National Physical Laboratory

Volatile organic compounds (VOCs)

World Meteorological Organisation Primary Standard

Global Atmosphere Watch logo

NPL is proud to announce that the 30 component EU Directive ozone precursor gas standard is now recognised by the World Meteorological Organisation (WMO) as their primary standard.

These standards are used as primary standards for the assessment of global ambient levels of VOCs. This is part of the Global Atmospheric Watch (GAW) international global programme that provides the technical basis for the assessment of the chemical composition of the atmosphere.

Volatile organic compounds (VOCs) play a key role in the chemical mechanisms that lead to the photochemical generation of ozone and control the oxidation capacity of the troposphere. NPL offers the following families of VOC mixtures:

30-component ozone precursor mixtures

Ozone precursors such as non-methane VOCs contribute to the formation of ground level ozone, which can have harmful effects on ecosystems and human health. The need to prevent or reduce these effects on the public and the environment has led to requirements for stable and accurate gas mixtures to facilitate the traceable calibration of analytical instrumentation.

NPL provides 30-component ozone precursor standards that contain all the hydrocarbon compounds listed in the European Directive covering ambient ozone measurements (Directive 2002/3/EC). These components are listed below. Mixtures containing these components are typically prepared at amount fractions of 4 nmol/mol, but other amount fractions from 1 to 100 nmol/mol are also available.

Ethane 1,3-butadiene 2,2,4-trimethylpentane
Ethene Pentane Benzene
Ethyne 2-methylbutane Toluene
Propane 1-pentene Ethylbenzene
Propene trans-2-pentene m-xylene
Butane 2-methyl-1,3-butadiene (isoprene) p-xylene
2-methylpropane Hexane o-xylene
1-butene 2-methylpentane 1,2,3-trimethylbenzene
trans-2-butene Heptane 1,2,4-trimethylbenzene
cis-2-butene Octane 1,3,5-trimethylbenzene

Since 2005, NPL has sold O3P Primary Reference Gas Mixtures (PRGMs) to monitoring stations, research institutes, NMIs, etc.

The O3P hierarchy formed the basis for the EURAMET 886 comparison exercise. The full report of the EURAMET 886 exercise is downloadable from the NPL website via this link: AS29, where extensive data on the stability of these mixtures can be found. This work has also recently been published: R J P Grenfell, M J T Milton, A M Harling, G M Vargha, C Brookes, P G Quincey and P T Woods, Standard mixtures of ambient voltaile organic compounds in synthetic and whole air with stable reference values, J Geophys Res, 115, D14302, 2010, doi:10.1029/2009JD012933.

Since 2008, NPL has been updating the ozone precursor hierarchy. The O3P MkII formed the basis of PRGMs disseminated from May 2009 to April 2012. Since then, the recently prepared MkIII hierarchy underpins all PRGMs disseminated. New technologies applied to the preparation of these multi-component VOC mixtures have led to improvements in the introduction of low vapour pressure, high molecular weight compounds. In the current hierarchy, gravimetric dilution steps have been refined with an expanded number of parent mixtures containing fewer components. This has led to improved measurements to validate the stability of the mixtures prepared.

These standard mixtures provide the basis for traceability for the following compounds in the GAW VOC network: ethane, propane, n-butane, n-pentane, acetylene, toluene, benzene and isoprene.

BTEX mixtures

Benzene, toluene, ethylbenzene and xylenes are volatile mono-aromatic hydrocarbons that are key pollutants resulting from combustion processes. BTEX components are regularly monitored in surveys of indoor air quality as they are common constituents of products such as paints and adhesives. In an industrial environment, regulation of exposure to BTEX components is controlled through the application of occupational exposure limits and COSHH regulations.

NPL provides a range of stable and accurate BTEX gas standards, which contain benzene, toluene, ethylbenzene, m-xylene, p-xylene and o-xylene at amount fractions from 1 to 10 nmol/mol. Mixtures containing these compounds and 1,3-butadiene are also available.

Bespoke VOC and halogenated hydrocarbon mixtures

NPL can also provide bespoke gas standards containing C2 to C10 hydrocarbons, chlorinated hydrocarbons and fluorinated hydrocarbons at a wide range of amount fractions.

Deuterated toluene standards

Gas standards of d8-toluene are often used as internal standards in GC-MS applications. NPL provides mixtures of d8-toluene at a range of amount fractions, most typically 5 µmol/mol.

Download User Guide for NPL VOC Standards Adobe Acrobat PDF file (65 KB)


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Last Updated: 19 Feb 2015
Created: 23 Sep 2013


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