Ambient air monitoring – ‘Air toxics’
Hazardous air pollutants
Volatile (vapour-phase) organic ‘air toxics’, also known as ‘hazardous air pollutants’ (HAPs), are monitored in many industrial and urban environments as a measure of air quality. They range in volatility from methyl chloride and propene to hexachlorobutadiene and the trichlorobenzenes, and include polar as well as non-polar compounds.
The sampling method chosen depends on the volatility and polarity range of the VOCs of interest, with sorbent tubes and canisters being two popular and well-validated methods. In either case, the vapours need to be concentrated prior to analysis by gas chromatography (GC).
What Markes can offer
In the last couple of decades, manufacturers of analytical equipment have responded to the increasing demand for measurement of air toxics. Markes has led the way in innovation in this field, offering sampling equipment for both tube users and canister users, as well as a TOF mass spectrometer suited to the specific challenges posed by air toxics.
One benefit of using sorbent tubes for monitoring air toxics is the ability to vary the sorbents according to the compounds needing to be monitored, with pumped sampling onto multi-bed tubes allowing the widest range of compounds to be retained and released. For example, retention volumes at 25°C for lightest components (propene and methyl chloride) in the TO-17 list are >2 L on ‘Air Toxics’ (ATA) tubes, and >1 L on ‘Universal’ tubes. For more information on the selection of sorbents, see Application Note 005.
|Tubes or canisters? || |
There are inherent advantages and disadvantages to using either tubes or canisters to measure air toxics, and deciding which to use can involve consideration of a number of factors, including volatility range and expected concentration, as well as reasons of historical investment.
For more advice about whether to monitor using tubes and canisters, contact our Environmental Applications Specialists, or see Application Note 079.
Markes’ tube-based UNITY-xr and TD100-xr thermal desorbers comply with the requirements of US EPA Method TO-17 by accommodating internal standard addition for precise quantitation. Also important is the ability to split sample flows, and collect the excess onto another sample tube, making possible repeat analysis by another GC method, or validation of recovery as required by standard methods, such as ASTM D6196-3.
Canister and on-line sampling
Air toxics and related compounds can also be monitored using canister sampling, which provides the simplest form of ‘grab’ sampling. While the regular-sized 6 L canisters remain widely used, small canisters (about 400 mL) have been growing in popularity for grab-sampling high-concentration volatiles (with vapour pressures greater than those of n-nonane).
However, pre-concentration/trapping is still required before analysis to selectively eliminate the bulk constituents of air, especially oxygen, which would otherwise adversely affect the performance of the GC column and detector. In addition, sample volumes must be minimised to prevent overload and/or contamination of the analytical system. Transfer of small volumes of air is usually done with gas sample loops (as on the CIA Advantage), in order to achieve quantitative transfer without introducing uncertainty.
For monitoring of trace-level compounds, a 6 L canister is typically used to collect the sample, with a large volume (1 L) being introduced to the analytical instrument in order to achieve good limits of detection.
Time-of-flight mass spectrometers for GC
Historically, the detection of very low-level compounds in air (<1 ppb) was possible using compound-specific detectors – such as flame photometric detectors (FPDs) for sulfur-containing compounds. Quadrupole mass spectrometers, when used in selected ion monitoring (SIM) mode with the latest thermal desorption (TD) trapping technology, can also provide very low detection levels.
However, in both these cases, compound identification relies on a limited number of characteristic ions and stable retention times. In this mode, to improve sensitivity, the vast majority of the spectral data is lost, so full characterisation of the sample is rarely possible in a single analysis. Time-of-flight (TOF) MS detectors for GC, such as BenchTOF, overcome this limitation by monitoring all ions simultaneously across the mass range, making them significantly more sensitive than scanning instruments.
- For more information on monitoring air toxics...
...using canisters according to US EPA Method TO-15, see Application Notes 081 and 099.
...using sorbent tubes according to US EPA Method TO-17, see Application Note 086.
- Other national and international standard methods for air toxics and related applications include ISO 16017 (Parts 1 and 2), ASTM D6196-03 and ASTM D5466-01 – see Application Note 003 for a full listing.
- The US EPA provides a short Q&A on air toxics and further resources on pollution monitoring.
- 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 a paper comparing the analysis of low loadings of 108 volatile compounds in ambient air by canisters and by direct sorbent preconcentration, see: E.H. Daughtrey, K.D. Oliver, J.R. Adams, K.G. Kronmiller, W.A. Lonneman and W.A. McClenny, A comparison of sampling and analysis methods for low-ppbC levels of volatile organic compounds in ambient air, Journal of Environmental Monitoring, 2001, 3: 166–174.
- For a recent example of a analysis of 47 VOCs in urban air according to US EPA Method TO-17 using a UNITY-ULTRA automated thermal desorption system, see: G.K.S. Wong, S.J. Ng and R.D. Webster, Quantitative analysis of atmospheric volatile organic pollutants by thermal desorption gas chromatography mass spectrometry, Analytical Methods, 2013, 5: 219–230.