Semi-volatiles and persistent organic pollutants
Semi-volatile organic compounds (SVOCs) have not historically been regarded as being relevant to ambient air monitoring.
However, there has been growing realisation that certain groups of compounds familiar as components of emissions from materials and indoor air are able to persist in the wider environment.
These compounds, often referred to as persistent organic pollutants (POPs), are normally dispersed in soil and water, but they can be detected in the air even in supposedly ‘pristine’ environments such as the polar regions.
They are therefore the focus of research, because they provide an insight into the physical and chemical mechanisms at play in their global dispersal, and their possible negative influences upon ecosystems.
Particular SVOCs of concern include:
- Polycyclic aromatic hydrocarbons (PAHs)
These are currently of high relevance because of their significant contribution to environmental pollution and because of their known or suspected carcinogenicity. PAHs are mainly anthropogenic in origin, with sources including the burning of coal, wood, oil and gas for residential heating and power generation, industrial processes, municipal incineration of waste, forest fires and traffic. However, natural processes also make a contribution to their presence in the environment.
|What are SVOCs? || |
SVOCs have a number of definitions, typically based on their boiling point, vapour pressure or photochemical reactivity.
However, a generally accepted definition is that SVOCs are composed mainly of hydrogen and carbon, have boiling points greater than 250°C, and have vapour pressures less than 0.1 mmHg at standard temperature and pressure.
As well as being present in the vapour phase, SVOCs are adsorbed to a significant degree onto airborne particles.
This means that analysts involved in pollution monitoring take both VOCs and particulate sources of SVOCs into account.
- Polychlorinated biphenyls (PCBs)
These were once widely used as dielectrics and coolants, but due to their toxicity, production was banned in the US in 1979, and under the Stockholm Convention on Persistent Organic Pollutants in 2001. However, their persistence in the environment means that they remain of concern.
- Phthalates (diesters of phthalic acid)
These have been widely used since the 1950s to soften plastics, but are now suspected of being endocrine disruptors, with the potential to result in a variety of knock-on health effects. There is therefore mounting pressure to monitor and control releases of phthalates into the environment.
Other semi-volatile compounds readily monitored by thermal desorption include high-boiling hydrocarbons (up to about n-C40), and certain pesticides.
What Markes can offer
Thermal desorption (TD) is usually associated with the analysis of volatile organic chemicals (VOCs), but can also be very successfully applied to semi-volatile organic compounds (SVOCs).
PAHs, PCBs and pesticides are often monitored using high-volume sampling on to polymer- or resin-based samplers, but the sample preparation is fairly laborious.
Sampling onto sorbent tubes and analysis by thermal desorption is an appealing alternative, as SVOCs are well-retained on even the weakest sorbents. During analysis, however, some thermal desorbers can struggle to achieve complete desorption of the least volatile compounds. Markes’ systems, such as the UNITY-xr and TD100-xr thermal desorbers, overcome this by using a short, inert, heated flow path to achieve quantitative recovery of compounds up to n-C40H82, six-ring PAHs, and didecyl phthalate.
- For more information on monitoring semi-volatiles, see Application Note 053.
- For an example of the detection of PAHs in vehicle exhaust, see Application Note 097.
- For example of the use of Markes’ UNITY–Air Server with GC–MS to measure airborne 4–6-ring PAHs collected onto quartz filters, see: M. Bates, P. Bruno, M. Caputi, M. Caselli, G. de Gennaro and M. Tutino, Analysis of polycyclic aromatic hydrocarbons (PAHs) in airborne particles by direct sample introduction thermal desorption GC/MS, Atmospheric Environment, 2008, 42: 6144–6151.