- Be able to analyze all the explosive compounds that are in active use today and preferably with one method of analysis.
- Improve the analysis sensitivity particularly for explosives compounds that are difficult to analyze such as the peroxide explosives.
- Improve the selectivity of explosive analysis via the enhancement of the molecular ions and/or high mass fragments in the explosives' mass spectra.
- Reduce the analysis time.
Explosives compounds are polar and certain explosives are very polar and can therefore saturate the GC column at few nanograms on-column amounts. For example, we found that HMTD saturates the DB5MS-UI column already at 5 ng on-column amount and HMX at well below this value. Thus, the analysis of HMX is further challenging in view of its limited column capacity and in view of the fact that it is the least volatile among the organic explosives thus requires higher injector, transfer line and ion source temperatures that could degrade the other more volatile explosive compounds. The ion source temperature problem is eliminated with the Cold EI fly-through ion source while the transfer line issue is resolved via its temperature programming. Injector temperature programming could resolve this issue at the injector but we found that with pulsed high pressure injection with a short column we can properly inject HMX even at 160ºC injector temperature. In figure 2, the total ion count mass chromatogram of RDX and HMX explosives mixture is shown (upper trace), as obtained with the 5975-SMB GC-MS with Cold EI. The Cold EI mass spectrum of HMX is shown at the bottom trace and while it is without a molecular ion even in Cold EI, unlike in standard EI the Cold EI MS of HMX is characterized by highly abundant m/z=222 fragment mass spectral peak that enables its selective detection in complex mixtures. We found that the Cold EI MS of HMX is easily identified by the NIST library with 75% identification probability. While we did not try to obtain a molecular ion for HMX by the use of cluster CI we assume that it will provide a protonated molecular ion in cluster CI as described for other explosives such as PETN and RDX in reference 1.
|Figure 2. The analysis of RDX and HMX explosives mixture with the 5975-SMB GC-MS with Cold EI. The total ion count is shown at the upper trace while the Cold EI mass spectrum of HMX is shown at the bottom trace. Click Image to Enlarge.|
We also evaluated the analysis of several other explosives with the 5975-SMB GC-MS with Cold EI and for example we found that ETN is relatively easy to analyze while similarly to PETN it does not have a molecular ion even in Cold EI. PETN provides a protonated molecular ion in cluster CI  while in Cold EI it provides a useful high mass fragment at m/z=240 and ETN similarly exhibits a useful high mass fragment at m/z=226. Urea nitrate is another challenging explosive which is an inorganic explosive compound. We found that contrary to some perception it can be analyzed by the 5975-SMB GC-MS with Cold EI as it probably degrade at the injector into urea and nitric acid and thus it is detected as a broad GC peak with a strong mass spectral peak at the urea mass of m/z=60.
|Figure 4. TAMI software generation of an elemental formula for HMTD. Only a one candidate elemental formula is generated in the TAMI table, the correct HMTD elemental formula. Click Image to Enlarge.|