2. GAS CHROMATOGRAPHY–MASS SPECTROMETRY
2.4. The Mass Spectrometry of Explosives Containing
This topic is discussed here only briefly. For a more extensive discussion, the reader is referred elsewhere [37,38]. The EI mass spectra of nitroaromatic com- pounds usually contain molecular ions or related ions in the high-mass region.
The EI spectra (including those of positional isomers) usually differ from each other to an extent that enables reliable identification. In addition to M⫹•, the loss of NO2from the molecular ion is characteristic to nitroaromatic compounds. The spectra of trinitroaromatic explosives, such as TNB, TNT, or picric acid, include diagnostic [M⫺ 3NO2]⫹ions. The abundance of the M⫹•ions in nitroaromatic compounds decreases when a nitro group is in an adjacent (‘‘ortho’’) position to a hydrogen-containing substituent such as methyl. The major process in these compounds, known as ‘‘ortho’’ effect, is the loss of hydroxyl radical from M⫹•, leading to an abundant [M⫺OH]⫹ion. In 2,6-DNT and 2,4,6-TNT, where two nitro groups are positioned ‘‘ortho’’ to the methyl group, the abundance of the molecular ions becomes negligible, and the base peaks are the [M⫺OH]⫹ions at m/z 165 and 210, respectively.
The CI mass spectra of nitroaromatic explosives usually contain highly abundant [M⫹H]⫹ions and little fragmentation. An important feature in these spectra is the existence of ions at m/z values that are lower by 30 than those of
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the corresponding [M⫹ H]⫹ions. Their appearance and abundance seemed to be dependent on experimental conditions, such as the type and pressure of the reagent gas, ion source design and temperature and the presence of water in the ion source. When first noticed, they had been mistakenly assumed to be formed by the unusual loss of the NO radical from the even-electron [M⫹H]⫹ion [39, 40]. It was later proved [41–44] that their formation was the result of reduction of the nitroaromatic compound to an aromatic amine (ArNO2→ArNH2), which could take place prior to the ionization. Thus, the ion of the protonated amine, formed in CI, is observed. It is important for the analyst to be aware of this reduction process, which can explain some unusual ions in the mass spectra of nitroaromatic com- pounds. Thus, the ion at m/z 198 in some GC–CI-MS analyzes of TNT [8,22] was probably the [M⫹H]⫹ion of the reduction product (amino-DNT).
NCI mass spectra of nitroaromatic compounds usually contain M⫺•and [M
⫺H]⫺ions, whose relative abundance depends on the specific compounds and the reactant ions. In compounds such as TNT, where the ‘‘ortho’’ effect may operate, an [M⫺OH]⫺is observed.
Electron-ionization mass spectra of nitrate esters are very similar to each other and usually contain ions in the low-mass region: at m/z 30 (NO⫹), 46 (NO2⫹), and 76 (CH2ONO2⫹). No ions in the molecular weight region are ob- served. While the combination of the three ions (at m/z 30, 46, and 76) is very helpful as group-diagnostic for the presence of a nitrate ester, an identification of a specific nitrate ester should not be made only on the basis of its EI spectrum.
Unlike their EI mass spectra, CI mass spectra of nitrate esters contain ions at the molecular weight region: [M⫹H]⫹and [M⫹H-HONO2]⫹. This makes CI an excellent complementary method to EI for the analysis of individual nitrate esters.
Figure 3 shows the EI and CI mass spectra of EGDN and NG [24]. The similarity between the EI spectra, in contrast to the difference between the CI spectra, can be clearly observed.
The NCI spectra of nitrate esters are characterized by two major ions in the low-mass region: at m/z 46 (NO2⫺) and 62 (ONO2⫺). Ions observed in the molecular weight region are M⫺•and [M⫹ONO2]⫺.
Electron-ionization mass spectra of the two heterocyclic nitramines RDX and HMX are similar, with their most abundant ions mainly at the low-mass region: at m/z 30 (NO⫹), 42 (CH2NCH2⫹), and 46 (NO2⫹). More diagnostic ions appear at m/z 120, 128, and 148 but the spectra lack molecular ions.
When CI spectra of RDX are taken with different reagent gases, they differ from each other not only in the degree of fragmentation (which is usual in CI), but also in the type of the fragment ions [45]. With reactant ions, which are weak Bro¨nsted acids, such as C4H9⫹ (in isobutane) or NH4⫹ (in ammonia), a set of fragment ions at m/z 84, 131, and 176 is observed. With strong Bro¨nsted acids, such as H3⫹(in hydrogen) or C2H5⫹(in methane) other major ions are observed
AnalysisofExplosives397
Figure 3 EI and CI mass spectra of EGDN and NG.
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at m/z 75 and 149. A mechanistic explanation, based on the energetics involved, was suggested [45].
The EI mass spectrum of tetryl has its base peak at m/z 241, corresponding to a loss of NO2from the molecular ion. This ion appears when tetryl is introduced into the mass spectrometer via a solid probe [46,47] but is completely absent in the spectrum of tetryl obtained by GC–MS, where tetryl is hydrolyzed to N- methylpicramide [24,25].
Some unusual adduct ions, corresponding to [M⫹NO]⫹and [M⫹NO2]⫹, appear in the mass spectra of nitrate ester explosives, such as NG and PETN, and nitramines, such as RDX and HMX [38, and references cited therein]. Their formation, which is favored by CI-MS conditions, especially in high sample pres- sures, is attributed to ion–molecule reactions between NO⫹and NO2⫹ions and the neutral molecules of the explosives. These adduct ions were also reported to be highly abundant in the EI spectra of nitrate esters [19], when EI was carried out in a tight, dual EI/CI ion source.