Gas standards and measurements

Gas standards and measurements Details of the traceable, high-accuracy gas standards and measurement solutions provided by the National Physical Laboratory

National Physical Laboratory Hampton Road Teddington Middlesex United Kingdom TW11 0LW Switchboard: 020 8977 3222 Web: www.npl.co.uk

Contents General information... 4-5 NPL gas standards... 4 Accreditation, international comparability and certificates... 5 Energy gases...6-11 Natural gas... 6 Odorants... 7 Refinery gas... 8 Liquid hydrocarbon standards... 9 Hydrogen purity...10 Biogas...11 GENERAL INFORMATION Environmental gases...12-17 Volatile organic compounds (VOCs)... 12-13 Oxygenated VOCs and semi-vocs...14 Terpenes...15 Gas standards for atmospheric monitoring...16 Gas standards for air quality monitoring...17 Industrial and emission gases...18-23 Gas standards for industrial emissions monitoring...18 Gas standards for vehicle emission measurements...19 Breath alcohol and interfering substances...20 Water vapour...21 Measurement of water vapour transmission rate...22 Noble gases...23 High purity gases... 24 Purity analysis...24 Other products and services...25-26 Calibration of ozone photometers...25 Gas metrology software...25 Proficiency testing schemes...26 3

GENERAL INFORMATION NPL gas standards NPL provides the following classes of gas standards: Primary Reference Gas Mixtures (PRGMs) PRGMs are provided to those customers requiring the lowest available uncertainties. These mixtures are prepared gravimetrically in accordance with ISO 6142 and are validated against in-house Primary Standard Gas Mixtures (PSMs). The certificate provided with a PRGM reports the gravimetric value with an uncertainty that represents the best achievable analytical uncertainty. It also contains an expiry date for the mixture, which is assigned from data obtained from stability studies of similar mixtures. Calibrated Gas Mixtures (CGMs) CGMs are mixtures that have been supplied by customers and individually certified against NPL s PSMs. The certificate provided with a CGM reports the certified value determined by the analytical method used, and an analytical uncertainty (which is typically larger than that provided for a PRGM). A stability guarantee is not normally given for a CGM due to a lack of knowledge of the history of the cylinder and the gas mixture. Secondary Gas Standards (SGSs) NPL also offers a limited range of SGSs. These mixtures, which typically contain reactive and unstable components, are prepared gravimetrically, and are provided with a certificate that reports the value obtained from the analytical certification process with an appropriate measurement uncertainty. 4

0002 0478 4002 0002 0478 4002 Accreditation, international comparability and certificates ISO Guide 34 and ISO 17025 accreditation NPL has an extensive scope of accreditation to ISO Guide 34 (Reference Materials) and ISO 17025 (Calibration) awarded by UKAS (the 0002 0478 4002 United Kingdom Accreditation Service). International comparisons NPL frequently participates in international comparison exercises with other National Measurement Institutes (NMIs). These comparisons, which are organised by the Consultative Committee for Amount of Substance (CCQM) of the International Committee for Weights and Measures (CIPM), provide a robust framework to demonstrate the global comparability of gas standard mixtures and measurements. Calibration and Measurement Capabilities Results of the comparisons operated by the CIPM CCQM are used to compile internationally-recognised Calibration and Measurement Capabilities (CMCs). A table of NPL s CMCs for gas analysis can be downloaded from: http://kcdb.bipm.org/appendixc/qm/gb/qm_gb_4.pdf Further information regarding international comparisons and CMCs can be found from the BIPM Key Comparison Database: http://kcdb.bipm.org CIPM Mutual Recognition Arrangement Certificates for gas mixtures covered by NPL s CMCs display the logo of the Mutual Recognition Arrangement (MRA) of the CIPM. Certificates containing this logo are mutually recognised by all NMIs who are signatories to the MRA. Units 0002 0478 4002 0002 0478 4002 0002 0478 4002 0002 0478 4002 ISO Guide 34 ISO 17025 0002 0478 4002 CIPM MRA The composition of gases on NPL certificates are stated in units of amount-of-substance fraction (mol/mol) or decimal submultiples thereof. Certificates may on occasion also contain other units if requested by the customer. The most common decimal submultiples of mol/mol used on NPL certificates are: mmol/mol (equivalent to 10-3 mol/mol) μmol/mol (equivalent to 10-6 mol/mol, often called parts-per-million or ppm) nmol/mol (equivalent to 10-9 mol/mol, often called parts-per-billion or ppb) GENERAL INFORMATION 5

