Improved Analysis of Environmental Samples Using Soft Ionization and Novel GCxGC-HR-TOFMS

Significance of the Topic

Rapid and reliable characterization of complex environmental matrices is essential for monitoring pollutants and protecting ecosystems. Comprehensive two-dimensional gas chromatography (GCxGC) coupled with high-resolution time-of-flight mass spectrometry (HR-TOFMS) significantly boosts separation power and mass accuracy. Incorporating soft ionization techniques further refines analyte identification by generating abundant quasi-molecular ions and reducing spectral interferences, thus improving confidence in formula assignments and trace-level detection.

Objectives and Study Overview

This work presents the development and evaluation of a novel multimode ion source for GCxGC-HR-TOFMS, capable of electron ionization (EI), positive chemical ionization (PCI), and negative chemical ionization (NCI) within the same unit. The main aims are to:

• Quantify dynamic range, sensitivity, and limits of detection for each ionization mode
• Assess chromatographic performance in GCxGC separations, including peak width and retention time stability
• Demonstrate complementary chemical information and enhanced responses for key analyte classes in environmental samples

Applied Methodology and Instrumentation

The GCxGC system comprised an Agilent 7890B gas chromatograph with a LECO quad-jet liquid-nitrogen-cooled thermal modulator. A 30 m × 0.25 mm Rxi-5SilMS primary column was coupled to a 1 m × 0.25 mm Rxi-17SilMS secondary column. Oven programming featured an initial 2 min hold at 50 °C, a 5 °C/min ramp to 300 °C, and an 8 min hold. The secondary oven and modulator were offset +15 °C, with a 5 s modulation period. Helium carrier gas flowed at 1 mL/min. The HR-TOFMS was a LECO Pegasus GC-HRT+ prototype with Encoded Frequent Pushing® capable of 200 full spectra/s acquisition. Environmental samples (Moscow snow) were prepared via EPA Method 8270 liquid–liquid extraction and analyzed under all three ionization modes.

Main Results and Discussion

Dynamic range improved in all modes, with 1D ranges spanning femtogram to nanogram levels. Negative CI achieved low-femtogram limits for octafluoronaphthalene, while positive CI and EI exhibited robust mid-range performance. Two-dimensional peak widths averaged 0.08 s, permitting retention time alignment across modes without chromatographic modification. NCI provided dramatic sensitivity gains for halogenated species, up to 200× enhancement for pentachlorobiphenyl and heptachlorobiphenyl. PCI proved particularly effective for alkane series (C15–C35), generating strong (M–H)+ signals and facilitating homologous identification.

Benefits and Practical Applications

Implementing a single multimode source streamlines workflows by maintaining consistent chromatography while delivering complementary ionization data. Enhanced detection of both polar and nonpolar analytes supports comprehensive profiling of environmental samples, including trace levels of PCBs, pesticides, and hydrocarbon residues. The approach reduces instrument switching and simplifies data interpretation by preserving retention indices across modes.

Future Trends and Applications

Ongoing developments may include automation of ionization mode switching, integration with advanced data-processing algorithms and machine learning for rapid compound classification, and expansion into other matrices like biota or industrial effluents. Coupling multimode GCxGC-HR-TOFMS with high-throughput sampling techniques could further elevate environmental monitoring capabilities.

Conclusion

The prototype multimode CI/EI ion source on GCxGC-HR-TOFMS demonstrates significant gains in sensitivity, dynamic range, and information content for environmental analysis. Its ability to operate in EI, PCI, and NCI without altering chromatographic conditions offers a versatile platform for comprehensive chemical characterization.

Instrumentations Used

• Agilent 7890B gas chromatograph
• LECO quad-jet liquid-nitrogen-cooled thermal modulator
• Rxi-5SilMS (30 m × 0.25 mm) primary GC column
• Rxi-17SilMS (1 m × 0.25 mm) secondary GC column
• LECO Pegasus GC-HRT+ (Folded Flight Path®) high-resolution time-of-flight mass spectrometer
• Data acquisition at 200 full spectra/s