Lyn Gameson

Methane Standards Made in Whole and Synthetic Air Compared by Cavity Ring Down Spectroscopy and Gas Chromatography with Flame Ionization Detection for Atmospheric Monitoring Applications

There is evidence that the use of whole air versus synthetic air can bias measurement results when analyzing atmospheric samples for methane (CH4) and carbon dioxide (CO2). Gas chromatography with flame ionization detection (GC-FID) and wavelength scanned-cavity ring down spectroscopy (WS-CRDS) were used to compare CH4 standards produced with whole air or synthetic air as the matrix over the mole fraction range of 1600-2100 nmol mol(-1). GC-FID measurements were performed by including ratios to a stable control cylinder, obtaining a typical relative standard measurement uncertainty of 0.025%. CRDS measurements were performed using the same protocol and also with no interruption for a limited time period without use of a control cylinder, obtaining relative standard uncertainties of 0.031% and 0.015%, respectively. This measurement procedure was subsequently used for an international comparison, in which three pairs of whole air standards were compared with five pairs of synthetic air standards (two each from eight different laboratories). The variation from the reference value for the whole air standards was determined to be 2.07 nmol mol(-1) (average standard deviation) and that of synthetic air standards was 1.37 nmol mol(-1) (average standard deviation). All but one standard agreed with the reference value within the stated uncertainty. No significant difference in performance was observed between standards made from synthetic air or whole air, and the accuracy of both types of standards was limited only by the ability to measure trace CH4 levels in the matrix gases used to produce the standards.

Preparation of accurate, low-concentration gas cylinder standards by cryogenic trapping of a permeation tube gas stream.

National and international measurements are underpinned by accurate, low concentration standards. These standards are typically produced gravimetrically, or volumetrically, by a series of dilutions of the pure material by the balance gas. This blend technique is time-consuming and may involve the handling of pure, hazardous material. These problems have been overcome by developing a novel blend technique whereby the process gas stream, from an appropriate permeation tube, was cryogenically trapped in an aluminum cylinder. The permeation rate of the component is monitored by real time mass determinations using a magnetic suspension balance system. With the combination of the real-time calculated permeation rate, plus the use of a dilution system, a one step production of a very low concentration of the minor component in nitrogen gas can be achieved. This method was used to prepare low μmol/mol standards of propane, a known stable compound. Analysis of a conventional gravimetrically prepared 10 μmol/mol propane standard and a cryogenically prepared standard via a permeation gas stream resulted in agreement between the two of <0.1% at 10 μmol/mol, confirming the accuracy of the permeation method. After confirmation of the validity of the permeation/cryogenic trapping system, the propane permeation tube was replaced with a methyl mercaptan tube (a toxic, reactive compound) in balance nitrogen. After cryogenically trapping the methyl mercaptan output stream from the permeation system into a cylinder, the output stream and the cylinder gas mixture were analyzed. The results showed agreement of <0.6% for methyl mercaptan at 5, 10, 15, and 20 μmol/mol to the expected blend concentration, thereby demonstrating the validity of the method.

Optical Measurement of Radiocarbon below Unity Fraction Modern by Linear Absorption Spectroscopy

High-precision measurements of radiocarbon (14C) near or below a fraction modern 14C of 1 (F14C ≤ 1) are challenging and costly. An accurate, ultrasensitive linear absorption approach to detecting 14C would provide a simple and robust benchtop alternative to off-site accelerator mass spectrometry facilities. Here we report the quantitative measurement of 14C in gas-phase samples of CO2 with F14C < 1 using cavity ring-down spectroscopy in the linear absorption regime. Repeated analysis of CO2 derived from the combustion of either biogenic or petrogenic sources revealed a robust ability to differentiate samples with F14C < 1. With a combined uncertainty of 14C/12C = 130 fmol/mol (F14C = 0.11), initial performance of the calibration-free instrument is sufficient to investigate a variety of applications in radiocarbon measurement science including the study of biofuels and bioplastics, illicitly traded specimens, bomb dating, and atmospheric transport.