ENERGY GASES Natural gas NPL s natural gas standards are typically used to calibrate gas analysers. This enables the composition of real natural gases to be determined, thus allowing accurate calculation of the calorific value and other physical properties of the gas. In addition to the components usually found in synthetic natural gas standards (C 1 to C 5 alkanes, n-hexane, nitrogen and carbon dioxide), NPL s standards can also contain C 6 to C 10 hydrocarbon components. Accurate measurement of these components is particularly useful when hydrocarbon dew point is to be calculated, reassuring the gas transporter that condensation of the mixture will not take place in the gas pipeline. The range of natural gas standards covered by NPL s UKAS accreditation is shown in the table below. Mixtures containing other components (including other C 6 and C 7 isomers) may be prepared on request. Amount fraction range (cmol/mol) * Amount fraction range (μmol/mol) Methane 55.0 to 99.9 Benzene 5 to 500 Nitrogen 0.02 to 25.2 Toluene 5 to 250 Carbon dioxide 0.04 to 25.0 Cyclohexane 10 to 400 Ethane 0.008 to 18.0 Methylcyclohexane 10 to 400 Propane 0.008 to 8.0 n-heptane 10 to 500 i-butane 0.004 to 1.7 n-octane 5 to 200 n-butane 0.004 to 1.7 n-nonane 1 to 120 neo-pentane 0.0005 to 0.50 n-decane 1 to 20 i-pentane 0.0025 to 0.60 n-pentane 0.0025 to 0.60 n-hexane 0.0008 to 0.50 Helium 0.0014 to 0.50 *1 cmol/mol is equal to 0.01 mol/mol and is equivalent to the unit % mol/mol often used by the natural gas industry. 6

Odorants Sulphur odorants Sulphur-containing compounds are added to natural gas to provide a distinctive odour, thus allowing any potentially hazardous leaks to be detected. The very low odour thresholds of these compounds enable detection by the human nose at minute concentrations. NPL s sulphur odorant standards are available as either binary or multi-component mixtures, in a matrix of methane or nitrogen. The range of standards covered by NPL s UKAS accreditation is shown in the table below. ENERGY GASES Amount fraction range (μmol/mol) Amount fraction range (μmol/mol) Hydrogen sulphide 0.4 to 5000 Methanethiol 0.4 to 200 Carbonyl sulphide 0.4 to 5000 Ethanethiol 0.4 to 200 Carbon disulphide 0.4 to 200 2-propanethiol 0.4 to 200 Dimethyl sulphide 0.4 to 200 1-propanethiol 0.4 to 200 Ethyl methyl sulphide 0.4 to 200 2-methyl-2- propanethiol 0.4 to 200 Diethyl sulphide 0.4 to 200 Tetrahydrothiophene 0.4 to 200 Non-sulphur odorants NPL also supplies a range of non-sulphur odorant mixtures in order to provide traceability for the measurement of components which are increasingly being used to odorise natural gas across Europe. Standards are available containing the components most commonly used as non-sulphur odorants: methylacrylate, ethylacrylate and 2-ethyl-3-methylpyrazine. 7

