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Note 95: Detection of Explosives on Clothing Material by Direct and AirSampling Thermal Desorption GC/MS

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By Ronald E. Shomo, Robert Frey, and John J. Manura, Scientific Instrument Services, 1027 Old York Road, Ringoes, NJ 08551
(presented at ASMS2007)

Introduction

The post 9/11 environment has necessitated the development of better explosive detectors. Virtually all commerce, travelers, and their associated luggage are screened by some method in order to detect explosives, biological or radioactive contamination. Thermal desorption GC/MS is another potential tool in the government's arsenal to identify this type of contraband, in this case explosive residues. Direct and air sampling thermal desorption GC/MS was used to detect a mixture of commonly found explosives and their precursors on various clothing materials. Thermal desorption provides for fast analysis with no carryover problems that can be associated with other techniques.

Materials and Methods

Four types of clothing materials were utilized in this work. Sample A was a white tee shirt composed of 100% cotton. Sample B was a yellow tee shirt composed of 50% cotton and 50% polyester blend. Sample C consisted of a pair of blue jeans composed of 100% cotton denim material. Sample D was a 100% cotton precleaned wipe.

The clothing samples were spiked with an explosive mixture obtained from Accustandard Inc. (catalog number M-8330-R).

The explosive mixture was diluted in Methanol to a total concentration of 40 ng/μl. This yielded a 2.85 ng/μl level for each individual component. Direct Thermal Extractions were carried out by placing 3 mm x 75 mm of each sample into separate pre-conditioned desorption tubes and spiking with 40 ng of explosive mix and desorbing at 180 °C.

Air sampling desorption tubes (4 mm stainless steel id x 4 inch) were packed with 150 mg of Tenax® TA adsorbent and heat conditioned in an SIS 24 port conditioning oven for 4 hours at 320 °C with an ultra-pure Nitrogen gas purge at 25 ml/min. Tubes were removed from the heat conditioning oven then allowed to cool for 10 minutes and capped with stainless steel caps with a PTFE seal to avoid adsorption of contaminants.

Air sampling was conducted by placing a 6 mm x 75 mm strip of sample spiked with 40 ng of explosive mix and placed in an enclosed glass tube and a flow of nitrogen (75 ml/min) was passed over the sample for fifteen minutes. The volatiles were collected onto a desorption tube packed with Tenax TA adsorbent.

The desorption tube was then connected to an SIS ADS2000 Thermal Desorption System. A 35 mm desorption needle was connected to one end of the desorption tube. The needle acts as the transfer line when desorption takes place, ensuring the highest transfer of sample to the GC and eliminating any carryover effects observed in many other desorption systems.

The desorption tube was placed in a 12 tube capacity autosampler. The Thermal Desorption unit (Figure 1) picked up the desorption tube from the autosampler, performed a 1 minute He purge using the GC's carrier gas, and then injected the needle through the septum of the GC injection port. After the injection port pressure stabilized, all of GC carrier gas flow was diverted through the desorption tube.

Figure 1 - schematic of
SIS AutoDesorb, Automated Short Path Thermal Desorption

Desorption blocks heated to 180 °C then closed around the desorption tube and heated the tube for 5 minutes. During the desorption process, a cryofocusing unit (SIS CryoTrap) inside the GC oven cooled a 2" section of a guard column (0.53 mm ID deactivated fused silica) to -45°C with CO2. This trapped the desorbed compounds during the desorption process, improving the chromatography by cryofocusing the analytes.

Thirty seconds after the desorption process was completed, the cryogenic cooling was stopped and the CryoTrap ballistically heated to 200°C and held for 3 minutes to elute the trapped components at the head of the capillary column.

