Stop Flow Modulated GCxGC

Importance of the Topic

Comprehensive two dimensional gas chromatography enhances the resolving power of conventional GC by delivering orthogonal separation of complex mixtures. This capability is crucial for the analysis of biodiesel and similar samples where fatty acid methyl esters, aliphatic hydrocarbons, aromatic compounds, and polycyclic species coexist.

Objectives and Study Overview

The study introduces a cost effective stop flow modulation setup implemented on an Agilent 7890 GC to achieve comprehensive GCxGC performance. The goal is to demonstrate enhanced peak capacity using differential flow modulation while maintaining accessibility for routine laboratory use.

Methodology and Instrumentation

A quasi stop flow modulator accumulates effluent from the first dimension in an internal capillary before reinjecting onto a polar second column. Key elements include:

• Primary column: nonpolar phase, 60 m × 0.25 mm × 0.25 µm
• Secondary column: polar phase, 3 m × 0.25 mm × 0.25 µm
• Modulation conditions: 2 s cycle, 0.1 s injection pulse, 21.6 psi auxiliary pressure
• Detection: flame ionization with hydrogen, air and helium makeup
• Control: programmable logic controller integrated with GC event interface

Main Results and Discussion

Analysis of a biodiesel sample diluted to 5000 ppm in dichloromethane produced a two dimensional heat map revealing distinct regions for aliphatics, aromatics, fatty acid methyl esters and PAHs. The system delivered second dimension peak widths from 64 ms to 201 ms (median 95 ms) and improved peak capacity from around 250 in one dimension to over 750 in GCxGC, representing more than a threefold increase.

Benefits and Practical Applications

Use of stop flow modulation provides:

• Significant enhancement of resolution for complex fuel and environmental samples
• Reduced cost compared to commercial GCxGC systems
• Ease of operation through remote method control and ability to switch between one and two dimensional modes

Future Trends and Possibilities

Further developments may include integration with mass spectrometric detection, automation of modulation sequences and expansion to other complex matrices such as petrochemicals, food extracts and forensic residues. Advances in software processing will also accelerate data interpretation.

Conclusion

The cost effective stop flow modulated GCxGC approach successfully multiplies chromatographic peak capacity and offers an accessible route for routine multidimensional separations of complex samples like biodiesel.

References

1. Xiaosheng Guan, Jim Luong, Ziwei Yu and Hai Jiang. Quasi Stop Flow Modulation Strategy for Comprehensive Two Dimensional Gas Chromatography. Analytical Chemistry. 2020;92(9):6251 6256. DOI 10.1021/acs.analchem.0c00814.