|Figure 1. Total ion chromatograms of Gasoline 2, Gasoline 3 and Gasoline 4 which is 50% vaporized Gasoline 2. Click on the Image to zoom.|
In Figure 2 below we demonstrate isomer distribution analysis (IDA) of the gasoline samples 2, 3 and 4 using m/z=184.4 of C13H28 as indicated in the figure. As clearly observed, gasoline 2 and 4 samples exhibit practically identical RSIM traces while gasoline 3 exhibits a different isomer distribution hence it belongs to a different fuel from a different source with much greater amount of branched hydrocarbons. This figure demonstrates that fuel vaporization practically does not affect the isomer distribution, particularly for late eluters hydrocarbons such as C13H28. Consequently, IDA brings another important tool for arson analysis that enables the correlation of the fuels to their source.
|Figure 2. Isomer distribution analysis of the three Gasoline fuels as above on the molecular ion of C13H28 m/z=184.3. Click on the Image to zoom.|
In many cases the arson accelerants used are heavier fuels such as Kerosene and or Diesel fuel which are harder to diagnose for their original source. Here again isomer distribution analyses can effectively serve to provide fuel fingerprint.
|Figure 3. Total ion chromatograms of a local diesel Fuel that was contaminated with oil (upper trace) and of a highly vaporized Kerosene (bottom trace). Click on the Image to zoom.|
In Figure 4 below we show and compare isomer distribution analysis of the diesel fuel and vaporized kerosene samples of Figure 3. We performed the IDA via RSIM on the molecular ion of C18H38 m/z=254.3 which eluted at the high elution time side of the mass chromatograms to minimize potential small residual vaporization effect on the isomer distributions. As clearly demonstrated, the two IDA traces are very different hence we conclude that these fuels are from a different source and the vaporized kerosene sample does not relate to the diesel. Furthermore, these IDA traces can serve to find the local source of both of these fuels for improved arson analysis conclusions. We note that the big branched isomer peak that eluted near 6.85 min is probably of an isoprenoid hydrocarbon according to its Cold EI mass spectrum (side methyl every four carbon atoms). We found that these isoprenoid branched hydrocarbons are particularly abundant for C19H40 (pristane) and C20H42 (phytane) whose ratio relates to the origin and time of production of the original oil. The relative concentration of these isoprenoid branched hydrocarbons was found to be a sensitive composition parameter of fuels that serve well as their fingerprint.
Conclusions and General Observations Summary
- We demonstrated that isomer distribution (abundance) analysis can serve for fuel characterization and as its fingerprint for improved arson analysis with the Aviv Analytical 5975-SMB GC-MS with Cold EI.
- Isomer distribution analysis requires GC-MS with Cold EI and can not be properly performed with standard GC-MS in view if its pre-requisite of having molecular ions for branched hydrocarbon isomers.
- Enhanced molecular ions were found for all the fuel hydrocarbons, including amplified isomer mass spectral effects.
- Reasonable chromatographic separation was achieved among the various (many) isomers that was sufficient to provide clear fuel fingerprints.
- The whole IDA analysis took 15 min, about twice faster than usual GC-MS analysis via the use of higher column flow rates hence higher than usual temperature programming rate, yet this fast analysis time was sufficient to elute oil.