ENERGY GASES Refinery gas NPL provides high-accuracy gas standards for the analysis of a range of refinery gases. These gases, which include syngas, coke oven gas, reformer gas and blast furnace gas, are produced and emitted during the production and refining of petroleum products and solid fuels. NPL s refinery gas standards are typically used to calibrate gas chromatographs (GCs), enabling the composition of real refinery gases to be determined. This allows the user to calculate the carbon content and other physical parameters of the gas, thus ensuring compliance with carbon trading legislation. The range of refinery gas standards covered by NPL s UKAS accreditation is shown in the table below. Mixtures containing other components may be prepared on request. Amount fraction range (cmol/mol) * Amount fraction range (cmol/mol) * Nitrogen 0.1 to 95 Propene 0.04 to 7 Carbon monoxide 0.1 to 11 i-butane 0.1 to 4 Carbon dioxide 0.3 to 8 n-butane 0.1 to 6 Oxygen 0.2 to 2.5 1-butene 0.015 to 1.55 Hydrogen 1 to 70 i-butene 0.018 to 1.2 Helium 5 to 10 trans-2-butene 0.015 to 0.85 Methane 1 to 85 cis-2-butene 0.015 to 0.35 Ethane 0.3 to 35 1,3-butadiene 0.01 to 0.65 Ethene 0.1 to 12 i-pentane 0.05 to 0.8 Ethyne 0.025 to 2 n-pentane 0.05 to 0.8 Propane 0.1 to 18 * 1 cmol/mol is equal to 0.01 mol/mol and is equivalent to the unit % mol/mol often used by the refinery gas industry. 8

Liquid hydrocarbon standards The quantity of liquefied petroleum gas (LPG) and liquefied natural gas (LNG) being imported into Europe is increasing as natural gas resources decline. Traceable standards for the measurement of liquid hydrocarbon mixtures such as LPG and LNG are required in order to support this important industrial sector these standards can be used to calibrate instruments measuring the composition of LPG and LNG, thus allowing the physical properties of the mixtures to be calculated. NPL s liquid hydrocarbon standards are typically provided in one litre constant pressure (piston) cylinders. These cylinders employ a piston as a physical barrier maintained by an over-pressure of an inert gas, which ensures that the mixture remains in the liquid phase. An internal cylinder mixer enables that the mixtures are homogenous. Examples of some typical components in NPL s liquid hydrocarbon standards are listed below. Other components may be included on request. ENERGY GASES Ethane n-butane cis-2-butene Propane 1-butene 1,3-butadiene Propene i-butene n-pentane i-butane trans-2-butene n-hexane 9

ENERGY GASES Hydrogen purity Fuel cells require high purity hydrogen for operation as even trace levels of impurities such as carbon monoxide and sulphur compounds can lead to the deactivation of fuel cell catalysts. In some cases, these trace levels of impurities cannot be measured by routine analytical techniques. Using a selection of techniques and methods incorporating several GCs and analysers, NPL can measure impurities in hydrogen at parts-per-million or parts-per-billion levels. Hydrogen samples are analysed against specially prepared traceable calibration gas standards, which are also available to customers. As an indication of the measurements required, the hydrogen purity specification given by the international standard ISO 14687-2 (Hydrogen fuel Product specification Part 2: Proton exchange membrane (PEM) fuel cell applications for road vehicles) is shown in the table below: Amount fraction (μmol/mol) Amount fraction (μmol/mol) Water 5 Carbon monoxide 0.2 Total hydrocarbons 2 Total sulphur compounds 0.004 Oxygen 5 Formaldehyde 0.01 Helium 300 Formic acid 0.2 Nitrogen 100 Ammonia 0.1 Argon 100 Carbon dioxide 2 Total halogenated compounds 0.05 10

Biogas Biogas and biomethane are set to play a key role in ensuring that Europe meets its stringent renewable energy targets. These gases can be blended with natural gas in transmission networks and used as vehicle fuels. A European specification for biomethane for these applications is being developed - this will set limits on the physical properties of the gas and will also specify maximum levels for a range of toxic impurities. NPL provides traceable gas mixtures for biogas analysis, and also carries out measurements of real biogas samples. ENERGY GASES Biogas composition Multi-component gas standards are available containing a range of components including hydrocarbons, hydrogen, carbon monoxide, carbon dioxide, nitrogen and oxygen. Trace-level impurities Traceable gas standards containing each of the following components are available: Siloxanes (multi-component mixtures of the siloxanes most commonly found in biogas) Hydrogen sulphide Ammonia 11

