Time-of-Flight Mass Spectrometry

Importance of the Topic

Styrene is a key monomer in polymer production but may contain trace amounts of benzene, a regulated carcinogen. Industry targets benzene levels below 10 ppm to ensure product safety and compliance. Conventional single-column GC methods, such as ASTM D5135, only quantify benzene down to 1000 ppm and cannot fully separate benzene from similar boiling C8/C9 hydrocarbons. A high-resolution, sensitive analytical approach is therefore critical for quality control, process monitoring, and regulatory compliance.

Objectives and Overview

This study demonstrates a two-dimensional gas chromatography (2-D GC) method using an Agilent 6890N GC equipped with a Deans switch to quantify trace benzene in styrene. The method supports two modes: prefractionation, which transfers all low-boiling components before benzene to a second column, and heart-cutting, which selectively transfers only the benzene fraction. Both modes aim to achieve better separation, lower detection limits, and higher precision than single-column GC.

Methodology

The sample is injected into a split/splitless inlet and first separated on a nonpolar HP-1 column. In prefractionation mode, all components eluting before benzene are sent to a polar INNOWax column. In heart-cutting mode, a fluidic switch diverts just the benzene fraction to the INNOWax column at the precise retention time window. Electronic pneumatic control (EPC) and Deans Switch Calculator software set accurate flows, pressures, and restrictor dimensions. An oven program ramps from 50 °C to 220 °C, and flame ionization detection (FID) is used on both columns.

Instrumentation

• Agilent 6890N GC with EPC control
• Capillary split/splitless inlet (HP-1, 15 m × 0.53 mm, 3 µm)
• Fluidic Deans switch kit
• Secondary INNOWax column (60 m × 0.53 mm, 1 µm)
• Flame ionization detectors (FID A and B)
• Agilent 7683 autoinjector and ChemStation data system

Main Results and Discussion

Heart-cutting mode detected 1 ppm benzene in styrene with a signal-to-noise ratio > 19. Retention time RSD was below 0.1 % and area RSD ≤ 0.8 % over five replicates. Linearity from 1 to 1000 ppm yielded R²=1.0000. Prefractionation mode showed comparable precision and sensitivity for trace levels. A gasoline-spiked styrene sample illustrated that heart-cutting dramatically reduces co-eluting hydrocarbon interferences versus prefractionation, enhancing selectivity.

Benefits and Practical Applications

• Quantification of benzene down to 1 ppm with high precision
• Reduced interferences through targeted heart-cut cuts
• Faster method development via Deans Switch Calculator
• Applicability to final product inspection, process control, specification setting, and R&D

Future Trends and Potential Applications

Advances in two-dimensional GC hardware, such as automated switching valves and integrated mass spectrometry, will further improve sensitivity and selectivity. Real-time online monitoring of reactive streams and extension of the approach to other trace contaminants in petrochemical and environmental matrices represent promising directions.

Conclusion

The Agilent 6890N with Deans switch offers a reliable 2-D GC solution for trace benzene analysis in styrene. Both prefractionation and heart-cutting modes deliver exceptional sensitivity, precision, and interference reduction, meeting stringent regulatory and quality requirements.

References

• Proposed ASTM Standard Test Method for Trace Benzene in Aromatic Hydrocarbons by Capillary Gas Chromatography
• ASTM D5135-02 Standard Test Method for Analysis of Styrene by Capillary Gas Chromatography, ASTM
• W. Bertsch, Two-Dimensional Gas Chromatography. Concepts, Instrumentation, and Applications – Part 1: Fundamentals, J. High Resol. Chromatogr. 22 (1999) 647–665