A Chemometric Approach for Ambient Air Monitoring Using Thermal Desorption GC/MS

Significance of the topic

Monitoring volatile organic compounds (VOCs) and semivolatile organic compounds (SVOCs) in ambient air is critical for assessing air quality, understanding pollution sources, and protecting public health. The Bureau of Indian Standards methods IS 5182-27 and IS 5182-28 introduce standardized procedures for passive (diffusive) and active (pumped) sampling of a wide range of organic pollutants. By coupling these sampling techniques with thermal desorption gas chromatography–mass spectrometry (TD GC/MS) and chemometric analysis, analysts can achieve sensitive, reproducible, and comprehensive characterization of hazardous air toxics.

Objectives and overview

This application note aims to demonstrate a TD GC/MS method compatible with BIS regulations for VOC monitoring, using an Agilent 8890 GC system with 5977C GC/MSD and Markes International TD100-xr thermal desorber. It compares passive and active sampling at industrial, residential, traffic, and petrol station sites. Chemometric tools, including principal component analysis (PCA) and hierarchical clustering, are applied to reveal differences among sampling locations and durations. Quantitative analysis of BTEX compounds is also presented.

Methodology and instrumentation

Sampling approaches:

• Passive sampling (IS 5182-27): Sorbent tubes with diffusion caps exposed for 24 hours, one week, or two weeks.
• Active sampling (IS 5182-28): Multibed tubes with pumped flow at 20 mL/min for one hour.

Chemical analysis:

• Thermal desorption: Markes TD100-xr two-stage desorption (tube desorb at 300 °C, trap focusing at –30 °C, backflush at 300 °C).
• GC/MS: Agilent 8890 GC with J&W HP-5ms column (60 m × 0.25 mm × 0.25 µm), helium carrier gas, oven ramp from 40 °C to 300 °C, Agilent 5977C MSD scanning 33–500 amu.
• Data processing: Agilent Mass Profiler Professional software; compound identification via NIST library matching.

Main results and discussion

Active sampling revealed over 70 VOCs across four locations. Traffic and petrol station sites exhibited the highest number and intensities of compounds such as benzene, toluene, ethyl benzene, and xylenes. Chemometric clustering and PCA differentiated site-specific VOC profiles and identified unique compounds at each location. Passive sampling at an industrial site showed that one-week exposure yielded sufficient analyte uptake compared to 24 h and two-week durations. PCA confirmed consistent replication for one-week samples. BTEX quantitation using sorbent tube calibration (20–400 ng) achieved R2 > 0.99. Concentrations ranged from 4–19 ng/L in active samples and 12–80 ng/L in passive samples.

Benefits and practical applications

• Standardized sampling in compliance with BIS ensures comparability across locations and time.
• TD GC/MS provides high sensitivity, low detection limits, and tube reuse capability.
• Chemometric analysis streamlines large-data interpretation, highlighting spatial and temporal trends.
• BTEX quantitation aids regulatory compliance and health risk assessments.

Future trends and applications

Emerging directions include integration of real-time sensors with TD GC/MS, machine learning for source apportionment, miniaturized sorbent systems for personal exposure studies, and expanded libraries for ultratrace pollutants. Combining high-throughput sampling networks with advanced data analytics will enhance urban air quality management and epidemiological studies.

Conclusion

The combined use of BIS-compliant passive and active sampling, thermal desorption GC/MS, and chemometric tools offers a robust framework for ambient air VOC monitoring. This approach enables sensitive detection, effective data interpretation, and reliable quantitation across diverse environments.

Reference

1. U.S. EPA Compendium Method TO-17 Determination of VOCs in Ambient Air Using Active Sampling onto Sorbent Tubes 1999
2. Mas S. de Juan A. Tauler R. Olivieri AC. Escandar GM. Application of Chemometric Methods to Environmental Analysis of Organic Pollutants A Review Talanta 2010 80(3) 1052–1067
3. Massart DL. Vandeginste B. Buydens L. De Jong S. Lewi P. Smeyers-Verbeke J. Handbook of Chemometrics and Qualimetrics Part A Elsevier 1997
4. U.S. EPA Technical Overview of VOCs https://www.epa.gov/indoor-air-quality-iaq/technical-overview-volatile-organic-compounds accessed 2025-06-09
5. Sharma S. Singhal A. Venkatramanan V. Verma P. Pandey M. Variability in Air Quality Ozone Formation Potential by VOCs and Associated Health Risks for Delhi Inhabitants RSC 2024
6. Bureau of Indian Standards IS 5182 Part 27 2024 Vapour-Phase Organic Chemicals Vinyl Chloride to nC22 Hydrocarbons by Diffusive Sampling
7. Bureau of Indian Standards IS 5182 Part 28 2025 Vapour-Phase Organic Chemicals C3 to nC30 Hydrocarbons by Pumped Sampling