ENVIRONMENTAL GASES Volatile organic compounds (VOCs) 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 NPL s ozone precursor mixtures are recognised as the World Meteorological Organisation (WMO) primary standard for a number of their components. They are used to assess global ambient levels of VOCs as part of the Global Atmosphere Watch (GAW) programme that provides the technical basis for the assessment of the chemical composition of the atmosphere. 12

Volatile organic compounds (VOCs) BTEX mixtures Benzene, toluene, ethylbenzene and xylenes are volatile mono-aromatic hydrocarbons that are key pollutants resulting from combustion processes. BTEX compounds 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 compounds 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. ENVIRONMENTAL GASES Bespoke VOC and halogenated hydrocarbon mixtures NPL can also prepare bespoke gas standards containing C 2 to C 10 hydrocarbons, chlorinated hydrocarbons and fluorinated hydrocarbons at a wide range of amount fractions. Deuterated toluene standards Gas standards of d 8 -toluene are often used as internal standards in GC-MS applications. NPL provides mixtures of d 8 -toluene at a range of amount fractions, most typically 5 µmol/mol. 13

ENVIRONMENTAL GASES Oxygenated VOCs and semi-vocs Oxygenated VOCs Oxygenated VOCs (oxy-vocs) are a class of compounds which include alcohols, ketones and aldehydes, many of which occur both anthropogenically and biogenically. For example, methanol is the second most abundant organic compound in the Earth s atmosphere and is a common industrial solvent. Oxy-VOCs are found in biomass burning plumes and their emissions are closely monitored due to their influence on the HO x cycle and their possible contribution towards tropospheric ozone, which affects air quality. NPL offers a range of traceable gas standards containing oxy-vocs at an amount fraction of 5 μmol/mol. One typical multi-component mixture contains the components shown in the list below; mixtures containing other oxy-vocs (including acetaldehyde) are available on request. Methanol Ethanol Acetone n-hexane Propane Benzene Another important oxygenated compound is formaldehyde, which is a key atmospheric species that readily photo-dissociates to create radicals that initiate oxidation reaction chains. The measurement of formaldehyde is also an important part of indoor air monitoring. NPL provides a range of static and dynamic formaldehyde standards with amount fractions from 1 to 10 μmol/mol. Semi-VOCs Semi-volatile organic compounds (semi-vocs) include a diverse range of compounds often emitted from building materials and furnishings. As a result, these compounds can build up in indoor environments and may have a detrimental impact on human health. These compounds are therefore of significant concern and NPL are researching novel methods to provide traceable measurements of semi-vocs at trace levels. 14

Terpenes Total emissions of biogenic carbon into the atmosphere are considered to be approximately ten times larger than the sum of all anthropogenic emissions. The most abundant compounds in these emissions are isoprene, α-pinene, β-pinene, Δ3-carene, camphene and myrcene. These compounds play a central role in the oxidation processes in the atmosphere and in the formation of particles. The accurate measurement of these compounds is essential to underpin the need to quantify the emission rates of isoprene and monoterpenes in the atmosphere. These measurements are crucially dependent on traceable gas standards, and NPL offers the following terpene standards: ENVIRONMENTAL GASES Four-component terpene mixture A mixture of the following monoterpenes and terpenoids at amount fractions of 2 nmol/mol: (+/-)-α-pinene 1,8-cineole (eucalyptol) (+)-3-carene R-(+)-limonene 33-component ozone precursor and terpene mixture This mixture, which is available at a range of amount fractions from 2 to 10 nmol/mol, contains all the compounds in the 30-component ozone precursor mixture (see page 12) as well as (+/-)-α-pinene, (+/-)-β-pinene and R-(+)-limonene. Bespoke terpene mixtures Bespoke mixtures containing a range of terpenes are also available at amount fractions from 2 to 100 nmol/mol. Examples of the terpenes that can be included in these mixtures are listed below. Other components may be included on request and these terpenes can also be combined with other ozone precursors and BTEX components. (+/-)-α-pinene (+/-)-β-pinene (+)-3-carene 1,8-cineole (eucalyptol) Limonene Myrcene Camphor p-cymene cis-ocimene 15

