The accurate measurement of odorant gases is key in supporting the role of local authorities in improving air quality. Although not necessarily harmful in nature, the release of odorant gases from landfill sites can cause alarm for local residents – accurate and rapid measurement is essential to address these concerns, and to allow any necessary preventative action to be instigated.

Odour measurements also support legislation – such as the Landfill (England and Wales) Regulations 2002 – which implements the European Landfill Directive 1999/31/EC. This aims to prevent, or to reduce as far as possible, the negative environmental effects of landfill.

The Environment Agency and the Scottish Environmental Protection Agency have identified 25 priority trace components of landfill gas for which measurement is required. Of these 25 components, 12 are categorised as having an impact on odour (see table, p19).

The National Physical Laboratory (NPL) is the UK’s national standards laboratory and a centre for the development and application of highly accurate measurement techniques. NPL’s analytical science team has built up a capability for gas measurements in a wide range of applications, including ambient air quality, stack emissions, vehicle emissions, volatile organic compounds and natural gas.

Setting standards

To support the measurement of odorant species, NPL provides a range of high-accuracy gas standards. These gas mixtures are traceable to national standards and are prepared gravimetrically in appropriately passivated cylinders.

They are ideally suited for odour-monitoring companies which require a reliable, accurate calibration for their analytical instruments.

Sampling for odorant species can be carried out on a routine basis, or when an unexpected or unusual smell is detected. Samples are most simply taken as grab samples, using a suitably evacuated vessel, such as a cylinder or canister, or a Tedlar bag. Grab samples enable a snapshot of the smell to be taken, before being returned to the laboratory for analysis.

An alternative sampling method is to use automated thermal desorption (ATD) tubes – these contain sorbent materials appropriate to trap the species of interest. Gas is drawn through the tubes by use of a pump and the sorbent

materials selectively extract and concentrate the target analytes, allowing others to pass through. The tubes are again returned to the laboratory for analysis.

Odour samples are analysed at NPL by either GC-MS (gas chromatography – mass spectrometry) or GC (gas chromatography), the former having the benefit of being able to identify unknown species.

Grab samples in a bag, cylinder or canister are analysed either by slightly pressurising the sample, or by pumping through the GC-MS or GC. If, as is often the case, very low levels of the species are detected, an additional pre-concentration step may be required. Here, the sample is focused on to a cold trap, where it is cooled with liquid nitrogen before analysis.

Samples collected on ATD tubes are analysed in a similar manner – the analytes are purged off the tubes by heating and flushing with dry nitrogen, before concentration (if required) and analysis.

Study into sampling

NPL carried out research to determine the most suitable method for taking grab samples. The study showed that samples captured in bags were more prone to decay than those stored in passivated cylinders or canisters. This decay is caused by the species reacting, decomposing, or being adsorbed to the surface of sampling bags. This is of vital importance for those odorant species that are reactive in nature – false results may be obtained if the sample has decayed in the time period between sampling and analysis.

To accompany this, a new system has been developed to reduce any losses of the sample when transferring to the GC-MS or GC. The system has three components: a cylinder valve; 15.9mm diameter Silcosteel or Sulfinert treated stainless steel pipework; and a flow restriction valve. Routine use of the new set-up at NPL has allowed the analysis of odorant gases to be carried out much more accurately and reliably.

To support UK odour monitoring laboratories, NPL has recently held the first round of an odour proficiency testing scheme. Participating laboratories analysed six samples, including three synthetic landfill mixtures, and the data from the scheme enabled the participants to assess their capability to carry out accurate odour measurements.

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