Quantitative Detection of Trace Explosive Vapors by Programmed Temperature Desorption Gas Chromatography-Electron Capture Detector

Summary

Trace explosive vapors of TNT and RDX collected on sorbent-filled thermal desorption tubes were analyzed using a programmed temperature desorption system coupled to GC with an electron capture detector. The instrumental analysis is combined with direct liquid deposition method to reduce sample variability and account for instrumentation drift and losses.

Abstract

The direct liquid deposition of solution standards onto sorbent-filled thermal desorption tubes is used for the quantitative analysis of trace explosive vapor samples. The direct liquid deposition method yields a higher fidelity between the analysis of vapor samples and the analysis of solution standards than using separate injection methods for vapors and solutions, i.e., samples collected on vapor collection tubes and standards prepared in solution vials. Additionally, the method can account for instrumentation losses, which makes it ideal for minimizing variability and quantitative trace chemical detection. Gas chromatography with an electron capture detector is an instrumentation configuration sensitive to nitro-energetics, such as TNT and RDX, due to their relatively high electron affinity. However, vapor quantitation of these compounds is difficult without viable vapor standards. Thus, we eliminate the requirement for vapor standards by combining the sensitivity of the instrumentation with a direct liquid deposition protocol to analyze trace explosive vapor samples.

Introduction

Gas Chromatography (GC) is a core instrumental analysis technique of Analytical Chemistry and is arguably as ubiquitous as a hot plate or balance in a chemistry laboratory. GC instrumentation can be used for the preparation, identification, and quantitation of a multitude of chemical compounds and can be coupled to a variety of detectors, such as flame ionization detectors (FIDs), photo-ionization detectors (PIDs), thermal conductivity detectors (TCDs), electron capture detectors (ECDs), and mass spectrometers (MS), depending on the analytes, methodology, and application. Samples can be introduced through a standard split/splitless inlet when working with small sample solutions, specialized headspace analysis inlets, solid phase micro-extraction (SPME) syringes, or thermal desorption systems. GC-MS is often the standard technique used in validation and verification applications of alternative or emerging, detection techniques because of its utility, flexibility, and identification power with established chemical databases and libraries.^1–7 GC and its related sampling and detecting components is ideal for routine chemical analysis and more specialized, challenging analytical applications.

An analytical application of increasing interest to military, homeland security, and commercial enterprises is trace explosive vapor detection, with detection including identification and quantitation. Trace explosive vapor detection is an unique analytical chemistry challenge because the analytes, such as 2,4,6-trinitrotoluene (TNT) and cyclotrimethylenetrinitramine (RDX) have physical properties that make them especially difficult to handle and separate using broader, more generic chemical analysis methodologies. The relatively low vapor pressures and sub parts-per-million by volume (ppm_v) saturated vapor concentration, combined with relatively high sticking coefficients, necessitate special sampling protocols, instrumentation, and quantitation methods.^8–12 A GC coupled to an Electron Capture Detector (ECD) or mass spectrometer (MS) is an effective method for quantitating explosive analytes, specifically dinitrotoluene (DNT), TNT, and RDX.^6,13–17 GC-ECD is particularly useful for nitro-energetic compounds because of their relatively high electron affinity. The U.S. Environmental Protection Agency (EPA) has created standard methods for explosive analyte detection using GC-ECD and GC-MS, but these methods have focused on samples in solution, such as ground water, and not samples collected in the vapor phase.^2,18–23 In order to detect explosive vapors, alternative sampling protocols must be used, such as vapor collection with sorbent-filled thermal desorption sample tubes, but quantitative detection remains difficult due to lack of vapor standards and calibration methods that do not account for sample tube and instrumentation losses.

