Breakthrough in Negating the Impact of Adsorption ... - ACS Publications

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Breakthrough in Negating the Impact of Adsorption in Gas Reference Materials Paul J. Brewer, Richard J C Brown, Eric B. Mussell Webber, Sivan van Aswegen, Michael K.M. Ward, Ruth Elizabeth Hill-Pearce, and David Robert Worton Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b00175 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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Analytical Chemistry

Breakthrough in Negating the Impact of Adsorption in Gas Reference Materials Paul J Brewer*, Richard J C Brown, Eric B Mussell Webber, Sivan van Aswegen, Michael K M Ward, Ruth E Hill-Pearce and David R Worton National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW, UK.

*Corresponding author. Tel +44 20 8943 6007; fax: +44 20 8614 0423. Email address: [email protected] Abstract. We have shown that an exchange dilution preparation method reduces the impact of surface adsorption of the target component in high pressure gas mixtures used for underpinning measurements of amount-of-substance fraction. Gas mixtures are diluted in the same cylinder by releasing an aliquot of the parent mixture. Additional matrix gas is then added to the cylinder. This differs to conventional methods where dilutions are achieved by transferring the parent mixture to another cylinder which then stores the final reference material. The benefit of this revolutionary approach is that losses due to adsorption to the walls of the cylinder and the valve are reduced as the parent mixture pacifies the surface with only negligible relative change in amount-of-substance fraction. This development allows for preparation of gas reference materials with unprecedented uncertainties beyond the existing state of the art. It has significant implications for the preparation of high accuracy gas reference materials which underpin a broad range of requirements, particularly in atmospheric monitoring of carbon dioxide, where understanding the adsorption effects is the major obstacle to advancing the measurement science. It has the potential to remove the reliance on proprietary surface pre-treatments as the method provides an insitu and consistent alternative.

Keywords: Gas metrology, atmospheric monitoring, adsorption, passivation, SI traceability. Introduction High accuracy gas reference materials produced by National Metrology Institutes (NMIs) and Designated Institutes (DIs) provide the primary realisation of the mole and underpin a vast range of measurement applications.1 For example, fiscal trading of natural gas, legislative air quality, government emission inventory measurements and roadside evidential breath alcohol tests all critically depend on primary gas reference materials and traceability. Until recently, gas reference materials for calibration of amount-of-substance (later referred to as amount) fraction with uncertainties of 0.1 % were state of the art. This has improved in recent years, driven by the needs of stakeholders for higher accuracy.2 A prominent example comes from the requirements for a measurement infrastructure to support legislation and initiatives towards tackling climate change.3,4,5 Based on the research and infrastructure activities governed by the World Meteorological Organisation’s Global Atmosphere Watch Programme,

1 Environment ACS Paragon Plus

Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

reference materials for carbon dioxide must be realised with expanded uncertainties of better than 0.025 % to observe trends in atmospheric composition.6 NMIs and DIs are developing capability in collaboration with expert laboratories to improve the accuracy of the major contributors in the uncertainty budget. These include gravimetric methods for gas mixture preparation, purity analysis of the source gases, the precision of spectroscopic techniques for validation and addressing commutability issues due to matrix composition and isotope ratio. A recent key comparison (CCQM-K120),7 coordinated by the BIPM, has shown the state of the art has increased since its predecessor (CCQM-K52) in 2006,8 with laboratories achieving typical expanded uncertainties of 0.05 %. Although this illustrates a significant improvement, further work is required to meet the requirements for global monitoring. The major obstacle to reducing the uncertainties further, is to address a long-standing issue of adsorption of the target component to the internal surface of the valve and cylinder in which these reference materials are stored.9 NMIs and DIs are currently reliant on commercial organisations to provide a proprietary pre-treated cylinder and valve surface to reduce the adsorption of the target component. However as there is not one solution for all components, a myriad of products exists on the market with limited information available on the chemical processes involved.10,11 In some cases, products with the same name but supplied from a different country have a significant impact on performance. An example of this was discovered in the preliminary work to an international comparison on formaldehyde (CCQM-K90).12 This makes it very difficult for laboratories providing primary traceability, to make an informed decision on the optimum solution for a specific target component and compromises international comparability. As NMIs and DIs have no control of these proprietary techniques, this can lead to large variability within batches of produced reference materials. Availability of certain passivation treatments is also an issue when a bespoke and commercially sensitive process is developed within an organisation that is unable to meet global demand. For reference materials provided in accordance with ISO 6142-1:2015,13 the reference value is determined from the gravimetric process. This means that any adsorption of the target component will result in the reference value being higher than the true value. With the example of carbon dioxide, adsorption to the walls of the cylinder and valve can lead to a bias of the order of 200 nmol mol

