Effect of variation in argon content of calibration gases on determination of atmospheric carbon dioxide

Carbon dioxide (CO₂) is a greenhouse gas that makes by far the largest contribution to the global warming of the Earth's atmosphere. For the measurements of atmospheric CO₂ a non-dispersive infrared analyzer (NDIR) and gas chromatography are conventionally being used. We explored whether and to what degree argon content can influence the determination of atmospheric CO₂ using the comparison of CO₂ concentrations between the sample gas mixtures with varying Ar amounts at 0 and 18.6 mmol mol⁻¹ and the calibration gas mixtures with Ar at 8.4, 9.1, and 9.3 mmol mol⁻¹. We newly discovered that variation of Ar content in calibration gas mixtures could undermine accuracy for precise and accurate determination of atmospheric CO₂ in background air. The differences in CO₂ concentration due to the variation of Ar content in the calibration gas mixtures were negligible (<±0.03 μmol mol⁻¹) for NDIR systems whereas they noticeably increased (<±1.09 μmol mol⁻¹) especially for the modified GC systems to enhance instrumental sensitivity. We found that the thermal mass flow controller is the main source of the differences although such differences appeared only in the presence of a flow restrictor in GC systems. For reliable monitoring of real atmospheric CO₂ samples, one should use calibration gas mixtures that contain Ar content close to the level (9.332 mmol mol⁻¹) in the ambient air as possible. Practical guidelines were highlighted relating to selection of appropriate analytical approaches for the accurate and precise measurements of atmospheric CO₂. In addition, theoretical implications from the findings were addressed.     

Continuous dynamic-gravimetric preparation of calibration gas mixtures for air pollution measurements

 The preparation of calibration gas mixtures for air pollution measurements by the dynamic-gravimetric method was investigated using sulphur dioxide in nitrogen as a model. The target mole fraction was 200×10^–9 mol/mol, with the option of also getting smaller mole fractions. Thermal mass flow meters calibrated with reference mass flows were used to measure the dilution gas flow (nitrogen). The relative standard uncertainty of the dilution gas flows between 10 mg/s (approx. 500 ml/min) and 40 mg/s (approx. 2000 ml/min) was 0.15%. The mass flow of the target component measured as the permeation rate was determined via the quasi-continuous observation of the loss in the permeation tube mass during the measuring time. A magnetic coupling system and an adapted microbalance were used for this purpose. The results presented show permeation rates measured over the lifetime of a tubular permeation source. The measurement cycles took between 3 days and 7 h at least. The relative standard uncertainty of the mixture composition did not exceed 2%. First comparisons with gas mixtures prepared by the static-gravimetric method show compatibility. The applicability of the system is not restricted to the SO₂/N₂ mixture. It can also be used for preparing other gas mixtures in this field of application. Received: 26 April 2000 / Accepted: 12 September 2000     

A Uniformly Oriented MFI Membrane for Improved CO2 Separation

Membrane separation of CO₂ from natural gas, biogas, synthesis gas, and flu gas is a simple and energy‐efficient alternative to other separation techniques. But results for CO₂‐selective permeance have always been achieved by randomly oriented and thick zeolite membranes. Thin, oriented membranes have great potential to realize high‐flux and high‐selectivity separation of mixtures at low energy cost. We now report a facile method for preparing silica MFI membranes in fluoride media on a graded alumina support. In the resulting membrane straight channels are uniformly vertically aligned and the membrane has a thickness of 0.5 μm. The membrane showed a separation selectivity of 109 for CO₂/H₂ mixtures and a CO₂ permeance of 51×10^?7 mol m^?2 s^?1 Pa^?1 at ?35 °C, making it promising for practical CO₂ separation from mixtures.     