Recently, quantitation methods using thermal desorption systems with a cooled inlet system (TDS-CIS), coupled to a GC-ECD have been developed for TNT and RDX vapors.^24,25 The losses associated with the TDS-CIS-GC-ECD instrumentation for trace explosive vapors were characterized and accounted for in example calibration curves using a direct liquid deposition method onto sorbent-filled thermal desorption sample tubes. However, the literature focused on instrumentation characterization and method development but never actually sampled, analyzed, or quantitated explosive vapors, only solution standards. Herein, the focus is on the protocol for sampling and quantitating explosive vapors. The protocol and methodology can be expanded to other analytes and trace explosive vapors, such as Pentaerythritol tetranitrate (PETN).

Protocol

1. Instrument Preparation

1. Ensure the instrument, oven, and detector are at RT. Turn off gas flow to the inlet and detector.
2. Remove the TDS from the GC. Consult the manufacturer’s user manual for the instrument-specific procedure.
3. Remove the TDS adaptor from the CIS inlet and remove the liner from the CIS.
4. Inspect the CIS inlet for particles and debris while the liner is removed. Clean any visible debris with compressed air, or preferably nitrogen.
5. Attach a new graphite ferrule to a new CIS liner using the manufacturer provided tool and instructions for ferrule-to-liner binding.
6. Insert the liner with the attached graphite ferrule into the CIS. Replace the TDS adaptor and re-mount the TDS.
7. Remove a new column from its packaging and remove the silicone protection from the ends of the column.
8. Insert a nut and ferrule onto each end of the column. Use an ECD detector nut and ferrule for one end of the column and a CIS ferrule for the opposite end of the column.
9. Using a ceramic column cutting tool, remove approximately 10 cm from each end of the column. Ensure the nuts and ferrules remain on the column but away from the end of the column to avoid clogging and debris.
10. Secure the column into the oven using the instrument manufacturer guidelines. Insert the column into the inlet. Connect the other end of the column to the detector port. The depth of insertion is specific to instrument, inlet, and detector manufacturer. See the user manual and specifications for the exact column insertion depth.
    NOTE: A pre-bake may be required for the column before connecting the opposite end of the column to the detector ports. Consult the column and instrument manufacturer documentation to determine if a pre-bake is required.
11. Gently hand-tighten nuts and ferrules onto their respective ports for the inlet and detector. Using a wrench, tighten with approximately a quarter turn of rotation the nuts and ferrules. Too much force or over-tightening will damage the ferrules causing leaks or the column to break and clog.
12. Bake out the TDS, inlet, column, and detector. A typical bake out consists of setting the temperature for all zones to just below the maximum operating temperature (typically 300 °C) while flowing carrier gas for at least 2 hr.
13. Cool all zones and retighten all nuts and ferrules to ensure leak-free operation. Heating and cooling during the bake out will cause the nuts and ferrules to loosen, which can introduce leaks.
14. Load, or reload, the instrument method using the software interface. Verify correct temperatures and flow rates have been achieved. Instrumentation is ready for analysis.

