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....

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 t....

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