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Ambient air monitoring – ‘Air toxics’

Hazardous air pollutantsHazardous 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.

Tube sampling

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.

As for sorbent tube samples, humidity in canister and on-line air streams needs to be minimised in order to ensure good chromatography and reliable responses across the full analyte range, but this can be difficult with many analytical systems. Markes' new Kori-xr module deals with this challenge using a simple but highly efficient cold trap upstream of the analyte pre-concentrator, which condenses out moisture efficiently, even from air with 100% relative humidity. A full assessment of a CIA Advantage setup with Kori-xr for TO-15-compliant canister monitoring can be found in Application Note 081.

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.

TO-15 and TO-17

A number of methods have been published by the US Environmental Protection Agency (US EPA) for the monitoring of VOCs and SVOCs. The two most popular of these, TO-15 and TO-17, relate to the analysis of VOCs by GC, and are now used as benchmark methods for VOC sampling by analysts around the world:

TO-17-type methods can be applied to ambient indoor and outdoor air samples, and facilitate simultaneous analysis of a wide range of polar and non-polar compounds, including very volatile, volatile and semi-volatile organic compounds.

Further information

Markes International Ltd
Gwaun Elai Medi-Science Campus
Tel: +44 (0)1443 230935
Markes International GmbH
Bieberer Straße 1-7
63065 Offenbach am Main
Tel: +49 (0)69 6681089-10
USA (West)
Markes International, Inc.
2355 Gold Meadow Way
Gold River, Sacramento
California 95670
Tel: +1 866-483-5684 (toll-free)
Markes Instruments (Shanghai) Co., Ltd
Unit 1002, Building 1
No.418, Guilin Road
Shanghai 200233
P.R. China
Tel: +86 21 5465 1216
Part of Schauenburg International

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