ENVIRONMENTAL GASES Gas standards for atmospheric monitoring Traceable, accurate and stable gas standards for high impact greenhouse gases are required to develop our understanding of the increasing influence of human activity on the global atmosphere, address the effects of climate change and provide the basis for stable and comparable measurements of these gases. NPL provides gas standards for calibrating a range of ambient atmospheric analysers, including cavity ring-down spectrometers, tunable diode laser absorption spectrometers, mid-infrared absorption spectrometers and gas chromatographs. Gas mixtures can be prepared in a matrix of whole air or synthetic air. The table below gives an indication of the available mixtures; others may be available on request. Matrix gas Approximate amount fraction Methane Air 1.8 μmol/mol Carbon dioxide Air 380 μmol/mol Carbon monoxide Air 100 nmol/mol 16

Gas standards for air quality monitoring Routine measurements of air pollutants such as sulphur dioxide, oxides of nitrogen, carbon monoxide and benzene must be performed to fulfil the requirements of the EC Directive on ambient air quality and cleaner air for Europe (Directive 2008/50/EC). NPL provides gas standards for calibrating air quality monitors. Mixtures that meet the uncertainty requirements of Directive 2008/50/EC are shown in the table below; other mixtures may be available on request. ENVIRONMENTAL GASES Matrix gas Amount fraction range From To Sulphur dioxide Nitrogen or air 50 nmol/mol 10 mmol/mol Nitrogen monoxide Nitrogen 50 nmol/mol 10 mmol/mol Nitrogen dioxide Nitrogen or air 50 nmol/mol 5 mmol/mol Carbon monoxide Nitrogen or air 1 μmol/mol 500 μmol/mol Standards of benzene and other hydrocarbon compounds are also available see pages 12 & 13. 17

INDUSTRIAL AND EMISSION GASES Gas standards for industrial emissions monitoring Monitoring of industrial emissions is required in order to demonstrate conformity to legislation, such as the European Directive on industrial emissions (Directive 2010/75/EU). NPL provides gas standards for calibrating a wide variety of analysers for measuring stack and other industrial emissions gases. Binary and multi-component gas mixtures can be prepared to the requirements of the customer. The table below gives an indication of the available mixtures; others may be available on request. Matrix gas Amount fraction range From To Carbon monoxide Nitrogen or air 1 μmol/mol 750 mmol/mol Carbon dioxide Nitrogen or air 1 μmol/mol 750 mmol/mol Oxygen Nitrogen 1 μmol/mol 750 mmol/mol Nitrogen monoxide Nitrogen 1 μmol/mol 10 mmol/mol Nitrogen dioxide Nitrogen 1 μmol/mol 5 mmol/mol Ammonia Air 1 μmol/mol 1 mmol/mol 18

Gas standards for vehicle emission measurements National and European legislation, such as vehicle emissions regulations, dictates that emission gases are routinely measured to exacting standards. In order to achieve this, it is necessary to utilise accurate and traceable gas standards. These standards can also be used for petrol, diesel and gas turbine engine research, as well as a wide range of development and certification work. NPL provides gas standards for calibrating vehicle emissions monitors, which can be prepared to the requirements of the customer. The table below gives an indication of the available mixtures; others may be available on request. INDUSTRIAL AND EMISSION GASES Matrix gas Amount fraction range From To Carbon monoxide 5 mmol/mol 150 mmol/mol Carbon dioxide 50 mmol/mol 150 mmol/mol Nitrogen Propane 500 μmol/mol 5 mmol/mol Oxygen 5 mmol/mol 210 mmol/mol Carbon monoxide Nitrogen or Air 1 μmol/mol 50 mmol/mol Carbon dioxide Nitrogen or Air 1 μmol/mol 50 mmol/mol Oxygen Nitrogen 1 μmol/mol 50 mmol/mol Propane Air 1 μmol/mol 50 mmol/mol Nitrogen monoxide Nitrogen 1 μmol/mol 10 mmol/mol Nitrogen dioxide Air 1 μmol/mol 5 mmol/mol n-hexane Nitrogen 1 μmol/mol 1 mmol/mol 19