-1

(equivalent to 0.05 % of a 400 µmol mol-1 in air mixture) being

reported.14,15 There are further issues when the gas mixtures are used at lower pressures due to the molecules desorbing from the cylinder walls and valve, resulting in a higher amount fraction.15,16 The impact of adsorption effects is also seen for the primary realisation of carbon dioxide composition using manometry. Interactions of carbon dioxide with manometer surfaces complicate interpretation of results. The Scripps Institution of Oceanography has documented small changes in results from their Constant-volume Mercury-column Manometer following cleaning to remove grease and residual mercury.17 The National Oceanic and Atmospheric Administration is investigating the possibility that carbon dioxide adsorbs to O-rings.18 In this paper we describe an exchange dilution approach for negating the effects of adsorption in gas reference materials. This has been developed from a general preparation principle referred to in annex A of ISO 6142-1:2015 (section 4.2b).13 This approach involves preparing


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a gas mixture from a dilution performed in the same cylinder. The cylinder is initially filled with a parent mixture at a high amount fraction. An aliquot is then released, and the mass of the remaining gas mixture is determined by the gravimetric difference. The pressure that the parent mixture is reduced to is referred to here as the reduction pressure. The mixture is then diluted by introducing the desired mass of the matrix gas. This differs to conventional methods where dilutions are achieved by transferring the parent mixture to another cylinder. Previous work has suggested that the absolute amount of the target component adsorbed to the surface will not change as a function of the amount fraction of the mixture.15,16 Hence by exposing the surfaces to the higher amount fraction parent mixture in this method, the relative change from adsorption is negligible after diluting to the final amount fraction. This work has profound implications for enabling the preparation of reference materials with superior uncertainties to underpin the immense challenges the world currently faces such as monitoring the influence of human activity on climate. It also has the potential to remove the requirement for surface pretreatments, as the method provides an in situ and consistent alternative. Preparation of the reference materials All mixtures were prepared by gravimetry, in accordance with ISO 6142-1:2015,13 in 10 litre aluminium cylinders (Luxfer) with a DIN1 outlet diaphragm valve (Hale Hamilton, UK). Cylinders were treated internally with a BOC proprietary Spectraseal passivation process. 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). Reference mixtures of carbon dioxide in matrix of air or nitrogen were prepared from carbon dioxide (BOC, 99.999 %), nitrogen (Air Products, BIP+), oxygen (BOC speciality gases, N 6.0) and argon (Air Products, BIP+). A selection of the mixtures was prepared in dry whole air (Scott Marrin Inc), sampled from the ambient atmosphere and with the carbon dioxide removed catalytically.



Figure 1 A schematic showing the difference between conventional preparation of a reference material (method I) and the novel procedure presented here (method II). Figure 1 is a schematic of two preparation schemes used to prepare mixtures of carbon dioxide in either an air or nitrogen matrix at amount fractions of 380 and 400 mol mol-1. Method I presents the conventional method where a parent mixture is produced from pure carbon dioxide by dilution in a cylinder with the matrix gas. An aliquot of the parent mixture is then transferred to another cylinder and diluted further. Method II depicts the novel approach described here where only one cylinder is used to prepare the parent mixture and then perform the dilution to the final mixture. The parent mixture is made in the same way as in method I. An aliquot of the mixture (marked blue) is then released from the cylinder and replaced with the matrix gas. The mass removed is determined from the mass difference before and after the gas is released. For both methods, components were added via a 1/16” tube (Swagelok, electro-polished stainless steel) which was purged several times with the component to be added.19,20 The cylinder was weighed before and after the addition of each component using a balance (Mettler Toledo ID7). Results and discussion A Cavity Ring Down Spectrometer (CRDS) (Picarro G2301) was used in the analysis of the carbon dioxide mixtures. The instrument employs a laser, a high-precision wavelength monitor, a high finesse optical cavity with three high-reflectivity mirrors (> 99.995 %), a photodetector and a computer. Measurements were performed by flowing the gas mixture into the gas cell of the analyser at a nominal flow rate of 0.5 L min-1 (controlled by the instrument) and setting up an excess flow to vent (0.5 L min-1).

Figure 2 Transient CRDS analyser response to alternate injections of four reference materials containing carbon dioxide (filled circles) at a nominal amount fraction of 380 µmol mol-1. The open circles show the mean of each data set after steady state has been achieved.