The Thermal Desorption controller then sent a remote start signal to the GC to begin the temperature ramp from 95°C to 182°C at a rate of 8°C per minute and then to 250°C at 10°C for a total GC run time of 42.68 minutes. The MS transfer line was held at 280°C. The GC (Agilent 6890) oven contained either a capillary column Phenomenex ZB-1701, (14% cyanopropylphenyl-methylpolysiloxane) 0.25 mm ID x 30 m length with a 0.25 μm film thickness, or an Agilent HP-5MS (5% phenyl-polysiloxane) 0.25 mm ID x 60 m length with a 0.25 μm film thickness. The GC injection port was held at 180°C and the GC was run in "split" mode set at a 2:1 ratio. Total helium flow through the desorption tube was 7.0 ml/minute for the length of the 5 minute desorption run. Helium flow through the GC capillary column was 1.4 ml/minute. A Mass Spectrometer (Agilent 5973MSD) was operated in Electron Impact Mode (EI) and scanned over a mass range between 50 and 500 daltons.

All analytes were identified using Agilent ChemStation software with the NIST AMDIS software using the NIST mass spec library as well as the Wiley NBS mass spec library running in the full scan mode.

Results and Discussion

The standard explosive mix chromatogram is illustrated in Figure 2 utilizing the ZB-1701 and Figure 3 utilizing the HP-5MS capillary columns. The components are labeled consistent with Table 1.

Table 1
1. Nitrobenzene7. 2,4-Dinitrotoluene
2. 2-Nitrotoluene8. TNT
3. 3-Nitrotoluene9. 1,3,5-Trinitrobenzene
4. 4-Nitrotoluene10. 4-Amino-2,6-dinitrotoluene
5. 2,6-Dinitrotoluene11. 2-Amino-4,6-dinitrotoluene
6. 1,3-Dinitrobenzene

The limit of detection for this mix under Direct Thermal Extraction was determined to be 2.85 ng.

The two tee shirts (Samples A & B) and pair of jeans (Sample C) were all previously worn and laundered clothing worn by the presenter. The cotton wipe (Sample D) was taken fresh from its package. The Direct Thermal Desorption results are shown in Figures 4-7. In each case TNT and all of its related components are easily detected under these conditions. One can also note that the Thermal Desorption process uncovered many additional components present in the clothing materials. There are numerous materials associated with laundry detergents, such as limonene and lilial used as fragrance in some detergents and fabric softeners. The presence of homosalate, commonly found in sunscreens, was also detected in the samples, not surprising as the clothing was worn for yard work activities. In Figure 3, the presence of methadone was detected in this tee shirt (Sample A). The clothing was washed in the same washing machine as a set of scrubs worn by a nurse who works at a methadone clinic--proof that very little is missed by this analytical technique!

Figure 2 - GC chromatogram of explosives standard on ZB-1701
Figure 3 - GC chromatogram of explosives standard on HP-5MS
Figure 4 - GC chromatogram of Sample A - white tee shirt
Figure 5 - GC chromatogram of Sample B - yellow tee shirt
Figure 6 - GC chromatogram of Sample D - wipe
Figure 7 - GC chromatogram of Sample C - blue jeans air sample
Figure 8 - GC chromatogram of Sample A - white tee shirt air sample

Figure 6 shows the less complicated chromatogram of the spiked cotton wipe (Sample D). This is the type of wipe that is generally used in airport screening devices, and it dramatically reduced the number of potentially interfering components that were the result of detergents and other materials present in normal clothing samples.

Air sampling Thermal Desorption results are shown in Figure 8. A portion of the white tee shirt (Sample A), spiked with 40 ng of the explosive standard, was exposed to a total of 375 ml of Nitrogen gas. The gas was collected onto a desorption tube packed with Tenax TA. All eight of the TNT and related components were detected as observed in the resulting chromatogram.

Conclusion

We have shown that TNT and its related components are easily detected by both direct thermal and air sampling thermal desorption on several types of clothing materials representative of a majority of fabrics worn in modern society. The sensitivity and versatility of the ADS2000 allows for a complete analysis of the materials without having to resort to utilizing SIM detection mode. This advantage allows one to screen for virtually the entire spectrum of contraband used by terrorists and smugglers in one analysis giving yet another tool to help authorities keep pace with the ever changing post 9/11 world.

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