The analysis concentrates and focuses the chemicals of interest on sorbents before injection into the GC. For air, gas and direct emissions measurements from materials, this occurs in a single stage process, however volatile chemical may also be sampled and analysed in a two-stage process by sampling air or vapours onto solid sorbents and subsequently desorbing and reconcentrating them on the focusing trap in the GC.
The US EPA Method TO-17 method is based on sampling of volatile organic compounds in air using a solid sorbent packed into a tube. The tube is then analysed in the laboratory, where the compounds are introduced into a GC via thermal desorption and re-concentrated on the instrument focusing trap, before chromatographic identification and quantification by mass spectrometry (GC-MS). Note that the analytical process involved in the canister sampling and analysis Method TO-15 is identical, except that TO-15 is a single stage TD method, with the air sample being passed directly onto the focusing trap of the analytical system, then thermally desorbed into the GC.
Key Australian Environmental guidance documents and legislation such as the 2011 Air Toxics NEPM and the 2013 NEPM for the Assessment of Contaminated Sites recommend these methods for ambient air and soil vapour. Whilst the Method TO-15 is the recommended and most widely accepted method in Australia, Method TO-17 can be used as a screening or alternative method and may be the preferred option for many projects.
Method and LOR information
ALS METHOD CODES EP102 (See Analysis suites for analytes)
- LIMIT OF REPORTING (LOR) 0.5ppbv
- METHOD REFERENCE: US EPA TO-17
Thermal desorption applications
While the original EPA TO-17 method was intended for ambient air applications, TD analysis is also commonly used for many other kinds of air and vapour samples, including soil vapour assessment, occupational hygiene monitoring, product emission testing and biological emission and human breath analysis. The canister approach has historically been more prominent in US and Australia Environmental guidelines, however in 2015 the US EPA issued Method 325, which is a TD application for petroleum refinery perimeter monitoring for benzene, indicating a growing international acceptance of the analytical approach. Increasing use and acceptance of passive TD samplers and techniques has also promoted acceptance of TD methods, providing lower cost screening and geographical profiling of contaminated sites and pollution plumes. The thermal desorption analytical approach is also used in NIOSH and UK HSE Occupational Hygiene monitoring methods, offering far more sensitivity than comparable solvent desorption methods and generally at a lower cost than equivalent canister analyses.
Advantages and limitations of TD analysis
TD tubes and passive samplers are seen as a compact, light-weight alternative to canisters, with a broad range of sorbents available for specific target compounds. The selection of specific sorbents and sampling setups allows sampling of a wide analyte volatility range and sample matrices, including high humidity samples across broad concentration ranges. Extensive literature is available on both sampling and analysis, for environmental, workplace and industrial applications. TD offers limits of reporting that are typically 3 orders of magnitude lower than comparable solvent desorption based sorbent methods.
The technique has the potential to be applied to the more volatile SVOCs as well as VOCs in air. It is generally compatible with all organics less volatile than ethane and more volatile than n-C20. The sensitivity of the method can also be enhanced by sampling larger volumes of air or extending passive sampling times, within the breakthrough and exposure limitations of the selected samplers.
To calculate concentrations in air for target chemicals, reliable sampling data is required, which may include the volume sampled or the sampling period and flow or uptake rates, as well as temperature and atmospheric pressure. Measurement of these parameters necessitates additional quality controls which must be addressed in sampling procedures.
Thermal desorption methods are not suitable for particulates, highly reactive compounds, oxides of carbon, nitrogen and sulfur, ozone, methane and other permanent gases. Filter, canister, chemical or direct testing methods remain the preferred option for analysis of these contaminants.
Repeated analysis and dilution for tube samples
Historically, one of the key limitations of TD tube analysis methods has been that the process was limited to a single instrument injection from each tube. ALS methods and instrumentation now allow the splitting and re-analysis of samples, allowing either dilutions of high concentration chemicals or repeated analysis under modified instrument conditions or for different target chemicals. Re-analysis is quantitative and may be performed for any target chemical that is not impacted by residual artefacts from the sorbents used in the TD tube or sampler. Where analytical artefacts interfere with re-analysis, semi-quantitative, qualified results may still be reported. For adsorbent cartridges not provided by ALS, it is recommended that blank (unused) adsorbent cartridges be submitted to assess the potential for interferences from artefacts.