2. Preparation of Standards

1. Remove 1,000 ng µl^-1 3,4-DNT, 10,000 ng µl^-1 TNT, and 10,000 ng µl^-1 RDX from the freezer or refrigerator and allow the three stock solutions to reach RT.
2. Dispense 100 μl of stock 1,000 ng µl^-1 3,4-DNT and add 900 µl of acetonitrile into an amber sample vial.
3. Dispense 100 μl of the 100 ng µl^-1 3,4-DNT solution from Step 2.2 and add 900 µl of acetonitrile into an amber sample vial.
4. Dispense 150 µl of the 10 ng µl^-1 3,4-DNT solution from Step 2.3 and 4,850 µl of acetonitrile into an amber sample vial. This is the internal standard for direct liquid deposition.
5. Dispense 100 μl of stock 10,000 ng µl^-1 TNT solution, 100 μl of stock 10,000 ng µl^-1 RDX solution, and 800 µl of acetonitrile into an amber sample vial.
6. Dispense 100 μl of the 1,000 ng µl^-1 TNT and RDX solution in Step 2.5 and 900 µl of acetonitrile into an amber sample vial.
7. Dispense 100 μl of the 100 ng µl^-1 TNT and RDX solution from Step 2.6 and 900 µl of acetonitrile into an amber sample vial.
8. Dispense 100 μl of the 10 ng µl^-1 TNT and RDX solution from Step 2.7 and 900 µl of acetonitrile into an amber sample vial. This creates the 1.0 TNT/1.0 RDX ng μl^-1 solution standard ready for direct liquid deposition onto sample tubes.
9. Dispense 60 µl of the 10 ng µl^-1 solution in Step 2.7 and 940 µl of acetonitrile into an amber sample vial. This creates the 0.6 TNT/0.6 RDX ng μl^-1 solution standard ready for direct liquid deposition onto sample tubes.
10. Dispense 40 µl of the 10 ng µl^-1 solution in Step 2.7 and 960 µl of acetonitrile into an amber sample vial. This creates the 0.4 TNT/0.4 RDX ng μl^-1 solution standard ready for direct liquid deposition onto sample tubes.
11. Dispense 20 µl of the 10 ng µl^-1 solution in Step 2.7 and 980 µl of acetonitrile into an amber sample vial. This creates the 0.2 TNT/0.2 RDX ng μl^-1 solution standard ready for direct liquid deposition onto sample tubes.
12. Dispense 100 µl of the 1.0 ng µl^-1 solution in Step 2.8 and 900 µl of acetonitrile into an amber sample vial. This creates the 0.1 TNT/0.1 RDX ng μl^-1 solution standard ready for direct liquid deposition onto sample tubes.

3. Sample Collection

1. Connect one sorbent-filled thermal desorption sample tube to a sample pump or similar equipment using a small piece of flexible silicone tubing. A red arrow is provided on the sample tubes indicating the air flow direction for sample adsorption, and it should be pointing in the direction of the silicone tubing and sample pump.
2. Attach a piston flow meter to the sample tube at the opposite end from the sample pump attached in Step 3.1. Adjust the flow rate on the sample pump, or similar equipment, such that the flow rate is approximately 100 ml min^-1 through the sample tube according to the readings from the piston flow meter. The flow rate should be set to ±5.0 ml min^-1 of the 100 ml min^-1 desired set point.
3. Disconnect the piston flow meter from the sample tube and temporarily shut off the sample pump but leave the sample tube connected to the pump. The sample pump will be reactivated to begin sample collection. The sample tube is ready for collection.
4. Place the sample tube with the still connected sample pump in the explosives vapor stream. The vapor source could be the headspace above a solid sample, an open environment, or a variety of analyte vaporization systems.
5. Set a timer based on the approximate sampling times listed in Table 2. The sampling times are listed as a general guideline based on the suspected concentration of material in the vapor phase. These sampling times, with a flow rate of 100 ml min^-1, will generally yield a mass in the center of the calibration curve, which is ideal for quantitation.
6. Activate the sample pump and start the timer. Wait until the timer has stopped and shut off the sample pump. Disconnect the sample tube from the pump and place it in the packaging provided with the sample tube. Cap the tube and store for analysis.
7. Record the unique serial number stamped onto each sample tube, the sample time, and the flow rate for the sample tube in a laboratory notebook. These values will be important for quantitation.

