Ozone Precursor Analysis Using a Thermal Desorption- GC System

Ozone Precursor Analysis Using Thermal Desorption–GC System

Topic Significance

Air quality regulations worldwide require precise monitoring of volatile organic compounds (VOCs) that contribute to ground-level ozone formation. Effective measurement of C2–C12 ozone precursors is vital for compliance with Clean Air Act standards in the U.S. and similar directives in Europe. High sensitivity, unattended operation, and rapid throughput are key requirements for field-based analyzers.

Objectives and Study Overview

This white paper presents a PerkinElmer automated system for hourly determination of 54 U.S. EPA and 23 EU target VOCs without using liquid cryogens. Developed in collaboration with the U.S. EPA, the study evaluates system design, methodology, chromatographic performance, precision, and field deployment results.

Methodology and Instrumentation

• Online sampling into a Peltier-cooled multi-sorbent trap (–30 °C) eliminating liquid nitrogen.
• Thermal desorption of analytes into a dual-column GC: a dimethylsiloxane capillary for heavier compounds and an alumina PLOT film for C2–C4 carbonyls.
• Heart-cut device employing pressure-balanced switching to route early fractions to the PLOT column while heavier components continue on the main column.
• Dual flame ionization detectors recording parallel separations, controlled via TotalChrom software and TurboMatrix remote control.
• Nafion membrane dryer removes water and polar interferences prior to trap, maintaining retention-time stability.
• Unattended sequence automates hourly sampling (15 mL/min for 40 min), desorption, GC analysis (48 min), system calibration, and data processing.

Key Results and Discussion

• Chromatographic Resolution: Critical separations achieved for low-level isobutene artifacts, iso-/cyclopentane, methylpentanes, and toluene reference peak.
• Retention-Time Stability: Relative standard deviations below 0.15% on the dimethylsiloxane column (n=15) and below 0.14% on the PLOT column (n=14), even after humidity shifts.
• Precision: Peak-area RSDs under 5% for most analytes at 10 ppb levels, with n-dodecane at 6.5% attributed to condensation effects.
• Comparison to Reference Methods: Strong correlation with EPA subambient GC data and legacy ATD-400 systems, confirming analytical equivalence without cryogen.
• Field Performance: Multiyear benzene and 1,3-butadiene trends measured roadside in London; a transient 1,3-butadiene release tracked at a UK port demonstrates real-time response.

Benefits and Practical Applications

• Cryogen-free operation simplifies logistics and lowers cost for remote sites.
• Fully automated, unattended hourly sampling and analysis support regulatory monitoring networks (PAMS, NAAQS).
• Dual-column, parallel chromatography enhances throughput and resolution without subambient ovens.
• Integrated data handling and remote control capabilities ensure continuous operation and rapid data delivery.

Future Trends and Applications

The growing focus on smaller VOCs, greenhouse gas co-monitoring, and real-time source apportionment will drive further enhancements in field analyzers. Integration with meteorological data, mobile platforms, and low-power designs will expand deployment in emerging markets and urban sensor networks.

Conclusion

The PerkinElmer online thermal desorption GC system provides reliable, high-precision analysis of ozone precursors in ambient air without cryogens. Its automated operation, robust chromatographic performance, and proven field record meet stringent regulatory requirements and support ongoing air quality management.

Reference

Technical Assistance Document for Sampling and Analysis of Ozone Precursors, EPA/600-R-98/161, U.S. EPA, 1998.