Characterization of Two Wastewaters for Explosives Using On-Column Injection and Gas Chromatography—Time-of-Flight Mass Spectrometry

Significance of the topic

Explosives and their degradation products in water and soil pose serious environmental and health risks. Reliable detection and quantitation are vital for site assessment, remediation monitoring and regulatory compliance. Conventional GC/ECD methods suffer from coelutions and thermal degradation of labile nitrated esters, limiting specificity and sensitivity. Integrating fast gas chromatography with high-speed time-of-flight mass spectrometry (GC–TOFMS) and on-column injection addresses these challenges, enabling comprehensive, accurate profiling of trace explosives and by-products.

Objectives and overview of the study

This work evaluates a combined on-column injection, fast GC and TOFMS approach for the analysis of EPA Method 8095 explosives in standards and real wastewater extracts. Key goals include minimizing thermal degradation of nitrate esters, achieving baseline resolution of all target analytes in a single run, and demonstrating automated peak finding and deconvolution for low-level components in complex matrices.

Methodology

The study employed EPA Method 3535A solid-phase extraction on styrene-divinylbenzene cartridges to isolate explosives from 200 mL wastewater samples. Extracts were eluted with acetonitrile, diluted to 5 mL, and filtered to remove precipitate. Two wastewater aliquots were compared: one refrigerated for six months and another frozen upon collection.

Used instrumentation

• Gas chromatograph: Agilent 6890 with programmable temperature vaporizer for on-column injection.
• Retention gap: 5 m × 0.53 mm Siltek-deactivated guard column, connected via Press-Tight fitting.
• Analytical column: 10 m × 0.32 mm × 0.91 µm Rtx-TNT phase.
• Carrier gas: Helium at 5 mL/min constant flow.
• Injection: 1 µL on-column at 85 °C (5 °C above oven temperature).
• Oven program: 80 °C (0.5 min), ramp to 260 °C at 15 °C/min.
• Mass spectrometer: LECO Pegasus III GC-TOFMS, 70 eV EI, source 180 °C, m/z 45–350, 10 spectra/s.
• Data analysis: LECO ChromaTOF with automated Peak Find and Deconvolution.

Main results and discussion

Using the Rtx-TNT column at high linear velocity (~140 cm/s), all seventeen Method 8095 compounds were baseline resolved except 2,6-dinitrotoluene and 1,2-dinitrobenzene, which were distinguished by mass spectra. Fast analysis times (e.g., tetryl elution ~8 min, HMX ~11 min) preserved thermally labile nitrated esters like nitroglycerin and PETN. Total ion chromatograms of wastewater extracts revealed TNT in the frozen sample only; both contained aminodinitrotoluenes, while the refrigerated sample also showed diaminonitrotoluenes, indicating further degradation. Automated deconvolution successfully identified minor coeluting compounds, exemplified by resolving 3,5-dinitroaniline in the presence of a dominant aminodinitrotoluene peak.

Benefits and practical applications

• Comprehensive detection of explosives and degradation products in a single fast run.
• Reduced thermal decomposition of labile nitrate esters via on-column injection.
• Enhanced specificity through high-speed TOFMS and deconvolution for trace-level components.
• Streamlined workflows for environmental monitoring, forensics and remediation verification.

Future trends and potential applications

Advances in ultrafast columns and higher-resolution TOFMS or Q-TOF systems may further reduce analysis time and improve mass accuracy. Coupling with automated sample prep and data-processing pipelines will enable high-throughput screening of environmental samples. Emerging software algorithms could enhance library searches for novel degradation products and transformation products in complex matrices.

Conclusion

The integration of on-column injection, fast GC and TOFMS provides a robust, selective and rapid method for the comprehensive analysis of explosives and their by-products in environmental samples. High linear velocities minimize thermal degradation, while TOFMS acquisition speed and deconvolution deliver reliable identification of both major and trace components.

Reference

• EPA Method 3535A. SW-846 Test Methods for Evaluating Solid Waste, Solid-Phase Extraction (SPE).
• EPA Draft Update IVB, Method 8095: Explosives by Gas Chromatography, SW-846 Test Methods for Evaluating Solid Waste.
• EPA Method 8330. Nitroaromatics and Nitramines by HPLC, SW-846 Test Methods for Evaluating Solid Waste.