INDUSTRIAL AND EMISSION GASES Breath alcohol and interfering substances Traceable measurements of breath alcohol and interfering substances are required to underpin drink driving, airline and other workplace legislation. Gas standards containing these compounds can be used, for example, to calibrate evidential breath analysers as specified by the OIML (International Organization of Legal Metrology) International Recommendation OIML R 126 (Evidential breath analyzers). Gas standards containing ethanol and interfering substances can be prepared to the requirements of the customer. Examples of the most common components in these mixtures are given below; other interfering substances (such as those listed in OIML R 126) may be available on request. Matrix gas Amount fraction range From To Ethanol Nitrogen or air 20 μmol/mol 1.6 mmol/mol Methanol Nitrogen or air 10 μmol/mol 100 μmol/mol Acetone Nitrogen or air 10 μmol/mol 100 μmol/mol Toluene Nitrogen or air 10 μmol/mol 100 μmol/mol Methyl ethyl ketone Nitrogen or air 100 μmol/mol 1 mmol/mol Ethanol 20 μmol/mol 1.6 mmol/mol Nitrogen or air Carbon dioxide 10 mmol/mol 100 mmol/mol 20

Water vapour is one of the most difficult impurities to remove from gases, and it affects a number of manufacturing processes even at trace amount fractions. Instrumentation dedicated to measuring trace levels of water is therefore of paramount importance. The accuracy of these instruments can only be maintained through regular calibration to traceable reference standards. Standards of water vapour provide an accurate method for calibrating instrumentation. Static reference standards of water (at amount fractions greater than 20 μmol/mol) in nitrogen are prepared in cylinders. Calibrations are also available at lower amount fractions using NPL s unique trace water vapour facility. Static standards INDUSTRIAL AND EMISSION GASES Water vapour NPL provides gas mixtures containing water vapour in a matrix of nitrogen. These mixtures are available with the following amount fractions: Matrix gas Water Nitrogen Amount fraction range From To 20 μmol/mol 100 µmol/mol Trace water vapour facility The NPL trace water vapour facility is capable of generating an adjustable amount fraction of trace water (between 2 and 2000 nmol/mol) by using continuous accurate measurements of mass loss from a permeation device coupled with a dilution system based on an array of critical flow orifices. This is achieved with a relative expanded uncertainty of less than 3%. 21 E-mail: gases@npl.co.uk Telephone: +44 20 8943 8715

INDUSTRIAL AND EMISSION GASES Measurement of water vapour transmission rate A major obstacle to introducing flexible organic devices into the commercial market is their limited lifetime when exposed to water. Ingress of water poses a particular challenge for flexible displays, since the polymeric substrates from which they are constructed are highly permeable. To reduce water ingress, thin inorganic coatings (approximately 10 nm in thickness) can be applied to the flexible polymeric substrates. To assess the efficacy of such barrier layers, measurements of water permeability are required. These are carried out using the calcium test, radioactive methods with tritiated water or, most commonly, the MOCON test. The industry goal for water permeation is 10 6 g m 2 day 1 and, with recent reports of flexible barriers achieving 10 7 g m 2 day 1, the question of how to measure water permeation at such low levels is critical. NPL has developed a new approach to measuring water vapour transmission rate directly, based on cavity ring-down spectroscopy. The unique facility provides accurate and traceable measurements with a detection limit significantly below 10 4 g m 2 day 1. The system operates with a dry chamber separated from a wet chamber of known temperature and relative humidity by the barrier material under test. Water vapour permeating through the film is collected by dry nitrogen and measured by cavity ringdown spectroscopy. The measurement cell uses a novel design to ensure optimum sealing conditions in order to eliminate water ingress to the dry chamber. The capability is underpinned by NPL s trace water vapour facility (see page 21). 22