A system for high‐quality CO2 isotope analyses of air samples collected by the CARIBIC Airbus A340‐600

In 2007, JRC‐IRMM began a series of atmospheric CO₂ isotope measurements, with the focus on understanding instrumental effects, corrections as well as metrological aspects. The calibration approach at JRC‐IRMM is based on use of a plain CO₂ sample (working reference CO₂) as a calibration carrier and CO₂‐air mixtures (in high‐pressure cylinders) to determine the method‐related correction under actual analytical conditions (another calibration carrier, in the same form as the samples). Although this approach differs from that in other laboratories, it does give a direct link to the primary reference NBS‐19‐CO₂. It also helps to investigate the magnitude and nature for each of the instrumental corrections and allows for the quantification of the uncertainty introduced. Critical tests were focused on the instrumental corrections. It was confirmed that the use of non‐symmetrical capillary crimping (an approach used here to deal with small samples) systematically modifies δ¹³C(CO₂) and δ¹⁸O(CO₂), with a clear dependence on the amount of extracted CO₂. However, the calibration of CO₂‐air mixtures required the use of the symmetrical dual‐inlet mode. As a proof of our approach, we found that δ¹³C(CO₂) on extracts from mixtures agreed (within 0.010‰) with values obtained from the ‘mother’ CO₂ used for the mixtures. It was further found that very low levels of hydrocarbons in the pumping systems and the isotope ratio mass spectrometry (IRMS) instrument itself were critical. The m/z 46 values (consequently the calculated δ¹⁸O(CO₂) values) are affected by several other effects with traces of air co‐trapped with frozen CO₂ being the most critical. A careful cryo‐distillation of the extracted CO₂ is recommended. After extensive testing, optimisation, and routine automated use, the system was found to give precise data on air samples that can be traced with confidence to the primary standards. The typical total combined uncertainty in δ¹³C(CO₂) and δ¹⁸O(CO₂) on the VPDB‐CO₂ scale, estimated on runs of CO₂‐air mixtures, is ±0.040‰ and 0.060‰ (2‐σ values). Inter‐comparison with MPI‐BGC resulted in a scale discrepancy of a similar magnitude. Although the reason(s) for this discrepancy still need to be understood, this basically confirms the approach of using specifically prepared CO₂‐air mixtures as a calibration carrier, in order to achieve scale unification among laboratories. As important practical application and as a critical test, JRC‐IRMM took part in the passenger aircraft‐based global monitoring project CARIBIC ( http://www.caribic‐atmospheric.com ). In this way, reliable CO₂ isotope data for the tropopause region and the free troposphere were obtained. From June 2007 to January 2009, approximately 500 CARIBIC air samples have been analysed. Some flights demonstrated a compact correlation of both δ¹³C(CO₂) and δ¹⁸O(CO₂) with respect to CO₂ concentration, demonstrating mixing of tropospheric and stratospheric air masses. These excellent correlations provide an independent, realistic data quality check. Copyright © 2009 John Wiley & Sons, Ltd.     

External calibration in gas chromatography-combustion-isotope ratio mass spectrometry measurements of endogenous androgenic anabolic steroids in sports doping control

An alternative calibration procedure for the Gas Chromatography–Combustion–Isotope Ratio Mass Spectrometry (GC–C–IRMS) measurements of the World Antidoping Agency (WADA) Accredited Laboratories is presented. To alleviate the need for externally calibrated CO₂ gas for GC–C–IRMS analysis of urinary steroid metabolites, calibration using an external standard mixture solution of steroids with certified isotopic composition was investigated. The reference steroids of the calibration mixture and routine samples underwent identical instrumental processes. The calibration standards bracketed the entire range of the relevant δ¹³C values for the endogenous and exogenous steroids as well as their chromatographic retention times. The certified δ¹³C values of the reference calibrators were plotted in relation to measured m/z¹³CO₂/¹²CO₂ (i.e. R(45/44)) mass spectrometric signals of each calibrator. δ¹³C values of the sample steroids were calculated from the least squares fit through the calibration curve. The effect of the external calibration on δ¹³C values, using the same calibration standards and set of urine samples but different brands of GC–C–IRMS instruments, was assessed by an interlaboratory study in the WADA Accredited Laboratories of Sydney, Australia and Athens, Greece. Relative correspondence between the laboratories for determination of androsterone, etiocholanolone, 5β-androstane-3α,17β-diacetate, and pregnanediacetate means were SD(δ¹³C) = 0.12‰, 0.58‰, −0.34‰, and −0.40‰, respectively. These data demonstrate that accurate intralaboratory external calibration with certified steroids provided by United States Antidoping Agency (USADA) and without external CO₂ calibration is feasible and directly applicable to the WADA Accredited Laboratories for the harmonization of the GC–C–IRMS measurements.     

