Non-Targeted Analysis of Complex Environmental Samples Using Thermal Desorption, Multidimensional Gas Chromatography, Multi-Mode Ionization Methods, and High-Resolution Time-of-Flight Mass Spectrometry

Importance of the Topic

Environmental monitoring demands comprehensive detection of diverse chemical contaminants across air, dust, soil, and food matrices. Persistent organic pollutants, endocrine disruptors, and emerging toxins often occur at trace levels within highly complex sample backgrounds. Advanced non-targeted workflows enable discovery and characterization of hundreds to thousands of compounds, supporting public health, regulatory compliance, and research initiatives.

Study Objectives and Overview

This study demonstrates the application of thermal desorption, pyrolysis, two-dimensional gas chromatography (GCxGC), multi-mode ionization, and high-resolution time-of-flight mass spectrometry (HR-TOFMS) for non-targeted analysis of complex environmental samples. Key aims include enhancing compound separation, improving annotation confidence, and extracting meaningful chemical information from rich chromatographic-spectral datasets.

Methodology

Sample introduction employed both thermal desorption for volatile organics and controlled pyrolysis for polymeric or refractory constituents. A dual-oven, dual-stage modulation GCxGC configuration provided orthogonal separations based on volatility and polarity. Multi-mode ionization included electron ionization (EI), positive chemical ionization (PCI), and electron-capture negative ionization (ECNI) to generate complementary ion populations. Data acquisition at high resolution (R ≥ 25 000) and fast rates (200 spectra per second) captured full mass ranges with sub-ppm mass accuracy.

Instrumentation Used

• Gas Chromatograph: Agilent 7890B with LECO Dual Stage QuadJet Modulator
• Sample Introduction: Thermal desorption (TD) 50 °C → 300 °C at 10 °C/s; Pyrolysis (Pyr) 50 °C → 600 °C at 60 °C/s
• Columns: Primary Rxi-5ms (30 m × 0.25 mm × 0.25 µm); Secondary Rxi-17sil ms (1.3 m × 0.10 mm × 0.10 µm)
• Carrier Gas: Helium at 1.4 mL/min constant flow
• Mass Spectrometer: LECO Pegasus HRT+ 4D; Source at 250 °C; Transfer line at 300 °C
• Ionization Modes: EI, PCI/ECNI (CH₄ reagent gas)
• Acquisition Parameters: m/z 30–1000; mass accuracy ≤ 1 ppm; acquisition rate 200 Hz

Major Results and Discussion

Comprehensive contour plots revealed thousands of peaks across sample types, including hydrocarbons, acids, aromatics, amines, alcohols, aldehydes, ketones, phenols, terpenes, fatty acids, phosphates, sterols, OTC drugs, illicit compounds, and polymer pyrolysis markers. Group clustering algorithms and mass defect plots simplified data interpretation. Extracted ion contour plots highlighted homologous series and target analytes. Chemical ionization spectra increased annotation confidence by providing protonated and adduct ions. More than twice as many compounds were annotated compared to EI-only workflows. Pyrolytic marker analysis identified polymer types (e.g., polyurethane, polyethylene terephthalate) and their characteristic fragments. Target analyte finding (TAF) workflows further isolated specific classes of toxins.

Benefits and Practical Applications

• Unbiased detection of known and unknown contaminants in air, dust, water, and food matrices
• Enhanced separation efficiency reduces coelution and improves signal-to-noise ratios
• Multi-mode ionization and high-resolution mass data support high-confidence compound identification
• Automated spectral analysis tools facilitate group clustering, database matching, and formula prediction
• Pyrolysis GCxGC enables polymer and additive profiling in environmental particulates

Future Trends and Possibilities

Integration of machine learning for automated feature extraction and compound classification promises to accelerate data processing. Real-time or near-real-time monitoring with portable GCxGC-HRMS platforms may expand field applications. Standardized spectral libraries tailored for environmental non-targeted screening will improve annotation consistency. Expanding multi-mode ionization reagents and coupling with ion mobility spectrometry could further enhance compound coverage.

Conclusion

The combination of thermal desorption, pyrolysis, GCxGC, multi-mode ionization, and high-resolution time-of-flight mass spectrometry provides a powerful discovery platform for non-targeted analysis of complex environmental samples. This workflow delivers improved chromatographic resolution, comprehensive compound coverage, and high-confidence identification—critical to monitoring emerging pollutants and supporting regulatory and research objectives.

Reference

No additional literature references were provided in the source document.