystals as well. The deuteration effect must play a role in the initiation chemistry. An isotope effect has been observed previously in the sensitivity of HMX and RDX to shock and thermal conditions [23]. 

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