Review: Biodegradation of Representative Ignitable Liquid Components on Soil
Emily C. Lennert
ignitable liquid, gas chromatography – mass spectrometry, GC-MS, biodegradation, microbial degradation
- Kindell, J. H.; Williams, M. R.; Sigman, M. E. Biodegradation of representative ignitable liquid components on soil. Forensic Chemistry. 2017, 6, 19-27.
- ASTM Standard E 1618, 2014, “Test Method for Ignitable Liquid Residues in Extracts from Fire Debris Samples by Gas Chromatography-Mass Spectrometry,” ASTM International, West Conshohocken, PA, 2014.
- ASTM Standard E 1412, 2016, “Practice for Separation of Ignitable Liquid Residues from Fire Debris Samples by Passive Headspace Concentration With Activated Charcoal,” ASTM International, West Conshohocken, PA, 2016.
The opinions expressed in this review are an interpretation of the research presented in the article. These opinions are those of the summation author and do not necessarily represent the position of the University of Central Florida or of the authors of the original article.
For more information on GC-MS, please refer to Review: A Fast and Reliable Method for GHB Quantitation in Qhole Blood by GC-MS/MS (TQD) for Forensic Purposes.
Ignitable liquids are flammable materials that, when ignited, produce sufficient heat to cause combustible materials (i.e. things that burn) in the vicinity to ignite as well. Ignitable liquids are a mixture of organic compounds that may be simple or complex, and may include normal alkanes, branched alkanes, cyclic alkanes, aromatics, polynuclear aromatics, and oxygenated compounds. These organic compounds may be subjected to microbial degradation if the sample is in an environment where microbes are present, such as in soil. The chemical structure of the molecule will affect its ability to resist microbial degradation. For example, branched and cyclic alkanes show greater resistance to degradation compared to normal alkanes.
Figure 1. a) Branched alkane b) Cyclic alkane c) Normal alkane.
The authors of this study had three main goals. The first goal was to determine whether ignitable liquids in soil produced biodegradation products that would be detectable by gas chromatography – mass spectrometry (GC-MS) and whether these byproducts complicated GC-MS data interpretation of the presence of ignitable liquids. The second goal of this study was to determine the approximate half-life of a simple hydrocarbon mixture, which contained compounds typically found in ignitable liquids. The third goal was to determine a half-life for each of seven ignitable liquids. Each ignitable liquid represented a class defined by the American Society for Testing and Materials (ASTM) E1618-14, Standard Test Method for Ignitable Liquid Residues in Extracts from Fire Debris Samples by Gas Chromatography-Mass Spectrometry.
A mixture of 14 hydrocarbons was prepared, containing: ethycyclohexane, n-nonane, n-undecane, n-tetradecane, 2-methylheptane, 2-heptanone, 2-butoxyethanol, p-xylene, 2-ethyltoluene, 2-methylnaphthalene, cis-1,3-dimethylcycohexane, trans-1,3-dimethylcyclohexane, 1,2,4-trimethylbenzene, and 2,2,5-trimethylhexane. Ten mL aliquots of the hydrocarbon mixture were then evaporated 50% and 90% by volume under nitrogen gas. Seven ignitable liquids from each ASTM class were also selected for analysis, and are listed in the table below alongside their respective ASTM E1618 classification.
|Ignitable Liquid||ASTM E1618 Class|
|Ortho Malathion 50 Plus Insect Spray Conc.||Aromatic|
|Phillips 66 Unleaded Boost||Gasoline|
|Pro-Gard Injector PLUS Intake Valve Cleaner||Medium Petroleum Distillate|
|STP Octane Boost||Miscellaneous|
|Lamplight Farms Citronella Torch Fuel||Napthenic-Paraffinic|
|Aura Lamp Oil||Normal Alkane|
Twenty microliters of pure hydrocarbon, hydrocarbon mixture (including evaporated mixtures), or ignitable liquid was applied to approximately 90 g of potting soil inside a quart sized can and mixed. Cans were stored for 0, 7, and 14 days for pure compounds. Hydrocarbon mixture cans were stored for 0, 2, 7, and 14 days. Ignitable liquids were stored for 0, 1, 3, 5, and 7 days. All cans were storeed at room temperature for the duration of the study. After the degradation period, extraction was performed following the ASTM E1412-12, Standard Practice for Separation of Ignitable Liquid Residues from Fire Debris Samples by Passive Headspace Concentration with Activated Charcoal. By this method, an activated charcoal strip is suspended in the headspace of the can and the can is placed in an oven and heated to 85 ˚C for 4 hours. After the can has been removed and cooled, the charcoal strip is removed and half of he strip is placed in a vial containing 0.5 mL of carbon disulfide (CS2) for extraction. For the evaporation samples, holes were punched from the charcoal strip to create a set of three replicate samples per strip, i.e. three replicates per degradation time. These replicates were individually extracted in CS2 containing a n-dodecane internal standard. The internal standard is a known amount of a material that serves as a reference point to allow for quantification of the hydrocarbon mixture as it degraded over time. Sample extracts were analyzed via gas chromatography – mass spectrometry (GC-MS).