Noble gases Noble gases are used in several important industrial applications such as lighting, welding, and space exploration. The range of noble gas standards provided by NPL is shown in the table below. Matrix gas Amount fraction range From To Helium Nitrogen, air or oxygen 500 nmol/mol 500 μmol/mol Neon Nitrogen, air or oxygen 500 nmol/mol 500 μmol/mol Argon Nitrogen, air or oxygen 500 nmol/mol 500 μmol/mol Krypton Nitrogen, air or oxygen 500 nmol/mol 500 μmol/mol INDUSTRIAL AND EMISSION GASES Gas standards for olfactometry Gas mixtures of compounds such as 1-butanol are regularly used for olfactometry testing applications. Examples of NPL s olfactometry gas standards are shown in the table below; other mixtures may be available on request. Matrix gas Amount fraction range From To 1-butanol Nitrogen or air 20 μmol/mol 100 μmol/mol 1-pentene Nitrogen or air 1 μmol/mol 80 μmol/mol 23

HIGH PURITY GASES Purity analysis A wide range of regulations and industrial requirements depend on the accurate and robust analysis of trace and ultra-trace levels of a wide range of components in pure gases. Examples of the areas where these analyses are highly important are atmospheric monitoring, ambient air measurements and indoor air measurements. They are also essential for process gases used in, for example, the health, micro-manufacturing, fuel cell and energy industries. The NPL trace gas analysis facility provides traceable measurements of key contaminants found in high-purity gases. Measurement traceability is achieved through instrument calibrations using in-house traceable gas standards. NPL s purity analysis capability is summarised in the table below. Approximate limit of detection (nmol/mol) Approximate limit of detection (nmol/mol) Nitrogen 10 Nitrogen monoxide < 1 Oxygen 10 Nitrogen dioxide < 1 Argon 10 Methane 3 Hydrogen 10 Other hydrocarbons < 1 Carbon monoxide 1 Water 2 Carbon dioxide 5 Hydrogen sulphide 5 Sulphur dioxide < 1 The NPL trace gas analysis facility is also available for instrument calibration and performance assessment work. 24

Calibration of ozone photometers NPL maintains the UK national ozone standard, a NIST standard reference photometer. This standard undergoes regular international comparisons with primary ozone standards held by other National Measurement Institutes. The UK national ozone standard is used to provide accredited calibrations of ozone photometers at amount fractions up to 1 μmol/mol with a relative expanded uncertainty of 2.5 %. Gas metrology software OTHER PRODUCTS AND SERVICES GravCalc2 GravCalc2 is Windows-based software that calculates the amount fraction and uncertainty of all components in gravimetrically prepared gas mixtures using the method described in ISO 6142. The program can be used for any gas mixture. Further details about GravCalc2 are available from the NPL website. XLGENLINE XLGENLINE is Excel-based software that allows the user to perform generalised least squares (GLS) regression analysis that is fully compliant with ISO 6143. XLGENLINE calculates the amount fractions and estimated uncertainties of unknown samples, displays a plot of the fitted regression function, and outputs the parameters of the fit for example, gradient, intercept and covariance. The software, which can also be used for ordinary least squares (OLS) regression analysis, is available as a free download from the NPL website. 25

OTHER PRODUCTS AND SERVICES Proficiency testing schemes Natural gas and refinery gas proficiency testing schemes NPL organises natural gas and refinery gas proficiency testing (PT) schemes to enable calibration and testing laboratories to achieve greater confidence in their analyses of the composition of these gases. These PT schemes involve the distribution of accurate natural gas standards of known composition to participating laboratories for blind testing. Participants analyse the mixtures and send their results back to NPL. At the conclusion of the scheme, a report is provided to each laboratory, which includes an anonymous comparison of all participants results. On-demand proficiency testing schemes Bilateral, on-demand PT schemes can also be organised. These are available for any gas mixture, and their scope and timing are arranged at the convenience of the participating laboratory. The procedure for an on-demand PT scheme is as follows: 1. NPL prepare and validate a Primary Reference Gas Mixture (PRGM). 2. The PRGM is sent (without the certificate) to the participating laboratory. 3. The external laboratory measures the composition of the PRGM and reports their results to NPL. 4. NPL provides the external laboratory with a comparison of their results against the gravimetric composition of the mixture, and a certificate showing the composition of the PRGM. 5. The external laboratory retains the PRGM for future use. 26

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