LECO's GCxGC Utilizing a Consumable- Free Modulator and Second Column Modulation

Significance of the Topic

Comprehensive two-dimensional gas chromatography (GCxGC) with thermal modulation is a powerful tool for resolving complex mixtures in environmental, petrochemical and industrial analyses. Transitioning from liquid-cryogen cooling to a consumable-free modulator (CFM) reduces operating costs, eliminates cryogen handling and extends the usable volatility range for less volatile analytes.

Objectives and Study Overview

This study compares LECO’s new CFM, which uses a silicone-oil dewar and closed-loop immersion cooler, against the conventional liquid nitrogen (LN₂) thermal modulator. Key goals:

• Highlight hardware and software differences
• Determine volatility limits and modulation performance in standard and high-volatility column configurations
• Assess peak shape, peak width and usable volatility range up to C44 and beyond

Methodology and Applied Instrumentation

Instrumentation and configuration:

• LECO Pegasus 4D GCxGC-TOFMS
• Consumable-Free Modulator using silicone oil chilled by a closed-loop immersion cooler (–80 °C limit)
• Dual-stage quad-jet design with fan-shaped cold jets
• Primary column: 30 m × 0.25 mm I.D. × 0.25 µm Rxi-1ms
• Secondary columns varied (0.10–0.18 mm I.D.)
• Flow: corrected constant 0.8 mL/min; modulation period 5 s; hot pulse 600–800 ms

Method development considered column alignment, cold-jet gas flow (10 L/min at 15 psi) and dew point requirements (≤–73 °C). Software updates allow dynamic hot-pulse duration and chiller temperature settings in ChromaTOF.

Main Results and Discussion

1. Hardware and Cooling

• CFM fan jets focus nitrogen more precisely at –80 °C vs. circular jets at –196 °C (LN₂)
• Precise column alignment critical for narrow jets and effective trapping

2. Volatility Range and Configurations

• Standard Column Configuration (SCC): modulation on second dimension; extends upper limit beyond n-C40 (demonstrated n-C44, FWHH 133 ms)
• High-Volatility Column Configuration (HVCC): required for quantitation down to n-C8 but sacrifices peak width and resolution, limiting upper range (~n-C20)

3. Peak-Width Trends and Volume Transfer

• Volume transfer effects in HVCC lead to >500 % increase in injected band length when moving from 0.25 mm to 0.10 mm, broadening peaks
• SCC avoids diameter transition before injection, preserving sharper peaks

4. Modulation Effectiveness

• Quantitative modulation threshold in SCC reached n-C12 at –80 °C, 800 ms hot pulse, 0.18 mm secondary column
• Performance trade-offs: shorter pulses and smaller diameter differences favor high volatility, whereas longer pulses and larger diameter differences favor less volatile analytes

Practical Benefits and Applications

The CFM-equipped GCxGC increases operational simplicity by removing liquid-nitrogen supply, reduces downtime, and extends upper volatility range. It is particularly advantageous for analyses of high-boiling hydrocarbons, polymers and lipid fractions, while still accommodating mid-range volatiles in SCC mode.

Future Trends and Potential Uses

Further enhancements may include:

• Variable chiller temperatures above –80 °C to fine-tune mid-range volatility trapping
• Adaptive hot-pulse programming for complex sample matrices
• Integration with automated method development tools to optimize column sets and flow rates

Emerging applications could span advanced fuel characterization, materials science and forensic metabolomics.

Conclusion

LECO’s consumable-free thermal modulator enables robust GCxGC analyses without cryogen, offers improved performance for high-boiling species beyond n-C40 in SCC, and maintains quantitative trapping for n-C8–n-C12 in HVCC. Method development must balance trapping and injection efficiency across the volatility range.