Each of the fourteen compounds were degraded individually on soil to examine the degradation behavior of the pure compound. For most samples, an overall decrease in abundance was observed between day 0 and day 14. Exceptions to this pattern of degradation were 1,3-dimethylcyclohexane, ethylcyclohexane, and 2,2,5-trimethylhexane, for which an increase of abundance was observed. This increase in abundance was attributed possibly to failure to homogenize the soil conditions and bacterial populations between sample cans or failure to quantitatively recover the day 0 sample. No partial degradation products were observed for any samples.
The abundance of the peak in the total ion chromatograms were evaluated to compare the undegraded evaporated samples. The total ion chromatograms for the 0%, 50%, and 90% evaporated samples can be seen in Figure 1 within the study. In the comparison of 0% and 50% evaporated samples, compounds eluting early, prior to the 6 min mark, showed a decrease in intensity. Compounds between 6 and 9 min showed no change. Compounds between 9 and 12 min showed slight increase in intensity, while compounds between 12 and 16 min showed significant increases in intensity. Comparing the 0% and 90% evaporated samples, compounds that had eluted prior to 14 min in the 0% chromatogram were no longer observed in the 90% chromatogram, with only 2-methylnaphthalene and n-tetradecane remaining after 14 min.
Degradation of the 0% evaporated hydrocarbon mixture was studied over the course of 0, 2, 7, and 14 days, as seen in Figure 2 within the study. At 0 days, sample extraction was performed 30 minutes after the application of the mixture to the soil. On this day, all compounds except 2-butoxyethanol were recovered. Since 2-butoxyethanol is an oxygenated compound, and oxygenated compounds are reportedly highly susceptible to microbial degradation, the lack of recovery was attributed to rapid microbial degradation of the oxygenate. At 2 days, significant degradation was observed due to overall reduced intensity on signal for all compounds. N-nonane, n-undecane, n-tetradecane, toluene, and 2-heptanone were most affected. This loss of analyte continued with increased degradation observed at 7 days and 14 days. Degradation studies of the 50% evaporated mixture were also discussed. Overall a similar trend was observed, with increasing degradation observed as time increased. For the 90% evaporated mixture, only two compounds were present at day 0: 2-methylnaphthalene and n-tetradecane. Similarly, these compounds showed increasing degradation as time passed.
Half-life calculations were approximated to provide guidance as to safe storage times for fire debris samples where microbial degradation is possible. A half-life is simply the time required for a specified property, such as concentration, to decrease by half of its original value. The approximate half-life for the unevaporated (0%) sample was calculated to be 3.15 days. However, the half-life for the 50% and 90% evaporated samples could not be determined since there were fewer peaks that were not significantly intense. This can provide the foundation on how long soil samples should be stored before the intensity of some the peaks, needed for classification, will disappear.
Overall, degradation of the ignitable liquids was observed during the degradation study of seven ignitable liquids. For example, gasoline, which contains a mixture of aromatics and alkanes, showed a reduction in overall abundance after one day. At 3 days, large degradation of aromatic compounds from toluene to 1,2,4-trimethylbenzene was observed. At 5 and 7 days, aromatics were no longer seen, and only branched alkanes were identified. A full explanation of the degradation trend of each individual ignitable liquid can be found in section 3.4 of the study. Half-life calculations were conducted, and it as found that only the aromatic, gasoline, and normal alkane samples had half-lives that ranged from 1.75 days to 5.89 days. The half-lives for the remaining ignitable liquids could not be determined.
- Overall, hydrocarbon mixtures and ignitable liquids showed degradation trends over the course of the degradation studies performed.
- Half-lives were calculated to range from 1.75-5.89 days for ignitable liquids following first order kinetic losses, indicating that microbial degradation of the samples was occurring within a short period of time.
Samples may be stored for long periods of time before processing. However, in cases where microbial degradation is a possibility, long storage of a sample may result in significant loss of evidence.
If a sample is stored at room temperature, as in this study, the sample should be processed within a day or steps should be taken to halt microbial degradation, such as freezing the sample.