Instrumental and Other Methods of Analysis
Once an explosive residue has been isolated from bomb debris then it should be further characterized and ultimately, identified. A variety of techniques can be used depending upon the amount and type of explosive available. Various types of chromatography and infrared spectrophotometry are widely used for this purpose.
Thin-layer chromatography Many explosives can be conveniently separated by thin-layer chromatography (TLC). These include many low and high explosives, including smokeless powder, dynamites, TNT, etc. Known explosives are normally spotted on the TLC plate along with the unknowns so that the identifications can be more certain. A number of specific stationary and mobile phase combinations have been used for various classes of explosives. Several visualizing reagents have also been employed. For example, Griess reagent has been employed extensively because they will color most nitrate-containing species bright red. Most explosives have nitro groups making Griess reagent very versatile. Figure 1 shows three thin-layer chromatograms of some explosives sprayed with Griess reagent. High performance liquid chromatography High performance liquid chromatography (HPLC) is also useful for the analysis of explosive residues. It has advantages over TLC in that it can be used for quantitative analysis, should this be called for and it doesn’t require the use of visualizing reagents. It is preferred of GC because high concentrations of explosives can cause problems with the high temperatures used in GC. Modern liquid chromatographs are outfitted with mass spectrometers as detectors, making the identification of explosives that have been separated from bomb debris much easier.
Capillary electrophoresis Capillary electrophoresis (CE) is similar in some ways to HPLC but operates on somewhat different principles. It has some advantages over HPLC, the major one being the amount of sample required. CE is very sensitive and only tiny amounts of material are needed for analysis. This can be a real advantage when only trace amounts of explosive residue are present.
Infrared spectrophotometry Because chromatographic methods provide only a tentative identification, a confirmatory test, IR, is often used. A variety of sample types can be used for IR including solids, liquids, mulls, and solutions. Similar compounds such as TNT and DNT (dinitrotoluene) can be differentiated by IR. See Figure 2 for the infrared spectrum of TNT.
FIGURE 1 Three thin layer chromatograms of explosive mixtures. The red spots are where explosives are. The plates were treated with Griess reagents, which react with nitrate-containing compounds to produce the red color. PETN, pentaerythritol tetranitrate; RDX, cyclotrimethylenetrinitramine; TNT, trinitrotoluene; NG, nitroglycerine.
FIGURE 2 Infrared spectrum of trinitrotoluene (TNT).