Ni(II)cyclam Catalyzed Reduction of CO2 – Towards a Voltammetric Sensor for the Gas Phase

The detection of CO₂ in the gas phase is possible in presence of oxygen with an amalgamated Au‐poly(tetrafluoroethylene) gas diffusion electrode and an internal electrolyte solution containing Ni(II)cyclam. For concentrations between 0.1 to 1% the electrochemical cell has a sensitivity of 3.58 mA %^?1 and the detection limit is 500 ppm. In preliminary experiments at rotating disk electrodes the optimum pH‐range was found to be between 3.5 to 6 and a selectivity ratio of the catalyst for CO₂/H⁺ of 5 : 1 could be determined. The relationship between reduction current and the square root of the angular speed is linear, indicating that the electrochemical process is limited by diffusion of CO₂. Tl and Pb are presented as alternative electrode materials at which the Ni(II)cyclam catalyzed reduction of CO₂ can be observed. Problems arise from fouling effects at the sensing electrode and a non‐linearity of the calibration plot at higher concentrations.     

The Electrocatalytic Oxidation of Thymine at α‐Cyclodextrin Incorporated Carbon Nanotube‐Coated Electrode

Electrocatalytic oxidation of thymine at α‐cyclodextrin (α‐CD) incorporated carbon nanotube‐coated electrode (CNT/CE) was thoroughly studied in alkaline media. CNT showed an electrocatalytic effect on the oxidation of thymine, formation of a supramolecular inclusion complex between α‐CD and thymine at CNT/CE further enhanced the sensitivity to thymine. The electrocatalytic behavior was further developed as a sensitive detection scheme for thymine by differential pulse voltammetry. A linear calibration over the concentration range from 2.5×10^?5 to 1.8×10^?3 mol/L in pH 10.8 NaHCO₃‐Na₂CO₃ buffer solution was obtained with a detection limit of 5×10^?6 mol/L.     

Comparison between hydride generation and nebulization for sample introduction in the determination of lead in plants and water samples by inductively coupled plasma mass spectrometry,using external calibration and isotope dilution

Four inductively coupled plasma mass spectrometric methods: nebulization sample introduction with external calibration; hydride generation with external calibration; isotope dilution with nebulization; and isotope dilution with hydride generation, have been tested and compared. Multimode Sample Introduction System (MSIS™) was employed in either nebulization or hydride generation mode. Best limits of detection (below 0.1 μg L^− 1) and accuracy were obtained for isotope dilution techniques in hydride generation and sample nebulization mode. A mixture of HNO₃ and H₂O₂ served both for microwave-assisted digestion as well as a medium for subsequent plumbane generation. Optimal reagent concentrations for hydride generation stage were 0.1 mol L^− 1 HNO₃, 0.28 mol L^− 1 H₂O₂ and 1.5% m/v NaBH₄. Critical effects of acidity, blanks and concomitants have been discussed. Analytical methods were validated by use of plant and water certified reference materials and spiked high-salt solutions (seawater and 20% m/v NaCl) at lead levels in nanograms per gram to micrograms per gram range.     

Determination of carbonate in the presence of hydroxide: Part 2. Evidence for the existence of a novel species from first-derivative potentiometric titration curves

An additional maximum has been detected in the differential potentiometric titration of mixtures of sodium hydroxide and sodium carbonate with hydrochloric acid using pH-responsive glass electrodes. This maximum is not detected under the following conditions: if ethanol (mol fraction >0.2) or t-butanol (mol fraction >0.1) is added; when the temperature is raised above 55°C; when hydrogen gas is bubbled through the solution; if a hydrogen gas electrode is used instead of the glass electrode; when carbonic anhydrase is added. The additional feature is ascribed to a new species HCO₃·H₂CO₃^- formed in solutions by a slow reaction.     

