Review: Investigating TNT Loss Between Sample Collection and Analysis

/in ,

Emily C. Lennert

 

Category

Chemistry

Keywords

trinitrotoluene, TNT, swab, collection, recovery, extraction, 2,4-dinitrotoluene, 2,4-DNT

Article Reviewed

Daeid, N. N.; Yu, H. A.; Beardah, M. S. Investigating TNT loss between sample collection and analysis. Science and Justice. 2017, 57, 95-100.

Disclaimer

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.

Summary

After an explosive event, explosive residues may be found in the surrounding area. These residues tend to have low persistence in the environment and therefore explosive residues must be sampled soon after the explosive event. Collection by swabbing, using cotton swabs and an organic solvent, is a common method. However, there is not an internationally agreed to best practice for swab collection and handling. Trinitrotoluene (TNT) is a commonly used high explosive that is inexpensive and safe to handle. TNT has high explosive power, chemical stability, and low shock sensitivity. For these reasons, TNT may be present in explosive residues found at the scene of a crime. Therefore, the authors of this study sought to assess how TNT recovery from swabs change with storage time, temperature, UV light exposure, and storage containers.

Swabs were spiked and stored either as a swab in a nylon bag or glass vial or extracted and stored in a glass vial. Samples were then either analyzed immediately or stored for 1, 2, or 4 weeks. Three storage conditions were used: in a freezer within a cardboard box, at room temperature on a benchtop and exposed to daylight, and at room temperature in a dark cupboard. Three replicates were made for each condition, as well as a negative control.

First, the extraction system was developed using 2,4-dinitrotoluene as a cheap alternative to TNT. Swabs were extracted with and without a sonication step to assess the benefit of a sonication step. Three sets of samples were prepared: with sonication, without sonication, and a positive control. For the sonicated samples, a clean cotton swab was placed into four glass vials, and 0.5 mL of 0.4 mg/mL 2,4-DNT was spiked onto three of the four samples. The fourth was kept blank to serve as a negative control. After spiking, 4 mL of ethyl acetate was added to each vial and the vials were sonicated for 10 minutes. After sonication, each swab was manually agitated with the tip of a pipette for 2 minutes, then the extract was drawn through the swab and placed in a 5 mL volumetric flask. The swab was rinsed with 1 mL of ethyl acetate and the rinse was transferred to the volumetric flask, then the volumetric flask was made to be 5 mL. Finally, 100 µL of 2 mg/mL Musk Tibetine (1-tert-butyl-3,4,5-trimethyl-2,6-dinitrobenzene) was added as an internal standard for gas chromatography – mass spectrometry (GC-MS) analysis. Unsonicated samples were prepared in the same manner, without the sonication step. The positive control was prepared by etracting the blank swab, then spiking the swab extract with 2,4-DNT. It was found that the sonication step was benefitted the recovery of 2,4-DNT. Sonication led to higher recovery of 2,4-DNT as well as more reproducible recovery from the swabs.

The procedure was reduced for TNT, due to the cost of TNT. One cotton swab was divided into five vials, with each piece weighing 0.13-0.16 g. Three sets were prepared, one with sonication, one without, and one as a positive control, following the procedures described for 2,4-DNT. Swabs were spiked with 0.1 mL of 0.4 mg/mL of TNT solution. Swabs were extracted in 1.0 mL of ethyl acetate. 20 µL of 2 mg/mL Musk Tibetine was used as an internal standard. Similar to 2,4-DNT, higher recoveries were observed in the samples that were sonicated. The authors note that TNT recoveries were lower than the recoveries of 2,4-DNT, suggesting that TNT may bind more tightly to the cotton swab than 2,4-DNT, even when extraction is performed immediately after spiking.

A cotton swab piece was placed into a nylon bag, and the swab spiked with 0.1 mL of 0.4 mg/mL TNT solution before the bag was sealed and placed in its given storage environment: freezer, benchtop, or cupboard. After the specified aging time, the swab was transferred to a 7 mL vial and the nylon bag was rinsed with 1 mL of ethyl acetate. Extraction then mirrored the previously described steps. Three replicates were prepared for each condition. Spiked swabs that were stored on the at room temperature on the benchtop and exposed to daylight showed slightly lower recoveries compared to those stored at room temperature in the cupboard. Those stored in the freezer gave the highest recoveries. These results suggested that the storage temperature has a large effect on the recovery of TNT. This may be due to TNT volatilization. The results implied that UV exposure played a less significant role in TNT recovery than temperature, but still had an effect on the recovery of TNT.

A cotton swab piece was placed into a clean glass vial and spiked with 0.1 mL of 0.4 mg/mL TNT solution. The vial was sealed and placed in its given storage environment for a specified amount of time. Three replicates were prepared for each condition. Extraction mirrored the previously described steps. A similar overall pattern was observed compared to what was observed with the nylon bag samples. Recovery from freezer samples remained stable over time. However, a large drop in TNT recovery was observed after just one week of storage at room temperature. Greater TNT recovery for benchtop samples was observed for nylon bag samples than for glass samples, suggesting that UV light penetration through glass was greater than through nylon bags. However, it was concluded that for the majority of cases, TNT recovery was not statistically significantly different between nylon and glass storage conditions.

A clean swab was extracted prior to spiking the solution with TNT to create simulated extracts. All gave high TNT recoveries, regardless of the storage conditions. The authors state that this suggests that volatilization plays a large role in the loss of TNT in swabs. There was also little difference between samples stored in daylight and in the dark, suggesting that the effect of UV degradation is minimized if the TNT is in a solvent. These results suggested that it is desirable to extract swabs immediately and store as an extract to minimize TNT loss. In most cases, significantly higher recoveries were reported for the simulated swab extracts compared to the swabs stored in nylon bags and glass vials.

Scientific Highlights

  • TNT recovery was lowest for samples stored in glass on the benchtop in room temperature conditions.
  • Samples stored as extracts gave significantly higher recoveries compared to samples stored in nylon or glass.
  • Temperature appeared to play a large role in the loss of TNT, with UV degradation playing a less significant role.

Relevance

Trace explosives must be preserved as best as possible to minimize sample loss.

Potential Conclusions

TNT samples should be extracted as soon as possible and sored in extract form to prevent the loss of TNT from samples.