Ana lysis of Explosives Using Infrared Spectroscopy FRANK PRISTERA, MICHAEL HALIK, ALEXANDER CASTELLI, and WALTER FREDERICKS Propellants Research Section, Propellants and Explosive Laborafory o f Picatinny Arsenal, Dover, N. J. A, compilation of 68 infrared spectrograms covering all common highexplosive compounds and many possible explosive ingredients, additives, and related compounds has been prepared. An examination of the prepared spectrograms had made possible the empirical deduction of severa! new assignments, which should facilitate the infrared structural investigation of unknown ingredients, such as newly synthesized compounds. Extensive detailed descriptions are presented for the preparation and qualitative use of infrared spectrograms of the explosive ingredients. There i s considerable information regarding the qualitative and quantitative analyses of mixtures of explosive ingredients by infrared and/or other methods. In the analysis of single or multicomponent high explosives, infrared spectroscopy i s useful either alone or in connection with other methods. Infrared spectroscopy makes available the advantages of speed, specificity, accuracy, and precision.
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modern explosives frequently contain several compounds, it is often necessary to &ablish the nature as ne11 as the amounts of the ingredients present. For control nork, where the nature and approximate amounts of the ingredients are known, qualitative analysis is not needed. For quantitative analysis in control nork, if the ehplosive contains more than one ingredient, many methods are usually available (4,7 , 8, 12, 15). For mihtures of similar or related compounds, encountered in nitration and purification processes, for example, there are usually no satisfactory chemical methods of analysis. I n the analysis of unknon n explosives, the chemical anti physLca1 methods described are of only liniited ralue (26-27). Thcie ale no simple detailed methods for the qualitative analysis of unknown explosives e x e p t general methods of organic analysis (9, ig, 22), nhich are usually based on clasi: tests and physical constants such as the melting points. llicrowopic analysis (3) is of some value but it can be used only for solid single compounds and requires specialized technique. Explosive substances usually fall into threie classes: nitrates, nitro compounds, and nitramines. ECAUSE
Furthermore, they are not very pure when used, as their physical constants are not sharp nor unique for positive identification purposes. For these reasons, the qualitative organic analysis of explosives by conventional methods is a rather long and involved procedure which requires thorough knowledge of the possible explosive ingredients and
their analytical chemistry. Because this system of analysis can be applied only t o pure compounds, prior separation and purification of the ingredients are needed if the explosive consists of more than one ingredient. Infrared spectroscopy had been successfully used in the analysis of propellants (19) and in other fields (6, io, VOL. 32,
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Table 1.
Melting Points of Explosive Ingredients
hleltbg Point, C.
Compound p-Nitroethylbenzene o-Nitroethylbenzene Diethylene glycol nitrate (DEGK) Ethylene glycol dinitrate o-Sitrotoluene Metriol trinitrate Nitrobenzene Nitroglycerin m-Nitrotoluene 2,4,6-Trinitroethylbenzene m-Kitroanisole o-Xitrobenzaldehyde p-Sitrotoluene p-Nitroanisole m-Sitrobenzaldehyde 2,6-Dinitrotoluene a-Xitronaphthalene m-Nitromethylaniline m-Sitrodimethylaniline 2,4,6-Trinitroanisole 2,4-Dinitrotoluene o-Sitroaniline 2,4-Dinitrobenzaldehyde Tetramethylolcyclopentanone tetranitrate (Fivonite) 2,4,6-Trinitrophenetole 2,4,6-Trinitrotoluene 4,6-Dinitro-o-cresol m-Dinitrobenzene 2,4Dinitroanisole Methylenedinitramine (MEDINA) 2,4,5-Trinitrotoluene 2,4,6-Trinitro-m-cresol 2,4Dinitroethylaniline 2,3,4-Trinitrotoluene Mannitol hexanitrate p-Nitrophenol 2,4--Dinitrophenol 1,3,5-Trinitrobenzene Trinitrophenol (picric acid) Tetr amet hylolcy clohexano1 pentanitrate (Sixolite) Trinitrophenylmethylnitramine (Tetryl) m-Nitrobenzoic acid Pentaerythritol tetranitrate (PETN) o-Nitrobenzoic acid p-Nitroaniline 2,4Dinitroresorcinol Ammonium nitrate 1,8-Dinitronaphthalene p-Dinitrobenzene 2,4Dinitromethylaniline Ethylenedinitramine (EDNA,) 2,4,6-Trinitroresorcinol (styphnic acid) 2,4Dinitrobenzoic acid Ethylenediamine dinitrate 2,4Dinitro~henvlhvdrazine Cyclotrimethylenetrinitramine IRDX) 3,5-Din&obenzoic acid Guanidine nitrate 1,5-Dinitronaphthalene 2,4,6-Trinitrobenzoic acid Hexanitrodiphenylamine p-Wtrobenzoic acid ~
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-32 - 23 -22.75 -20 -10.6 for a, - 4 . 1 for p -3 5.8 13.2, 2 for labile form 15.5 37 38 43.5-46 51.3 54 58
61 61.5 66.0 66 68.4 70.5 71.5 72 7-1 for a , 66.7 for p 80 81 87 90 95.2 101 104 109.5 112 112 113 114 114 122 122 122.5 130 141.4 142 147.5 147.5 148 170 173.5 174 177.5 179 180
183 187
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205 216 217.5 228.7 240-50 242.4 Cyclotetramethylenetetra- 279 nitramine (HMX) Ammonium picrate 280