4. Calibration Curve Generation

1. Pipet 5.0 µl of the solution standard directly on the glass frit of an unused, conditioned sample tube. Hold the sample tube and pipet upright with a gloved hand during deposition.
2. Repeat Step 4.1 for each of the six calibration standards onto three different sample tubes.
3. Deposit 5 µl of the 0.3 ng μl^-1 3,4-DNT on each of the tubes as well.
4. Allow the eighteen sample tubes (three per solution concentration, six solution concentrations) to sit at RT for at least 30 min to evaporate the solvent.
5. Use the twenty tube autosampler and the previously described TNT and RDX TDS-CIS-GC-ECD method to run and analyze all eighteen tubes O/N.^24,25 A summary of the TDS-CIS-GC-ECD parameters for the method is provided in Table 1.
6. Integrate the peaks associated with 3,4-DNT, TNT, and RDX in the chromatogram for each of the eighteen sample tubes. The 3,4-DNT, TNT and RDX peaks will occur at approximately 4.16, 4.49 and 4.95 min, respectively.
7. Note the 3,4-DNT, TNT and RDX peak areas for each of the eighteen tubes along with the corresponding mass of TNT and RDX that was deposited on the sample tube in a spreadsheet and laboratory notebook.
8. Normalize the peak areas for both TNT and RDX by dividing each peak area by the peak area for 3,4-DNT. Do this for all eighteen tubes.
9. Calculate the average and standard deviation of the normalized TNT and RDX peak areas for the six standard concentrations.
10. Plot the average normalized peak area versus mass of analyte present on the tubes for both TNT and RDX.
11. Add a linear trend line for both the TNT and RDX data points. Identify the slope and y-intercept for each analyte. Record the slope, intercept, and R² value in a spreadsheet and laboratory notebook.
12. Place used sample tubes in a tube conditioner for 3 hr at 300 ºC and 500 ml min^-1 nitrogen flow.

5. Sample Analysis

1. Deposit 5.0 µl of the 0.3 ng µl^-1 3,4-DNT on each of the sample tubes.
2. Allow the tubes to sit at RT for at least 30 min to evaporate the solvent from the internal standard.
3. Use the twenty tube autosampler and the previously described TNT and RDX method to run the tubes O/N on the TDS-CIS-GC-ECD.^24,25 A summary of the instrumentation parameters for the analysis method is provided in Table 1.
4. Integrate the peaks associated with 3,4-DNT, TNT, and RDX in the chromatogram for each of the eighteen sample tubes. The 3,4-DNT, TNT and RDX peaks will occur at approximately 4.16, 4.49 and 4.95 min, respectively.
5. Note the 3,4-DNT, TNT and RDX peak areas for each of the sample tubes in a spreadsheet and laboratory notebook.
6. Use the peak areas and calibration curve to calculate the vapor concentration in parts-per-billion by volume (ppb_v) for each analyte. See Equations 1-4.
7. Place used sample tubes in a tube conditioner for 3 hr at 300 ºC and 500 ml min^-1 nitrogen air flow.

Results

Obtaining quantitative results for trace explosive vapor samples begins with establishing a calibration curve for the TDS-CIS-GC-ECD instrumentation using the direct liquid deposition method of solution standards onto sample tubes to account for instrument losses and differences between solution standards and vapor samples. The TDS-CIS-GC-ECD instrumentation and method for TNT and RDX trace analysis has been previously described in detail elsewhere, but the instrument parameters are summarized in Table 1...

Discussion

Reproducibility is a critical attribute for the quantitation of trace explosive vapors using the direct liquid deposition method with TDS-CIS-GC-ECD instrumentation, and Relative Standard Deviation (RSD) is often used as a metric for reproducibility. We have experienced RSDs for inter- and intra-sample reproducibility of approximately 5% for TNT and 10% for RDX. Any RSD above 15% is used as an indicator to check common sources of variation that reduce the effectiveness of the protocol. Sources of variation that have led ...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
2,4,6-Trinitrotoluene (TNT)	Accu-Standard	M-8330-11-A-10X	10,000 ng μl-1
Cyclotrimethylenetrinitramine (RDX)	Accu-Standard	M-8330-05-A-10X	10,000 ng μl-1
3,4-Dinitrotoluene (3,4-DNT)	Accu-Standard	S-22988-01	1,000 ng μl-1
Tenax® TA Vapor Sample Tubes	Gerstel	009947-000-00	Tenax® 60/80
CIS4 Liner	Gerstel	014652-005-00	or equivalent
Transfer Line Ferrule	Gerstel	001805-008-00
Inlet Liner Ferrule	Gerstel	001805-040-00
CIS4 Ferrule	Gerstel	007541-010-00
ECD Detector Ferrule	Agilent	5181-3323
DB5-MS Column	Res-Tek	12620