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Figure 2 shows some example experimental data, where four different gas mixtures of differing composition are passed through the analyser for a period of approximately 5 minutes. The mean of the data set is taken after approximately 1 minute (shown with open circles). The relative difference between each mixture and one of the ensemble, selected as the reference (dotted line in figure 3), is then determined. Three gas mixtures (A, B and C) prepared in aluminium cylinders pre-treated with a BOC Spectraseal process are displayed by the filled circles. In each case, the amount fraction of carbon dioxide present is 380 µmol mol-1 and the water present was less than 1 µmol mol-1. The mixtures were prepared in a synthetic air matrix from the pure components of oxygen, argon and nitrogen, blended in a ratio to replicate ambient composition. They were prepared from pure carbon dioxide via a 4000 µmol mol-1 carbon dioxide in air intermediate mixture. The comparative measurements to the reference (a mixture prepared in the same way) shows excellent repeatability from the measurement system (A, B and C show equivalence to the reference to within 0.001 % relative) in figure 3.

Figure 3 The relative difference between an internal reference (dotted line) and eight carbon dioxide reference materials prepared at nominal amount fraction of 380 µmol mol-1. Six were prepared using method I, three with a synthetic air matrix (mixtures A, B and C) and three with a whole air matrix (mixtures D, E and F). A further two mixtures were prepared at the same time in synthetic air, one with method I (mixture G) and one with method II (mixture H). Bars represent the standard uncertainties. Three further mixtures (D, E and F) shown with open circles were prepared 5 years ago with the same amount fraction of carbon dioxide but in whole air (Scott Marin Inc), containing similar background levels of water vapour to the synthetic air matrix. They were also prepared from a 4000 µmol mol-1 carbon dioxide in air parent mixture. The difference observed to those prepared in synthetic air (filled circles) is likely to result from a systematic bias introduced from assigning the amount fraction of argon in the whole air. These mixtures are also less consistent than mixtures A, B and C. This may be due to mixture E reducing in amount fraction over the 5 year time period since they were prepared. Mixture H (filled square) is a 380 µmol mol-1 carbon dioxide in synthetic air mixture prepared in the same cylinder from a 4000 µmol mol-1 parent mixture, following method II (figure 1). The reduction pressure was 8 bar. Some of the aliquot of the parent gas released to make the dilution in the same cylinder, was transferred to another cylinder for the conventional preparation to prepare mixture G. The results show a



difference of 0.04 % between the mixtures which is equivalent to 150 nmol mol-1. This difference is comparable to that observed from previous decant experiments14,15 and demonstrates the adsorption of carbon dioxide to the internal surfaces of the cylinder and valve for the mixture using method II is negligible. Figure 4 shows the modelled relative difference between the amount fraction of carbon dioxide in a 400 µmol mol-1 reference mixture prepared using method II and the true value, as a function of the pressure of the parent mixture before the final dilution step. The model assumes that the parent mixture is prepared at 100 bar and then an aliquot (magnitude dependent on the amount fraction) is replaced with the matrix gas to make a final 400 µmol mol-1 mixture at 100 bar. Previous data15 has been used for the dependence of the amount fraction of carbon dioxide in the mixture on the pressure of the mixture in the cylinder. The change in the amount fraction of the mixture due to adsorption of carbon dioxide has been fixed at 196 nmol mol-1 based on the mean of the data from decant experiments published previously.15 The dashed line presents the expected difference with the assumption that the mixture pressure has no influence on adsorption of carbon dioxide to the internal surfaces of the cylinder and valve. It shows that as the reduction pressure decreases, the influence of adsorption decreases, resulting in a final mixture closer to the true value. This is explained by there being a fixed number of carbon dioxide molecules that adsorb to the surface. So as the amount fraction of the parent mixture is increased to allow for a lower reduction pressure (to achieve a final mixture at 400 µmol mol1