High-accuracy 12 C 16 O 2 line intensities in the 2 µm wavelength region measured by frequency-stabilized cavity ring-down spectroscopy

Reported here are highly accurate, experimentally measured ro-vibrational transition intensities for the R-branch of the (20012) - (00001) 12C16O2 band near λ = 2 μm. Measurements were performed by a frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) instrument designed to achieve precision molecular spectroscopy in this important region of the infrared. Through careful control and traceable characterization of CO2 sample conditions, and through high-fidelity measurements spanning several months in time, we achieve relative standard uncertainties for the reported transition intensities between 0.15 % and 0.46 %. Such high accuracy spectroscopy is shown to provide a stringent test of calculated potential energy and ab initio dipole moment surfaces, and therefore transition intensities calculated from first principles.

Final report on international comparison CCQM-K71: Measurement of stack gas

Industrial stack gas emission measurements are important for process control, control of air pollution, and for implementing legislation regarding carbon dioxide emission rights. Measurements are typically performed using a range of process analysers for carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOx), sulphur dioxide (SO2) and miscellaneous hydrocarbons. The calibration of these analysers is often performed using a series of binary mixtures of each component in nitrogen. For reasons of efficiency as well as a better match with true stack gas, the use of multi-component mixtures for this purpose would be preferred. The aim of this key comparison is to evaluate the measurement capabilities of national metrology institutes for carbon monoxide, carbon dioxide, nitrogen monoxide, sulphur dioxide and propane in nitrogen. Ten laboratories participated in the key comparison and one in the associated study. The key comparison reference value is based on the gravimetric preparation data. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Issues with analyzing noble gases using gas chromatography with thermal conductivity detection

AbstractThe noble gases, namely neon, argon, krypton and xenon, have many uses including in incandescent and gas discharge lighting, in plasma televisions, shielding gas in welding, in lasers for surgery and semiconductors, and in magnetic resonance imaging (MRI) of the lungs. When incorporating these noble gases in industries, especially the medical field, it is important to know accurately the composition of the noble gas mixture. Therefore, there is a need for accurate gas standards that can be used to determine the noble gas amount-of-substance fraction in the appropriate mixture application. A recent comparison of mixtures containing four noble gases in a helium balance showed mixed results among National Metrology Institutes. Significant differences, 0.7 to 3.8% relative, were seen in the analytical amount-of-substance assignments versus the gravimetric value of the noble gases in the comparison mixture when using “binary standards”, i.e. neon in helium, argon in helium and krypton in helium, as applied by the National Institute of Standards and Technology. Post-comparison studies showed that when all four noble gases were included in the standards, the agreement between analytical and gravimetric values was within 0.05% relative. Further research revealed that different carrier gases (hydrogen, helium and nitrogen) resulted in varying differences between the analytical and gravimetric values assignments. This paper will discuss the findings of these analytical comparisons. Graphical abstractᅟ

NIST Standards for Measurement, Instrument Calibration, and Quantification of Gaseous Atmospheric Compounds

There are many gas phase compounds present in the atmosphere that affect and influence the earths climate. These compounds absorb and emit radiation, a process which is the fundamental cause of the greenhouse effect. The major greenhouse gases in the earths atmosphere are carbon dioxide, methane, nitrous oxide, and ozone. Some halocarbons are also strong greenhouse gases and are linked to stratospheric ozone depletion. Hydrocarbons and monoterpenes are precursors and contributors to atmospheric photochemical processes, which lead to the formation of particulates and secondary photo-oxidants such as ozone, leading to photochemical smog. Reactive gases such as nitric oxide and sulfur dioxide are also compounds found in the atmosphere and generally lead to the formation of other oxides. These compounds can be oxidized in the air to acidic and corrosive gases and contribute to photochemical smog. Measurements of these compounds in the atmosphere have been ongoing for decades to track growth rates and assist in curbing emissions of these compounds into the atmosphere. To accurately establish mole fraction trends and assess the role of these gas phase compounds in atmospheric chemistry, it is essential to have good calibration standards. The National Institute of Standards and Technology has been developing standards of many of these compounds for over 40 years. This paper discusses the development of these standards.

Final report on CCQM-K93: Preparative comparison of ethanol in nitrogen

This report presents the results of CCQM-K93, a key comparison between 13 National Measurement Institutes (NMIs), which tested the capability of the NMIs to prepare standard gas mixtures of ethanol at a nominal amount fraction of 120 µmol/mol in nitrogen. This composition is typical of the levels used to calibrate evidential breath analysers in many countries. Such standards fulfill the agreed requirements of the International Organization of Legal Metrology (OIML) for the calibration of evidential breath-alcohol analysers. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).