MOF Encapsulated AuPt Bimetallic Nanoparticles for Improved Plasmonic-induced Photothermal Catalysis of CO2 Hydrogenation

Exploring new catalytic strategies for achieving efficient CO₂ hydrogenation under mild conditions is of great significance for environmental remediation. Herein, a composite photocatalyst Zr-based MOF encapsulated plasmonic AuPt alloy nanoparticles (AuPt@UiO-66-NH₂) was successfully constructed for the efficient photothermal catalysis of CO₂ hydrogenation. Under light irradiation at 150 °C, AuPt@UiO-66-NH₂ achieved a CO production rate of 1451 μmol g_metal⁻¹ h⁻¹ with 91 % selectivity, which far exceeded those obtained by Au@Pt@UiO-66-NH₂ with Pt shell on Au (599 μmol g_metal⁻¹ h⁻¹) and Au@UiO-66-NH₂ (218 μmol g_metal⁻¹ h⁻¹). The outstanding performances of AuPt@UiO-66-NH₂ were attributed to the synergetic effect originating from the plasmonic metal Au, doped active metal Pt, and encapsulation structure of UiO-66-NH₂ shell. This work provides a new way for photothermal catalysis of CO₂ and a reference for the design of high-performance plasmonic catalysts.     

Characterization of the natural inorganic colloids from a reference granitic ground water

In order to achieve a better understanding of the potential transport of radionuclide pseudo-colloids, characterization of natural colloids from a reference granitic ground water was studied. The methods included pulsed ultrafiltration under a defined atmosphere of gas or gas mixtures (N₂ and CO₂) and examination of the colloids on the dry filter or in a concentrate after resuspension in a little double-distilled water. The filters were examined by means of scanning electron microscopy and x-ray spectrometry. Colloidal concentrates were examined by atomic absorption spectrometry (Si, Al, Ca) or, after evaporation on a sample carrier, by transmission electron microscopy/x-ray spectrometry. On the basis of average data sets for granitic ground water (density, pH, ionic composition, etc.), saturation indices can be predicted by using the MINEQL chemical speciation model for the reference ground water. These calculations indicate that in underground conditions (P(CO₂) = 0.1 atm.) Silica (quartz) and barium sulfate should precipitate; under “surface” conditions (P(COP₂) = 10^?3.5 atm.), SiO₂ (quartz), BaSO₄ and CaCO₃ should precipitate. In underground conditions, poorly crystallized silica (20-nm diameter) agglomerated in 100–1000-nm aggregates, was found by transmission or scanning electron microscopy. To a lesser extent, Ca associated with Si and Ba (with S) were also detected. In the surface conditions, calcite was found as colloidal material (<1 μm) and as larger particles (>1 μm).     

Preparation and characterisation of synthetic mixtures of lithium isotopes

Re-certification of the absolute isotopic composition of the natural lithium isotopic reference material (IRM), IRMM-016, requires measurements calibrated by means of synthetic mixtures of highly enriched lithium isotopes. Ten such mixtures were prepared by weighing and mixing of two well characterised, isotopically enriched, Li₂CO₃ compounds. The starting materials, 99.9981% enriched ⁶Li, and 99.9937% enriched ⁷Li, were purified by ion exchange, and the purified materials converted from LiOH to Li₂CO₃ by reaction with CO₂. Ten new mixtures were prepared by mixing different weighed amounts of these dissolved Li₂CO₃ carrier compounds. The compounds had an estimated level of impurities of 100 ± 100 μg · g^–1 (expanded uncertainty with a coverage factor of 2). In the ten mixtures, the n(⁶Li)/n(⁷Li) ratio varies from 0.025 to 14 and the achieved expanded relative uncertainty on the amount ratio prepared is typically 2 · 10^–4. These mixtures were then used to determine the correction factor, K, for mass discrimination of the measurement procedure and instrument concerned.     

Fourier transform infrared determination of CO2 evolved from carbonate in carbonated apatites

A quantitative method based on FTIR has been developed to determine carbonate in synthetic apatites. The method measures the evolved CO₂ after reaction of 50 mg apatite with 2 mL of hydrochloric acid (0.5 M) in a reaction vessel, heated to 40?°C. The CO₂ evolved was swept by a carrier of nitrogen to a laboratory-made infrared gas cell of 39 mm pathlength and 490 μL volume. The signals were recorded as a function of time and the areas of the chemigram peaks obtained from the measurements in the wavenumber range of 2500–2150 cm^–1, were interpolated using a calibration curve. The method can be used to study apatites with carbonate contents below 0.2% with a sampling frequency of 8 h^–1.     