Active sampling for TD analysis involves drawing a volume of air or gas through a tube packed with specific sorbents known to be suitable for the collection of the chemicals of concern. These sorbents may be used singly or in multi-sorbent packings. Tubes with more than one sorbent, packed in order of increasing sorbent strength are used to facilitate quantitative retention and desorption of VOCs over a wide volatility range. The higher molecular weight compounds are retained on the front, least retentive sorbent; the more volatile compounds are retained farther into the packing on a stronger adsorbent. The higher molecular weight compounds never encounter the stronger adsorbents, thereby improving the efficiency of the thermal desorption process.
The volume of air sampled can be varied to increase sensitivity or to prevent overloading of the sorbents. For active sampling of ambient and indoor air, flow rates may be as low as 20 ml/min, depending on the expected concentration of contaminants and the application. For odour investigations and when sampling industrial facilities, lower flow rates or shorter sampling periods are recommended relative to ambient or indoor air.
Distributed volume pairs
For the active sampling of ambient air onto sorbent tubes, the US EPA TO-17 method requires and ALS recommends (at least initially) the sampling of distributed volume pairs. The Distributed Volume Pair are two samples taken in parallel over the same period at different flow rates, resulting in different volumes being sampled and different concentrations of contaminants being absorbed on each tube. Comparison of the results of analysis for these tubes allows an assessment of precision, which can aid quality assessment and assessment of artefact formation, degradation on the sorbent, breakthrough or any other factor that is non-linear with sample volume.
Passive samples are collected by exposing a sorbent to the environment being sampled and allow passive diffusion of contaminants onto the sorbent. The sorbent is contained in a specific sampling cartridge, tube or other sampler body which determines the ‘uptake rate’ or rate of diffusion from the environment being sampled onto the sorbent. The uptake rate, combined with sampling data including the exposure period, temperature and pressure, can be used to convert the amount of contaminant absorbed into a concentration in the environment sampled. Passive sampling is a simple, powerful, relatively low cost method which is gaining increasing acceptance as both a screening tool for contaminated site characterisation and spatial mapping and as a non-invasive monitoring technique for Occupational Hygiene assessments.
The design of the Passive Sampler is generally targeted to the specific application for which it is intended, with slow uptake rates used in static/equilibrium conditions and faster uptake rates more suitable for time averaged measurements (eg ambient air) and shorter exposure periods. Radial sampling bodies have larger surface areas and faster uptake rates, whilst axial samplers have less surface exposed and therefore have slower uptake rates.
Uptake rates are specific for each target chemical and must be validated and/or provided by the supplier of the sampler.
The raw results of TD analysis is a quantity of each target chemical per sample, generally expressed in nanogram per sample (ng/sample). For active samples, if flow rates and sampling periods or calculated volumes are provided, the laboratory can calculate and report air concentrations at any given temperature and pressure. Similarly, for passive samples where the uptake rates are published or available for the chemicals of interest, concentrations in air may be reported. For all sample types, NATA accreditation only applies to the calculated concentration results if the sampling data (volume, temperature and pressure) is also NATA endorsed.
Certificate of analysis
ALS issues fully NATA endorsed Certificates of Analysis consistent with US EPA TO-17 method requirements. Results are reported in ug/sample and in ppbv or µg/m3 where volume data or uptake rates are available. Calculated concentrations may be NATA endorsed where the sampling is covered by NATA accreditation.
- EPA/625/R-96/010b Compendium Method TO-17: Determination of Volatile Organic Compounds in Ambient Air Using Active Sampling onto Sorbent Tubes http://www.epa.gov/ttn/amtic/files/ambient/airtox/to-17ar.pdf
- ASTM D7758 Standard Practice for Passive Soil Gas Sampling in the Vadose Zone for Source Identification, Spatial Variability Assessment, Monitoring, and Vapor Intrusion Evaluations
- National Environment Protection (Assessment of Site Contamination) Amendment Measure 2013 (No. 1)
- National Environment Protection (Air Toxics) Measure 2011