Rapid LC-MS/MS Analysis of 19 Explosives in Soil

Rapid LC-MS/MS Analysis of 19 Explosives in Soil — Summary

Significance of the topic

The reliable identification and quantitation of explosive residues in soil are essential for forensic investigations, environmental risk assessment, and post-incident remediation. Many explosives are thermally labile or present at low concentrations in complex soil matrices, so analytical approaches that preserve analyte integrity, provide sensitivity, and deliver rapid turnaround are highly valuable in practice.

Objectives and study overview

This study developed and evaluated a rapid LC-MS/MS method using a triple quadrupole system (LCMS-8060RX) coupled to UHPLC (Nexera X3) to simultaneously measure 19 target explosives spanning nitramines, nitroaromatics (including regioisomers), nitrate esters and organic peroxides in soil. The method aimed for short analysis time (~5 min), high sensitivity for thermally labile species, and chromatographic separation of nitrobenzene regioisomers.

Methodology

• Sample collection and pretreatment: Dried soil was sieved (10-mesh) and 10 g aliquots were extracted with 10 mL methanol following a protocol based on EPA Method 8330B. An internal standard (1,2-dinitrobenzene, 1,2-DNB) was added. Extraction steps: vortex 1 min, shaker 10 min, centrifugation, filtration (0.45 µm), concentration under N2 and reconstitution to 1 mL methanol.
• Calibration and spike levels: Calibration standards in methanol covered 1.0–1,000 ng/mL (weighted 1/C). Spiked soil QC at 10 ng/mL (n = 5) evaluated accuracy and repeatability. Limits of detection: most compounds detected at 1.0 ng/mL; PETN and TATP at 5.0 ng/mL.
• MS acquisition: Multiple reaction monitoring (MRM) with polarity switching to capture both positive- and negative-ion forming explosives. Transitions and collision energies were optimized for each compound.

Used instrumentation

• UHPLC: Nexera X3 with Shim-pack Velox Biphenyl column (100 mm × 2.1 mm I.D., 2.7 µm), column oven at 30 °C.
• Mobile phases: A = 5 mM ammonium formate + 0.1% formic acid in water; B = methanol. Flow rate 0.3 mL/min. Gradient: 60% B (0–1.5 min) → 90% B (5.0–6.0 min) → 60% B (6.0–8.0 min). Injection volume 2 µL.
• Mass spectrometer: Shimadzu LCMS-8060RX (triple quadrupole) with APCI source in positive/negative switching mode. Gas flows: nebulizing 4 L/min, drying 5 L/min. Temperatures: DL 150 °C, heat block 200 °C, interface 270 °C (compromise to reduce thermal decomposition while maintaining peak shape).

Main results and discussion

• Polarity behavior: Peroxides and nitramines predominantly formed positive ions; most nitroaromatics and nitrate esters formed negative ions. The method used polarity switching to monitor both types in a single run.
• Interface temperature optimization: A lower interface temperature reduces thermal decomposition of nitrate esters and peroxides, but excessively low temperatures caused peak tailing for nitramines. An interface temperature of 270 °C provided an acceptable compromise for sensitivity and peak shape.
• Chromatography: The biphenyl stationary phase and optimized gradient separated nitrobenzene regioisomers within a total run time of about 5 minutes, enabling rapid discrimination of structural isomers often encountered in forensic samples.
• Analytical performance: Calibration curves (1–1,000 ng/mL for most analytes; 5–1,000 ng/mL for PETN and TATP) showed excellent linearity with correlation coefficients R ≥ 0.995. Most compounds had detection at 1 ng/mL; PETN and TATP were reliably detected at 5 ng/mL. Intra-day accuracy for soil spikes at 10 ng/mL ranged from ~70% to 120% and repeatability (%RSD) was ≤ 9.1%, consistent with fitness for purpose for forensic and environmental screening.

Benefits and practical applications

• Fast throughput: Single-run (~5 min) analysis of 19 explosives reduces instrument time and increases sample throughput for forensic laboratories.
• Broader applicability: Simultaneous positive/negative ion detection captures a chemically diverse set of explosives (nitramines, nitroaromatics, nitrate esters, peroxides) without thermal degradation that can compromise GC-based methods.
• Forensic and environmental utility: Method sensitivity and robustness support trace-level detection in soil for evidence recovery and contamination assessment.
• Regioisomer separation: Effective resolution of nitrobenzene isomers improves compound identification and reduces potential misassignment during forensic interpretation.

Future trends and possible uses

• Sample cleanup improvements: Integration of selective SPE or automated on-line cleanup workflows could enhance matrix removal and reduce sample handling variability across diverse soil types.
• Broader validation: Expanded validation across soil matrices with varying organic content, moisture and co-contaminants, plus inter-laboratory studies, would strengthen method adoption as a forensic standard.
• High-resolution and hybrid workflows: Complementary use of high-resolution MS for non-target screening, structural confirmation, and identification of degradation products can extend information from targeted MRM assays.
• Field and rapid-response applications: Development of portable LC-MS or simplified sample prep kits could enable more immediate on-site screening following incidents.
• Stability-focused methods: Improved protocols for peroxide stabilization and peroxide-specific cleanup will be important for accurate quantitation of TATP and HMTD in environmental samples.

Conclusion

A rapid LC-MS/MS method using a triple quadrupole instrument with polarity switching and optimized LC/MS conditions enables simultaneous, sensitive analysis of 19 explosives in soil within approximately five minutes. The approach preserves thermally labile nitrate esters and peroxides, separates nitrobenzene regioisomers, and demonstrates suitable linearity, accuracy and repeatability for forensic and environmental applications.

Reference

1. U.S. EPA. Method 8330B (SW-846): Nitroaromatics, Nitramines, and Nitrate Esters by High Performance Liquid Chromatography (HPLC), Revision 2, 2006.