), the relative adsorption tends to zero. The line also shows that at 100 bar (i.e. when there is

no dilution of the parent simulating a conventional method I approach) the difference on the amount fraction is 0.05 %, as observed in figure 3. The data points show the same analysis with the influence of mixture pressure on adsorption (using the data from Brewer et al). This analysis shows that as the reduction pressure decreases (which is required to achieve the final dilution step), the amount fraction of carbon dioxide will increase due to desorption of carbon dioxide from the internal surfaces. This effect will oppose the initial adsorption losses (dashed line) and lead to a final amount fraction that is higher than the true value. However, this is only correct if the molecules that become desorbed, do not re- adsorb when the cylinder is taken back up to 100 bar in the final dilution step. If molecules re-adsorbed, for reduction pressures below the onset of when molecules start to desorb from the walls, the opposite effect would be observed resulting in mixtures more depleted in carbon dioxide (below the dashed line). For simplicity, the data presented for the four different types of cylinder pre-treatment are shown for the case that carbon dioxide is not re-adsorbed when the cylinder pressure is taken back to 100 bar with matrix gas. This effect has only a marginal influence and the noise on the data is larger than the deviation from the dashed line. These differences are smaller than expected from previous work15 as any increase in carbon dioxide at the reduced pressure is diluted out in the final stage.


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The most pronounced effect is for Spectraseal treated aluminium cylinders in the presence of 10 mol mol-1 water vapour in the mixture (filled circles).

Figure 4 The calculated relative difference between the amount fraction of carbon dioxide in a 400 µmol mol-1 reference mixture prepared using method II and the true value, as a function of the pressure of the parent mixture before the final dilution step. The parent mixtures are prepared at 100 bar and an aliquot (magnitude dependent on the amount fraction) is replaced with the matrix gas to make a final 400 µmol mol-1 mixture at 100 bar. Previous pressure dependence data with different cylinders (Spectraseal treated aluminium - circles and untreated aluminium - squares) is used. Open and filled symbols represent mixtures with < 0.05 mol mol-1 and 10 mol mol-1 water vapour respectively. Experimental data from 400 µmol mol-1 mixtures prepared using methods I and II are shown with plus symbols. A repeat preparation of mixtures G and H was performed with a slight change in the amount fraction of the final mixture (400 mol mol-1) and in a nitrogen matrix. The reduction pressure was similar (10 bar compared to 8 bar previously). The results are shown in figure 4 with plus symbols. The relative difference between the two mixtures was determined using CRDS. The value for the relative difference of the mixture prepared using method I was fixed at 0.05 % from the modelled true value (symbol in figure 4 corresponding to 100 bar on the x-axis). The difference between the mixture prepared using method II and the modelled true value was then determined (symbol in figure 4 corresponding to 10 bar on the x-axis). The value is 0.017 % larger than the prediction. This difference is small given that the calculations are dependent on previous adsorption studies on a limited dataset and the surface of the cylinders used in this work are likely to vary somewhat. Furthermore, effects of carbon dioxide desorption due to reduced pressure in the cylinder and thermal fractionation during transfer may account for the difference observed. Further work is required to study these to determine the biases these processes introduce and the accuracy of this new preparation method. Further studies may also lead to the refinement of the method outlined here. For example, if the reduction pressure does not influence the accuracy of the final mixture, then lower reduction pressures could be used to reduce the influence of adsorption. Alternatively, if the reduction pressure does have an influence on the accuracy of the final mixture then a double or triple dilution process could be applied using method II.



The benefit of using the method described here is illustrated further in figure 5 with application to the preparation of a 10 mol mol-1 water vapour in nitrogen gas reference material. The dotted line shows the Key Comparison Reference Value (KCRV) from CCQM-K116.21 The circles show the certified values for a travelling standard used in CCQM-K116, submitted by the National Physical Laboratory (UK). One value was determined by comparing the travelling standard to a Molbloc dilution system (filled circle).22 Another assignment was made from a 10 mol mol-1 hydrogen in nitrogen reference material, converted to water vapour using a chemical looping combustor.21 In both cases, a CRDS (LaserTrace F6000, Tiger Optics LLC) was used as the comparator. Both values agree within 0.3 % of the reference value. The diamonds show the result of certifying the travelling standard using two gravimetric reference materials using the conventional method I (open diamond) and the new method II (filled diamond). Both mixtures were prepared from pure water injected into a cylinder using a transfer vessel. Both mixtures involved the preparation of a parent mixture at a nominal amount fraction of 1000 mol mol-1 by addition of nitrogen. The mixtures were then diluted to prepare a 100 mol mol1

before the final dilution to 10 mol mol-1. The pressure of the mixture prepared using method

II was reduced to 10 bar during each dilution step. Both gravimetric mixtures determine the amount fraction of the travelling standard to be higher than the KCRV, which is commensurate with adsorption losses of water vapour. The mixture prepared using method II suffers significantly less adsorption losses with the certified value agreeing with the KCRV to within 1 % relative. This is compared to an almost 5 % relative difference for the mixture prepared by conventional methods.