Theoretical aspects of quantitative mass spectrometry of gas mixtures

The problem of calibration of mass spectrometers with standard gas mixtures is studied theoretically. The results obtained can be used in deciding the number and optimum composition of standard mixtures. An analogy is drawn between this calibration problem and the mathematical theory of experimental design when mixtures are considered. It is shown that calibration based on a number of standard mixtures is more accurate than calibration with pure gases. A procedure for correction of calibration coefficients is described; it can be applied during measurements on the composition of gas mixtures or gas flows. Application to gas mixtures containing CO, N₂ and CO₂ is discussed.     

Highly accurate gas chromatographic analysis of reference gases for use in vehicle emission measurements

 A highly accurate gas chromatographic analytical method has been developed for the determination of the composition of gas mixtures. It was tested using a reference gas as an example consisting of 3.5% of CO, 14% of CO₂, 0.2% of propane and residual N₂ intended for the use in vehicle emission measurements. The method is based on comparison measurements with samples of a calibration gas, whose composition is iteratively adapted to that of the sample investigated using a gravimetric gas mixing method. For the gas chromatographic measurement, a molecular sieve column and a polymer column are used in parallel and in isothermal operation. All gas components can be determined by a single gas chromatographic measurement, and the relative uncertainty of measurement achievable is ?0.4%. Received: 6 March 1996/Revised: 30 May 1996/Accepted: 6 June 1996     

Sensitive and selective determination of mercury by differential pulse stripping voltammetry after accumulation of mercury vapour on a gold plated graphite electrode

 A selective and sensitive method is proposed for the determination of mercury by anodic stripping voltammetry after its preconcentration from the gas phase. Mercury from the sample solution is reduced to elemental Hg by SnCl₂ and volatilized by the bubbles of a carrier gas. The gas containing mercury vapour is dried and passed through a capillary onto a gold coated graphite electrode. An anodic stripping voltammogram is recorded from 0.1 mol/l HClO₄+3×10^-3 mol/l HCl solution. The calibration curve is linear from 1×10^-9 to 4×10^-8 mol/l Hg(NO₃)₂. The absolute detection limit is 0.46 ng Hg. The relative standard deviations for 4×10^-9 mol/l and 2×10^-8 mol/l Hg(NO₃)₂ are 9.8% and 6.1%, respectively (n=5). Received: 18 December 1995/Revised: 16 April 1996/Accepted: 20 April 1996     

Pulsed TEA CO2 Laser Irradiation of Titanium in Nitrogen and Carbon Dioxide Gases

Surface changes created by interaction of transversely excited atmospheric carbon dioxide (TEA CO₂) laser with titanium target/implant in nitrogen and carbon dioxide gas were studied. TEA CO₂ laser operated at 10.6 μm, pulse length of 100 ns and fluence of ∼17 J/cm² which was sufficient for inducing surface modifications. Induced changes depend on the gas used. In both gases the grain structure was produced (central irradiated zone) but its forms were diverse, (N₂: irregular shape; CO₂: hill-like forms). Hydrodynamic features at peripheral zone, like resolidified droplets, were recorded only in CO₂ gas. Elemental analysis of the titanium target surface indicated that under a nitrogen atmosphere surface nitridation occurred. In addition, irradiation in both gases was followed by appearance of plasma in front of the target. The existence of plasma indicates relatively high temperatures created above the target surface offering a sterilizing effect.

