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Industrial air monitoring – Fenceline

Boundary monitoring of industrial sites

Fenceline monitoring has for many years been used to check the impact of large industrial sites on the wider environment – and continues to receive attention, with the release of US EPA Method 325 scheduled for summer 2015.

Though benzene, toluene, ethylbenzene and the xylenes are perhaps the most widely regulated group of compounds, the concentrations of other hydrocarbons, particularly olefins (alkenes), are also of interest in ambient and industrial air.

These include carcinogens such as butadiene, the so-called ozone precursors, ‘air toxics’ or hazardous air pollutants, and those listed in the UK ‘Protocol for the determination of the speciation of hydrocarbon emissions from oil refineries’.

What Markes can offer


Benzene, toluene, ethylbenzene and the three xylene isomers, collectively known as BTEX, are volatile aromatic hydrocarbons. As petrochemical products, they can enter the environment in a variety of ways, but a major route is through incomplete burning of gasoline (as from vehicle emissions), and spills or leakages at industrial sites.

Being indicative of polluted urban or industrial environments in general, BTEX is used by analysts as a benchmark when assessing new monitoring or analytical methods. The compounds are also of concern individually for their negative health effects, with benzene (a known human carcinogen) being subject to exposure limits in both workplace and ambient air in most industrialised countries.

BTEX emissions from industrial sites have also attracted attention recently, with fenceline monitoring and control of benzene concentrations at US oil refineries being proposed by the US EPA under the ‘residual risk’ program. Similar steps are anticipated in Europe and elsewhere.

Diffusive (passive) sampling using sorbent tubes, with analysis by thermal desorption (for example, using Markes’ UNITY-xr or TD100-xr thermal desorbers), has long been popular for fenceline monitoring. With no need for a pump or other bulky apparatus, passive samplers are convenient to use, low-cost and unobtrusive.

They are inherently suitable to fenceline sampling over an extended period (typically 1–4 weeks), and the abundance of data on uptake rates makes it straightforward to generate reliable concentration data.

Such an approach forms the basis of Markes' solution package for US EPA Method 325, for monitoring of benzene and other VOCs at refinery perimeters. The package comprises:

  • A 325 Field Station to protect the passive sampling tubes from the elements.
  • A 325 Container for secure transport of tubes to and from the sampling location
  • TubeTAG RFID tagging of the tubes to ensure robust chain-of-custody between the field and the laboratory
  • A TD100-xr automated thermal desorber for analysis of multiple tubes levels in a single sequence
  • A TC-20 multi-tube conditioning unit to improve laboratory productivity and reduce costs.

When sampling over shorter time periods is desirable (e.g. for monitoring discrete changes, or for sampling during process operation), sequential pumped-tube samplers such as Markes’ MTS-32 are more suitable.  

Further information

  • For an overview of options for the monitoring of benzene and other hydrocarbons in air by TD, see Application Note 049.
  • For a stepwise approach to complying with US EPA Method 325 for monitoring VOCs at refinery perimeters, see Application Note ​114.


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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