Figure 5 The relative difference between four methods used to determine the amount fraction of a 10 µmol mol-1 water vapour in nitrogen mixture used in CCQM-K116 and the KCRV. Bars show standard uncertainties. Conclusion We have developed a novel approach for reducing the impact of surface adsorption of the target component in high pressure gas reference materials used for underpinning measurements of amount fraction. Compared to conventional methods, the approach has resulted in the preparation of 380 mol mol-1 mixtures of carbon dioxide in air or nitrogen that differ to those prepared by conventional methods by 0.04 % relative. The difference has been attributed to negating the effect of adsorption of carbon dioxide to the walls of the cylinder and value by


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pacifying with a higher amount fraction mixture before dilution in the same cylinder to prepare the final mixture. Further work is required to fully characterise the influence of other effects such as thermal fractionation and pressure dependence on the amount fraction of the final mixture. This development promises to allow preparation of gas reference materials with unprecedented uncertainties beyond the existing state of the art and can be applied to other components of interest where adsorption is a major contributor to the uncertainty. This has been demonstrated for water vapour where the application of this approach has resulted in a change of almost 4 % relative for a 10 mol mol-1 mixture compared to the conventional transfer method. This work has the potential to remove the reliance on proprietary surface pre-treatments as the method provides an in-situ and consistent alternative. It also has the added advantage of requiring fewer cylinders in the preparation process, however at the cost of expending the parent mixture. This novel approach may also lend itself to the recent advances in gas reference materials to underpin isotope ratio measurements of carbon dioxide, methane and nitrous oxide where surface interactions may result in fractionation. Acknowledgements This work was funded by the UK Department of Business, Energy and Industrial Strategy National Measurement System’s Chemistry and Biology Knowledge Base measurement programme.



