A fast thermal desorption unit for micro thermal desorption tubes, Part II: Method development and validation for explosives and chemical warfare agent simulants

The goal of this study is to create and optimize a rapid and efficient thermal desorption flow field thermal gradient gas chromatography/mass spectrometry (FF-TG-GC/MS) method for the analysis of explosives and chemical warfare agent (CWA) simulants. Using low thermal mass micro thermal desorption tubes (TD-tubes), the study investigates desorption conditions, the effects of TD-tube deactivation, spatial temperature gradients, and cryofocusing to enhance analyte separation and detection.

The method achieves a total cycle time of 164 seconds, including cryofocusing, thermal desorption, separation, and cooling. The validated approach demonstrated detection limits as low as 0.17 ng/L, calibration curve R-values above 0.983 (except PETN), and repeatability within acceptable ranges, making it a robust tool for rapid, high-sensitivity analysis of hazardous substances.

The original article

A fast thermal desorption unit for micro thermal desorption tubes, Part II: Method development and validation for explosives and chemical warfare agent simulants

Miriam D. Chopra, Florian A. Menger, Benny Duong,Thomas M. Klapötke, Matthias Wüst, Peter Boeker

Journal of Chromatography A, Volume 1739, 4 January 2025, 465537

https://doi.org/10.1016/j.chroma.2024.465537

licensed under CC-BY 4.0

Selected sections from the article follow. Formats and hyperlinks were adapted from the original.

Highlights

• Micro thermal desorption tubes and thermal desorption unit, with low thermal mass.
• Enrichment by thermal desorption of explosives and chemical warfare agent simulants.
• Optimization of desorption conditions (temperature, split) and method validation.
• Impact of TD-tube deactivation, spatial temperature gradient on peak areas.
• New application for flow field thermal gradient gas chromatography (FF-TG-GC).

Abstract

TD-tubes, 1.4 mm I.D., 10 mg Tenax TA). In addition to the desorption conditions, the effects of deactivated TD-tubes, the spatial temperature gradient along the separation column and additional cryofocusing to refocus the analytes were evaluated. Limits of detection between 0.17 and 9.18 ng/L and R values  0.983 for calibration curves (with the exception of PETN) were obtained. Relative standard deviations of mainly 15% were achieved for within day repeatability. The fluctuations in the peak areas were mostly below 20% with between-day repeatability measured across three days.

1. Introduction

The detection of explosives and chemical warfare agents (CWA) is of great importance for public safety, e.g. at national borders, in aviation or public transportation, critical infrastructure or at large events. In recent years, various terrorist assaults as well as the Novichok attacks on Segei and Yulia Skripal (2018) and Alexei Navalny (2020) [1] demonstrate the threat to the global civilian population. Particularly in a suspected case, rapid analysis is essential for deciding on further action and initiating potential measures. In addition to the actual analysis, sample collection and introduction into the analytical system are further time-critical steps.

The aim of this study was to develop a comprehensive method for fast analysis of ten explosives and six CWA simulants. Sample introduction was carried out via micro thermal desorption tubes (μTD-tubes) and a corresponding thermal desorption unit (TDU) previously published in part I of this article [2]. Analysis were performed using a flow-field thermal gradient gas chromatograph (FF-TG-GC) with mass spectrometric detection (TOF-MS). Other common detection methods are e.g. ECD [3], [4], [5] or IMS [6], [7], [8]. This study continues previous projects on the thermal desorption of CWA simulants [9] and on explosives analysis using FF-TG-GC [10].

The analytes investigated have very heterogeneous chemical and physical properties and differ greatly in terms of vapor pressure, boiling point and decomposition temperature (Table 1), which has to be considered during method development. Structural formulas of the respective analytes are shown in Fig. 1. In addition to optimizing the desorption conditions, the effects of μTD-tube deactivation, the spatial gradient and cryofocusing for analyte refocusing were investigated.

Due to the high toxicity of CWAs, suitable simulants that mimic the chemical and physical properties are necessary for method development [9]. Dimethyl methylphosphonate (DMMP), diethyl methylphosphonate (DEMP) and triethyl phosphate (TEP) were used as simulants for sarin, soman and tabun. Malathion and tributyl phosphate (TBP) are chosen as simulants for VX and tris(2-ethylhexyl) phosphate (TEHP) for the Novichoks.

2. Material and method

2.2. Thermal desorption setup

For sampling and sample introduction into the FF-TG-GC low thermal mass micro thermal desorption tubes (μTD-tubes) and a thermal desorption unit (TDU, HyperChrom Deutschland GmbH, Alfter, Germany) were used (Fig. 2, Fig. 3). The construction and mode of operation are explained in part I of this publication [2].

