While the C-4 samples A includes mostly dioctyl adipate plasticizer in the form of three isomers in a unique isomer distribution pattern plus a little bit of dioctyl sebacate, sample B includes dioctyl sebacate in a single isomer form. However, it also includes two additional peaks of its homologous compounds. In Figure 2 we show the reconstructed single ion monitoring mass chromatograms (RSIM) of this C-4 explosives code name B on m/z=426.4 of dioctyl sebacate and m/z=440.4 and m/z=454.5 of its homologous compounds with the addition of CH2 and C2H4. These RSIM traces are made possible via the enhanced molecular ion in Cold EI and as demonstrated the second homologous compound further exhibits its own isomer distribution pattern. Note that the second homologous compound is at about 1% of the dioctyl sebacate. In addition, C-4 sample B further uniquely includes a few isomers of nonyl decanoate C19H38O2 as demonstrated in Figure 3. Thus, as shown, we found that the plasticizers of C-4 explosives provide a clear and unambiguous characterization of the C-4 source origin if a library of C-4 mass chromatograms would be constructed for comparison.
C-4 is a plastic explosives thus by definition it contains plasticizers that can serve for its characterization in order to build a library of C-4 plasticizers for its source identification. The underline assumption in this approach is that the plasticizers are technical grade materials thus include several source dependent isomers, homologous compounds and characteristic impurities that can fully differentiate any given C-4 explosives and possibly even C-4 from the same source but that was produced at different times. For the C-4 characterization we used the unique 5975-SMB GC-MS with Cold EI and our conclusions, based on limited two samples, include:
- We demonstrated that the 5975-SMB GC-MS with Cold EI can effectively serve for C-4 explosives characterization for its source identification via the analysis of its plasticizers, their isomers, homologous compounds and plasticizers related impurities.
- Cold EI MS provides enhanced molecular ions that are abundant for all the plasticizers, in contrast to standard EI. Molecular ions are by far the most selective ions in the mass spectra thus they can effectively serve for C-4 residue identification at trace level in complex matrices. While not tested yet, this method could potentially also serve for post-blast C-4 analysis via its plasticizers.
- The 5975-SMB GC-MS with Cold EI is characterized by improved sensitivity for the plasticizers with estimated femtogram level LOD at SIM mode.
- The demonstrated analysis time is about 3 times faster than with standard GC-MS and if essential it can be further reduced.