References 1 CCQM Working Group on Gas Analysis (CCQM-GAWG), https://www.bipm.org/en/committees/cc/wg/gawg.html (accessed, Jan 2019) 2 Brewer, P. J.; Brown, R. J. C.; Tarasova, O.; Hall, B.; Rhoderick, G.; Wielgosz, R. Metrologia 2018, 55, S174. 3 https://www.ipcc.ch/sr15/ (accessed Jan 2019) 4 Kyoto Protocol, United Nations Framework Convention on Climate Change. http://www.unfccc.int. (accessed Jan 2019) 5 http://www.cop21paris.org/about/cop21/ (accessed Jan 2019) 6 WMO, 19th WMO/IAEA Meeting on Carbon Dioxide, Other Greenhouse Gases and Related Tracers Measurement Techniques (GGMT-2017): Geneva, Switzerland, 2018 7 Flores, F.; Viallon, J.; Choteau, T.; Moussay, P.; Idrees, F.; Wielgosz, R. I.; Lee, J.; Zalewska, E.; Nieuwenkamp, G.; van der Veen, A. M. H.; Konopelko, L. A.; Kustikov, Y. A.; Kolobova, A. V.; Chubchenko, Y. K.; Efremova, O. V.; Zhe, B. I.; Zhou, Z.; Miller, W. R.; Rhoderick, G. C.; Hodges, J. T.; Shimosaka, T.; Aoki, N.; Hall, B.; Brewer, P. J.; Cieciora, D.; Sega, M.; Macé, T.; Fükő, J.; Szilágyi, Z. N.; Büki, T.; Jozela, M. I.; Ntsasa, N. G.; Leshabane, N.; Tshilongo, J.; Johri, P.; Tarhan, T. CCQM-K120 (Carbon dioxide at background and urban level) Metrologia 2019, 56: 08001. 8 Wessel, R. M.; van der Veen, A. M. H.; Ziel, P. R.; Steele, P.; Langenfelds, R.; van der Schoot, M.; Smeulders, D.; Besley, L.; de Cunha, V. S.; Zhou, Z.; Qiao, H.; Heine, H. J.; Martin, B.; Macé, T.; Gupta, P. K.; Di Meane, E. A.; Sega, M.; Rolle, F.; Maruyama, M.; Kato, K.; Matsumoto, N.; Kim, J. S.; Moon, D. M.; Lee, J. B.; Murillo, F. R.; Nambo, C. R.; Caballero, V. M. S.; Salas, M. J. A.; Castorena, A. P.; Konopelko, L. A.; Kustikov, Y. A.; Kolobova, A. V.; Pankratov, V. V.; Efremova, O. V.; Musil, S.; Chromek, F.; Valkova, M.; Milton, M. J. T.; Vargha, G.; Guenther, F.; Miller, W. R.; Botha, A.; Tshilongo, J.; Mokgoro, I. S.; Leshabane, N. International comparison CCQM-K52: Carbon dioxide in synthetic air Metrologia 2008, 45 08011. 9 Lee, S.; Kim, M. E.; Oh, S. H.; Kim, J. S. Determination of physical adsorption loss of primary standard gas mixtures in cylinders using cylinder-to-cylinder division Metrologia 2017, 54, L26-L33 10 Allen, N. D. C.; Worton, D. R.; Brewer, P. J.; Pascale, C.; Niederhauser, B. The importance of cylinder passivation for preparation and long-term stability of multicomponent monoterpene primary reference materials Atmos. Meas. Tech. 2018, 11, 6429. 11 Rhoderick, G. C.; Miller, W. R.; Cecelski, C. E.; Worton, D. R.; Moreno, S.; Brewer, P. J.; Viallon. J.; Idrees, F.; Moussay, P.; Kim, Y. D.; Kim, D. H.; Lee, S.; Baldan, A.; Li, J. Stability of gaseous volatile organic compounds contained in gas cylinders with different internal wall treatment Elementa 2019, pending. 12 Viallon, J.; Flores, E.; Idrees, F.; Moussay, P.; Wielgosz, R. I.; Kim, D.; Kim, Y. D.; Lee, S.; Persijn, S.; Konopelko, L. A.; Kustikov, Y. A.; Malginov, A. V.; Chubchenko, I. K.; Klimov, A. Y.; Efremova, O. V.; Zhou, Z.; Possolo, A.; Shimosaka, T.; Brewer, P. J.; Mace, T. CCQM-K90, formaldehyde in nitrogen, 2 μmol mol−1 Final report Metrologia 2017, 54, 08029. 13 International Organisation for Standardisation 2015 ISO 6142−1 Gas analysis - Preparation of calibration gas mixtures - Part 1: Gravimetric method for Class I mixtures (Geneva: ISO) 14 Miller, W. R.; Rhoderick, G. C.; Guenther, F. R. 2015 Investigating Adsorption/Desorption of Carbon Dioxide in Aluminum Compressed Gas Cylinders Anal Chem 2015, 87, 1957-1962. 15 Brewer, P. J.; Brown, R. J. C.; Resner, K. V.; Hill-Pearce, R. E.; Worton, D. R.; Allen, N. D. C.; Blakley, K. C.; Benucci, D.; Ellison, M. R. Influence of Pressure on the Composition of Gaseous Reference Materials Anal Chem 2018, 90, 3490-3495. 16 Leuenberger, M. C.; Schibig, M. F.; Nyfeler, P. Gas adsorption and desorption effects on cylinders and their importance for long-term gas records Atmos. Meas. Tech. 2015, 8, 5289-5299. 17 Keeling, R. F.; Guenther, P. R.; Walker, S.; Moss, D. J. 2016 Scripps reference gas calibration system for carbon dioxide-in-nitrogen and carbon dioxide-in-air standards: revision of 2012; Scripps CO2 Program; Scripps Institution of Oceanography: La Jolla, CA 18 Zhao, C. L.; Tans, P. P. Estimating uncertainty of the WMO mole fraction scale for carbon dioxide in air J. Geo. Res. Atmos. 2006, 111, D08S09. 19 Milton, M. J. T.; Woods, P. T.; Holland, P. E. Uncertainty reduction due to correlation effects in weighing during the preparation of primary gas standards Metrologia 2002, 39, 97-99. 20 Milton, M. J. T.; Vargha, G. M.; Brown, A. S. Gravimetric methods for the preparation of standard gas mixtures Metrologia 2011, 48, R1-9. 21 Brewer, P. J.; Gieseking, B.; Ferracci, V. F.; Ward, M.; van Wijk, J.; van der Veen, A. M. H.; Lima, A. A.; Augusto, C. R.; Oh, S.H.; Kim, B. M.; Lee, S.; Konopelko, L. A.; Kustikov, Y.; Shimosaka, T.; Niederhauser, B.; Guillevic, M.; Pascale, C.; Zhou, Z.; Wang, D.; Hu, S. International comparison CCQM-K116: 10 μmol mol-1 water vapour in nitrogen Metrologia 2018, 55, 08018. 22 Brewer, P. J.; Minarro, M. D.; Di Meane, E. A.; Brown, R. J. C. A high accuracy dilution system for generating low concentration reference standards of reactive gases Measurement 2014, 47, 607.


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