Additional μTD-tubes were fabricated from raw materials (tubing and volumetric mesh) that were deactivated with a SilcoNert coating by Silcotek (Bellefonte, USA) using the same procedure as described by Chopra et al. [2]. The volumetric mesh (VG 230, GKD - Gebr. Kuffenrath AG, Düren, Germany) is made of stainless steel and has a surface area of 236 dm/m, which corresponds to a surface area of 118.63 mm per 4 mm diameter disk. The inner surface of the μTD-tube without filling is 3.5 cm. 10 mg Tenax TA 80/100 mesh have a surface area of 3500 cm [22], which corresponds to an area ratio to the tube surface of 994.7.

Journal of Chromatography A, Volume 1730: Fig. 3. Schematic drawing of the FF-TG-GC. A - TDU; B - transfer oven; C1/C2 - purged connectors; D - transfer line; E - column fixture; F - potential cryospot; G, H - upper and lower IR-sensor; I - resistively heated capillary; J - transfer oven; K - transfer line to the detector; L - FID a - carrier gas supply of the TDU (phase I); b - split; c - carrier gas supply of the purged connector C1 (phase 0, II and III), d - carrier gas supply of the purged connector C2.

2.3. Functional principle

The TDU can be heated up and cooled down very quickly due to its small thermal mass. In combination with the μTD-tubes, a rapid desorption of the analytes is enabled.

The functional principle of the injector is described in detail in part I of the publication [2]. Only differences due to the different method are explained here.

The analysis of a μTD-tube with an additional refocusing step by cryofocusing is divided into five phases (Fig. 4):

Phases 0, II, III and IV are respectively equivalent to phases 0, I, II and III from Chopra et al. [2].

In phase I (Fig. 4, I), the cryospot is pre-cooled at the top of the separation column and temperatures of around −50 °C are reached.

Then, in phase II (Fig. 4, II) the TDU is heated up and thermal desorption occurs in reverse sampling direction. High helium flows in combination with an open split are used to accelerate the desorption process. In order to avoid band broadening of the more volatile analytes, the cryospot is used for refocusing [23].

At the end of this phase, desorption is completed and the analytes are trapped at the cryospot. Furthermore, the μTD-tube is conditioned and ready for reuse.

At the start of the GC temperature program (Fig. 4, phase III), the cryospot is re-heated by the resistively heated capillary. The analytes are released and separated. At the end of the FF-TG-GC temperature ramp the carrier gas supply is switched from the TDU to the purged connector C1 in order to backflush the transfer line and the injector to remove remaining impurities.

TDU and FF-TG-GC are cooled down in phase III and IV, respectively (Fig. 4).

2.6. Method development

To ascertain the influence of the SilcoNert coating on peak areas, comparative measurements were carried out as a tenfold determination using both a deactivated and a non-deactivated μTD-tube. This experiment was conducted for both liquid and gaseous sample application via ATIS. In both cases the μTD-tube was loaded with 1 μL of the explosive mix S2 (10 ng/mL TNT).

4. Conclusion

The development and validation of a comprehensive thermal desorption FF-TG-GC method for the explosives EGDN, 2-MNT, 4-MNT, PETN, NG, DNT, TNT, DNAN, RDX and tetryl as well as the chemical warfare agent (CWA) simulants DMMP, DEMP, TEP, TBP malathion and TEHP was the aim of this study. It is shown that a small amount of only 10 mg Tenax is enough for the adsorption of a sufficient amount of analyte for the analysis. Thus, long desorption times due to TD-tubes with high thermal masses are not necessary. Very fast measurements can be performed with a cycle time of 164 s including 4 s for pre-cooling the cryospot and only 35 s for thermal desorption.

Due to low decomposition temperatures, low vapor pressures or adsorption on surfaces, EGDN, PETN, NG, RDX, tetryl, malathion and TEHP are particularly challenging to analyze using thermal desorption. The application of deactivated μTD-tubes and the spatial temperature gradient, which lowers the elution temperature by an average of 42.2 °C, greatly improves the sensitivity of the method, especially for these analytes. Due to the slow heating of the cryospot, only the FWHM of the first two peaks, DMMP and DEMP, are reduced. To avoid broadening of the remaining signals, further optimization of the refocusing setup is necessary.

Low LODs between 0.17 ng/μL and 9.18 ng/L and coefficients of determination of 0.983 in linearity tests (exception PETN) were obtained. Relative standard deviations of 15% were achieved for all analytes except PETN and RDX in within-day repeatability tests. Between three different measurement days, the deviations mostly remain below 20%. Deviations up to 30% are observed for DNAN and Tetryl. However, huge deviations are observed for PETN, NG, RDX and TEHP.