     

Theoretical Investigations on MIL-100(M) (M=Cr,Sc, Fe) with High Adsorption Selectivity for Nitrogen and Carbon Dioxide over Methane

The removal of impurity gases (N₂, CO₂) in natural gas is critical to the efficient use of natural gas. In this work, the selective adsorption for N₂ and CO₂ over CH₄ on MIL-100 (M) (M=⁴Cr, ¹⁰Cr, ⁶Fe, ¹In, ¹Sc, ³V) is studied by density functional theory (DFT) calculations. The calculated adsorption energy of the large-size cluster model (LC) of MIL-100 (M) shows that the ⁴MIL-100 (⁴Cr) is the best at the refinement of natural gas due to the lower adsorption energy of CH₄ (−2.58 kJ/mol) in comparison with that of N₂ (−21.49 kJ/mol) and CO₂ (−23.82 kJ/mol). ¹MIL-100 (¹Sc) and ¹MIL-100 (⁶Fe) can also achieve selective adsorption and follows the order ⁴MIL-100 (⁴Cr)>¹MIL-100 (¹Sc)>¹MIL-100 (⁶Fe). In the research of the selective adsorption mechanism of MIL-100 (M) (M=⁴Cr, ¹Sc, ⁶Fe), the independent gradient model (IGM) indicates that these outstanding adsorbents interact with CO₂ and N₂ mainly through the electrostatic attractive interaction, while the van der Walls interaction dominates in the interaction with CH_4. The atomic Projected Density of State (PDOS) further confirms that CH₄ contributes least to the intermolecular interaction than that of CO₂ and N₂. Through the scrutiny of molecular orbitals, it is found that electrons transfer from the gas molecule to the metal site in the adsorption of CO₂ and N₂. Not only does the type of the metallic orbitals, but also the delocalization of the involved orbitals determines the selective adsorption performance of MIL-100. Both Cr and Sc share their orbitals with the gases, making ¹MIL-100 (¹Sc) another potential effective separator for CH₄. Additionally, the comparison of adsorption energy and PDOS shows that the introduction of ligands such as benzene impedes the electron donation from gas molecules (CO₂, N₂) to the metal site, indicating electron-withdrawing ligands will further favor the adsorption.     

Cost-effective and Facile Production of a Phosphorus-doped Graphite Electrode for the Electrochemical Determination of Pyridoxine

In this study; a sensitive, selective, and simple electrochemical sensor was developed to determine low concentration pyridoxine (Py) using a phosphorus-doped pencil graphite electrode (P-doped/PGE). Electrode modification was implemented using the chronoamperometry method at +2.0 V constant potential and 100 seconds in 0.1 mol L⁻¹ H₃PO₄ supporting electrolyte solution. The characterization processes of the P-doped/PGE were carried out using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), and atomic force microscope (AFM) methods. In the concentration study, using the differential pulse voltammetry (DPV) method, a linear calibration plot was acquired in the concentration range of 0.5 to 300 μmol L⁻¹ Py. The limit of quantification (LOQ) and limit of detection (LOD) of the developed method were calculated as 0.219 μmol L⁻¹ and 0.0656 μmol L⁻¹, respectively. Detection of Py has been successfully performed on the P-doped/PGE in the beverage samples. As a result, the method developed has been shown to have fast, low cost, and simple for the sensitive and selective detection of Py as an effective electrode.     

Preparation and characterisation of synthetic mixtures of lithium isotopes

Re-certification of the absolute isotopic composition of the natural lithium isotopic reference material (IRM), IRMM-016, requires measurements calibrated by means of synthetic mixtures of highly enriched lithium isotopes. Ten such mixtures were prepared by weighing and mixing of two well characterised, isotopically enriched, Li₂CO₃ compounds. The starting materials, 99.9981% enriched ⁶Li, and 99.9937% enriched ⁷Li, were purified by ion exchange, and the purified materials converted from LiOH to Li₂CO₃ by reaction with CO₂. Ten new mixtures were prepared by mixing different weighed amounts of these dissolved Li₂CO₃ carrier compounds. The compounds had an estimated level of impurities of 100 ± 100 μg · g^–1 (expanded uncertainty with a coverage factor of 2). In the ten mixtures, the n(⁶Li)/n(⁷Li) ratio varies from 0.025 to 14 and the achieved expanded relative uncertainty on the amount ratio prepared is typically 2 · 10^–4. These mixtures were then used to determine the correction factor, K, for mass discrimination of the measurement procedure and instrument concerned. Received: 6 November 1997 / Revised: 22 December 1997 / Accepted: 24 January 1998