Ozone precursors, odorous compounds and halogenated greenhouse gases
Monitoring ultra-volatile compounds is an important field of environmental monitoring, as it encompasses several key classes of compounds, including:
- Ozone precursors – Ranging in volatility from acetylene (ethyne) to trimethylbenzene, ozone precursors are believed to contribute to the formation of ground-level ozone, one of the main constituents of urban smog. Vehicle emissions are thought to be the main source of these compounds, and recent regulations require round-the-clock monitoring of key compounds to establish and monitor the link between pollution levels, traffic density, industrial emissions, and weather conditions.
- Odorous compounds – A number of ultra-volatiles, and sulfur compounds in particular, are associated with unpleasant, pungent odours, which can be noticeable even at low concentrations. These compounds have historically been difficult to analyse, as they are ‘thermally labile’ (sensitive to high temperatures), particularly when they come into contact with metal surfaces. The detection of odorous compounds at trace levels is critically important for applications including industrial emissions testing, and the monitoring of odours from sewage treatment plants and landfill sites.
- Halogenated greenhouse gases – In response to the Kyoto Protocol, regulations are being enacted that require the monitoring of greenhouse gases, including trace-level halogenated compounds. Some of these compounds are on the standard US EPA list of ‘air toxics’, but other compounds (some with high global warming potential) are not. These can present an analytical challenge due to their extreme volatility, and include carbon tetrafluoride, hexafluoroethane, chlorotrifluoromethane and sulfur hexafluoride.
What Markes can offer
Monitoring of ultra-volatiles typically requires on-line sampling, as most of them are too volatile to be retained on sorbent tubes at ambient temperature. Indeed, for certain ultra-volatiles thermal desorption is often the only way to provide the high degree of analyte concentration needed to address the low detection limits needed.
Many on-line thermal desorption systems are deployed remotely, or in mobile laboratories (e.g. for environmental emergency response), and use either:
- A single focusing trap, such as the UNITY–Air Server-xr, which samples air for a defined period of time followed by analysis.
- Two focusing traps working alternately, for continuous sampling. Systems such as the TT24-7 use wider-bore focusing traps to allow high sample loadings in short periods of time.
In either case, the use of inert flow paths is vital to ensure compatibility with highly labile analytes such as sulfur compounds.
- For more information on the application of thermal desorption to...
...ozone precursors, see Application Note 016.
...odorous sulfur compounds, see Application Note 032.
...perfluorinated greenhouse gases, see Application Note 087.
- For a full listing of all relevant standard methods, see Application Note 003.
- For an overview of the benefits of thermal desorption and time-of-flight mass spectrometry for a range of air monitoring situations, see: N. Watson, S. Davies and D. Wevill, Air monitoring: New advances in sampling and detection, The Scientific World Journal, 2011, 11: 2582–2598.
- For an example of the use of the Micro-Chamber/Thermal Extractor and the TD-100 automated thermal desorber for the detection of highly volatile compounds emitted from building materials and other products, see: V.M. Brown and D.R. Crump, An investigation into the performance of a multi-sorbent sampling tube for the measurement of VVOC and VOC emissions from products used indoors, Analytical Methods, 2013, 5: 2746–2756.
- For a description of the use of the UNITY–Air Server with GC–FID for analysing urban air for 18 oxygenated VOCs, see: J. Roukos, H. Plaisance, T. Leonardis, M. Bates and N. Locoge, Development and validation of an automated monitoring system for oxygenated volatile organic compounds and nitrile compounds in ambient air, Journal of Chromatography A, 2009, 1216: 8642–8651.
- For guidance on sampling for ozone precursors from the US EPA, see: Technical Assistance Document for Sampling and Analysis of Ozone Precursors (EPA/600-R-98/161).