Article Cite This: Anal. Chem. XXXX, XXX, XXX−XXX
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Influence of Pressure on the Composition of Gaseous Reference Materials Paul J. Brewer,* Richard J. C. Brown, Kate V. Resner, Ruth E. Hill-Pearce, David R. Worton, Nicholas D. C. Allen, Kevin C. Blakley, Daniel Benucci, and Matthew R. Ellison National Physical Laboratory, Hampton Road, Teddington, Middlesex TW11 0LW, United Kingdom ABSTRACT: We have shown that the amount fraction of carbon dioxide in a nitrogen or synthetic air matrix stored in cylinders increases as the pressure of the gas mixture reduces, while the amount fraction of methane remains unchanged. Our measurements show the initial amount fraction of carbon dioxide to be lower than the gravimetric value after preparation, which we attribute to the adsorption of a proportion of the molecules to active sites on the internal surface of the cylinder and the valve. As the mixture is consumed, the pressure in the cylinder reduces and the amount fraction of the component is observed to increase. The effect is less pronounced in the presence of water vapor. More dramatic effects have been observed for hydrogen chloride. These findings have significant implications for the preparation of high accuracy gaseous reference materials with unprecedented uncertainties which underpin a broad range of requirements, in particular atmospheric monitoring of high impact greenhouse gases.
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greater certainty and to better inform predictive models. In order to maintain short- and long-term stable values of greenhouse gases for analysis of trends in the atmosphere with confidence, measurements, and therefore gaseous reference materials, at challenging low levels of uncertainty are required. To address this, the World Meterological Organisation’s (WMO) Global Atmosphere Watch (GAW) program has set Data Quality Objectives (DQOs) for these components. Perhaps among the most challenging is the DQO of 100 and 50 nmol mol−1 for carbon dioxide in the northern and southern hemisphere, respectively.10 To achieve SItraceable reference materials that meet these targets is incredibly challenging given that up until recently the state of the art among National Metrology Institutes (NMIs) delivering routine gas reference materials was to provide an expanded uncertainty of 0.1% (four times the DQO for carbon dioxide in the northern hemisphere). Research in the European Metrology Research Programme project HIGHGAS11 has helped to close the gap by overcoming several challenges focused on improving the uncertainty in gravimetry, purity analysis of the starting materials, and stability and quantification of isotopic composition of carbon dioxide.12 Other work in the NMI community has focused on studying the changes in initial amount fraction of gaseous reference materials due to adsorption losses on the internal surfaces of the cylinder used for its storage.13 They have found that the difference can
limate change poses one of the greatest risks to society worldwide. In order to prevent irreversible changes to the Earth’s climate, it is essential that emissions are reduced. Industrial and domestic activities have been recognized by the Kyoto Protocol1 and more recently the Conference of Parties (COP21)2 as being major sources of greenhouse gases. COP212 agreed that each country would provide nationally determined contributions, pledges, to reduce emissions. Furthermore, in Europe, the Covenant of Mayors new signatories now pledge to reduce carbon dioxide emissions by at least 40% by 2030 and to adopt an integrated approach to tackling mitigation and adaptation to climate change. The perturbed global biogeochemical cycles of carbon dioxide, methane, and nitrous oxide are the driving force of current and future climate change. Some of these gases have been monitored for more than 40 years. The longest continual record is for carbon dioxide where measurements started in 1958.3,4 The concentrations of carbon dioxide and methane in the atmosphere are at the highest they have been in the past 25 million years. Current levels of carbon dioxide have increased by 30% from preindustrial times, and they continue to rise, as fossil fuel emissions climb substantially.5−8 Current levels of methane are nearly triple the preindustrial value.7,9 These changes are caused by human activities. There is an urgent requirement to provide a validated and traceable measurement infrastructure 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. Furthermore, there is pressure to reduce the uncertainty of the measurements made to allow trends to be identified with © XXXX American Chemical Society
Received: December 19, 2017 Accepted: January 30, 2018 Published: January 30, 2018 A
DOI: 10.1021/acs.analchem.7b05309 Anal. Chem. XXXX, XXX, XXX−XXX
Article
Analytical Chemistry sometimes be larger than the calculated uncertainty. Similar work reported losses of carbon dioxide to the walls of aluminum cylinders due to adsorption.14 This work reports the influence of the pressure of the gaseous reference material on the amount fraction of carbon dioxide and methane under a variety of experimental conditions. Similar experiments for hydrogen chloride are used to investigate a molecule with different chemical and physical properties to carbon dioxide and methane in order to improve our understanding of the underlying processes governing these pressure dependent characteristics. This research provides vital information to enable the production of reference materials that meet the demanding requirements for uncertainty, particularly in the field of atmospheric monitoring where these findings are imperative to the advancement of reference materials. Preparation of the Reference Materials. All mixtures were prepared by gravimetry, in accordance with ISO6142,15 in 10 L aluminum cylinders (Luxfer) with a DIN1 outlet diaphragm valve (Hale Hamilton, UK). A selection of the cylinders was treated internally with a BOC proprietary Spectraseal passivation process of the internal surface. The cylinders were evacuated using an oil-free pump [Scrollvac SC15D, Leybold Vacuum] and turbo molecular pump with magnetic bearing [Turbo vac 340M, Leybold Vacuum]. A gaseous reference mixture of methane and carbon dioxide in nitrogen was prepared at nominal amount fractions of 45 μmol mol−1 and 1 cmol mol−1, respectively, from pure carbon dioxide (BOC, 99.999%), pure methane (6.0, CK Gas Products), and pure nitrogen (Air Products, BIP+). Each was added to an evacuated cylinder in turn using a 1/16 in. tube (Swagelok, electro-polished stainless steel) which was purged several times with the component to be added.16,17 The cylinder was weighed before and after the addition of each component using a balance (Mettler Toledo ID7). Eight reference mixtures of methane and carbon dioxide in nitrogen (Air Products, BIP+) were then prepared from the same parent at a nominal amount fraction of 1.8 and 400 μmol mol−1, respectively, following the same procedure outlined earlier. Four standards (A, B, C, and D) were prepared in 10 L aluminum cylinders with no treatment and four others (E, F, G, and H), in 10 L cylinders treated with a passivation process (BOC Spectraseal). A nominal 130 μmol mol−1 of water in nitrogen standard was added to A, B, E, and F to ensure 10 μmol mol−1 of water in the final mixture. A Cavity Ring Down Spectrometer (Lasertrace 6000, Tiger Optics) was used to confirm the final amount fractions of water. It was also used to measure the amount fraction of water in the dry mixtures (C, D, G, and H), and these were certified at