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IS), and development and use of this tool for the analysis of explosive substances were considered desirable. Before infrared spectroscopy could be successfully applied in this field, however, infrared spectrograms of the various explosive ingredients had to be available. Accordingly, the infrared spectrograms of 68 ingredients were prepared shonn in the 17 accompanying figures. A compilation of these spectrograms, together with qualitative and quantitative methods for their analysis by using infrared spectroscopy, is included in this report. The substances which have been used, proposed, or suggested as ingredients of explosives are so numerous that it was considered beyond the scope of this investigation t o include them all. The 68 substances selected included all of the more commonly used ingredients. Also included were some possible impurities, starting materials, and intermediates formed or used in the manufacture of common explosives. (The infrared spectrograms of these substances are of great value in studying nitration and purification processes.) Several other substances related to common explosives were investigated for observing any possible spectral correlation in the explosive substances. For convenience, Table I lists the melting points of the ingredients in the order of increasing temperature, and Table I1 presents some of their physical constants. From the prepared spectrograms, a few new band correlation assignments nhich should be very valuable in the structural investigation of unknown explosive ingredients were made (Table 111). The most important of these assignments are: 9.25 microns for the sym-trinitro aromatic configuration and 11.0 microns for the m-dinitro aromatic configuration. GENERAL DISCUSSION
The infrared spectrogram of a n organic compound is unique, and can serve for its positive identification by comparing it with knov n spectrograms to establish the one t o which it is identical. This simple system becomes someLt hat involved n hen the unknown can be any one of a great many compounds. Then it is necessary to compare the spectrogram of the unknown with many knowns. For such a purpose, some mechanical means, such as I B M or Keysort punch cards, can be very useful. A useful method to classify unknown spectrograms is in terms of the presence of groups-Le., nitrates, amines, hydroxyls, carbonyls, and ethers. The standard s p e c t r e grams can then be similarly classified, rt ith the effect of reducing considerably the number of comparisons that have to be made. This latter device, together with a little experience, has been
found entirely satisfactory for the qualitative analysis of explosive ingredients. The following class characteristics have been found very useful for the infrared identification of explosive ingredients (20). Organic nitrates: two strong sharp bands a t 6.0 and 7.8 microns and a broad band a t about 12.0 microns Alcohols: a strong band a t about 2.9 microns Ethers: a strong band a t about 8.9 microns Phenyl groups: t F o bands at 6.23 and 6.70 microns Aromatic nitro: two strong bands at about 6.4 and 7.4 microns Carbonyl: a very strong band a t about 5.75 microns for esters and a t 5.8 microns or a little higher for acids, aldehydes, ketones, and amides Amines (mono and disubstituted only) : a strong band a t 3.0 microns Unsubstituted phenyl groups: a strong band a t about 14.3 microns I n using the attached compilation of infrared spectrograms, it should be realized that occasionally there may be differences in the spectrograms depending on n-hether the ingredient is mounted as a solid, as a liquid, or in the form of a solution. As a rule, the difference is only in the resolution. When the sample is mounted as a liquid or in solution, clearer sharper bands with little or no background are usually produced, whereas with samples mounted as solids-for instance, mulls-broader bands and a greater background are usually obtained. Occasionally, the exact position of one or more bands may be different (shift) with one type of spectrogram than with another. I n some cases, a band may exist in one type of spectrogram and not in another. [The mull spectrogram of cyclotetramethylenetetranitramine (HMX) contains two band a t 10.35 and 10.6 microns and no band at 10.9 microns, whereas the solution spectrogram of HMX has a band a t 10.9 microns and none a t 10.35 and 10.6 microns.] If there is any doubt as to the identity of a n ingredient, the spectrograms of both the unknown and the standard must be prepared in the same manner. Practically all explosive ingredients are compounds containing one or more nitro, nitrate, or nitramine groups. These groups supply the explosive poner. The identification of a n unknown explosive compound can be simplified by placing it within certain classes of compounds. I n modern explosives, however, it is not unusual to find mixtures of two or more explosive ingredients, and sometimes also one or more nonexplosive ingredients which may serve such functions as desensitizing, stabilizing, or plasticizing. The analysis of an unknown complex explosive by infrared spectroscopy alone
is not usually recommended because of sample mounting difficulties and uncertainties in the interpretation of the spectrogram obtained. Infrared identification of pure ingredients is more expedient and certain. Generally, separation of the ingredients, which usually occurs in complex explosives, is feasible and expedient by the use of common solvents. Such separation also yields desirable quantitative information on the mixture. A few simple preliminary tests and observations bearing on such factors as melting point, color, whether made by casting or pelleting, and solubility in nater, carbon tetrachloride, benzene, and acetone can usually establish whether the explosive is a single ingredient or a mixture. I n some cases. a complex explosive cannot be suitably separated into its fractions by the use of solvents. Mixturer of related compounds, such a$ aromatic nitro compounds or ester nitrates, usually fall into this category. Such mixturcs ares sometimes used because their melting point is lorn-er than that of any of the individual ingredients. They are also encountered in the study of nitration and purification processes. In general, infrared spectroscopy is extremely useful in the quantitative analysis of such mntures, even n hen several of the ingredients are isomers. I n most such cases infrared spectroscopy is either the beqt tool or the only applicable one (16, I
