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%.     

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     

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     

Use of calibration gases in the U.S. acid rain program

The United States Acid Rain Program continuous emission monitors (CEMs) have been successful in producing quality-assured data 95% of the time, and in meeting a relative accuracy standard of less than or equal to 10.0% at over 99% of the CEMs in the program. One key reason for this high accuracy is the required use of high quality calibration gases in certification and quality assurance/quality control (QA/QC) tests. An annual QA audit helps ensure high quality calibration gases. A third party purchases gases from gas vendors. An Environmental Protection Agency (EPA) laboratory analyzes the gases and compares the results with the tag value on the cylinder. The results are posted on an EPA website. This allows purchasers of calibration gases to buy gases from vendors producing the most accurate gases. Over time, we believe it also results in better accuracy from all gas vendors. Because of a change in SO₂ quantification methodology, SO₂ emissions were underreported by approximately 2% between 1989 and 1996. EPA, the National Institute for Standards and Technology and calibration gas vendors collaborated to produce a correction policy and a standard correction form to be used by affected electric utility plants. Calibration gas cylinder tag values were required to be corrected by 1 January, 1997. In the future, it is possible that cleaner, more varied sources will be regulated for greenhouse effect, ozone and toxic emissions control. This will probably require more accurate CEMs, lower calibration gas concentrations, and a broader menu of gas mixtures. Received: 23 December 1999 Accepted: 12 December 2000     

Traceability in routine chemical measurements: an example of application in the determination of CO2 at atmospheric concentration

In routine chemical measurements traceability can be achieved by using analytical instruments calibrated against primary reference materials. In the present work the calibration of a CO₂ non-dispersive infrared (NDIR) analyzer with measuring range 0–2000 μmol/mol of CO₂ and a resolution of 5 μmol/mol is reported. A procedure with working reference gas mixtures (WRMs) has been adopted, which requires seven calibration points. Primary reference gas mixtures (PRMs) are used to validate WRMs in a narrower range around the average atmospheric CO₂ concentration value. In this range the relative uncertainty reached is of the order of some parts in 10³ and the corrections are between 1 μmol/mol and 5 μmol/mol. Received: 16 March 2000 Accepted: 27 November 2000     

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.     

Development of membrane inlet mass spectrometry for examination of fermentation processes

Membrane inlet mass spectrometry (MIMS) is useful for on-line monitoring of fermentation processes. However, readings are affected by the complex and dynamic matrix in which biological processes occur, making MIMS calibration a challenge. In this work, two calibration strategies were evaluated for measurement of typical products of acidogenic fermentation, i.e., ethanol, H₂, and CO₂ in the liquid phase, and H₂ and CO₂ in the gas phase: (1) “standard calibration”, which was performed independent of fermentation experiments with sterile standards in water with a N₂ headspace, and (2) “in-process calibration” whereby fermentation was monitored concurrent with off-line analysis. Fermentation was operated in batch and continuous modes. In-process calibration was shown to be most effective for measurements of H₂ and CO₂ in both gas and liquid phases; standard calibration gave erroneous results. In the gas phase, this was due to a lower sensitivity during experiments compared to the independent standard calibration, believed to be caused by formation of a liquid film on the surface of the probe. In the liquid phase, moving from the standard calibration environment to the fermentation caused the linear relationship between the H₂ concentration and MIMS signal to change in intercept, and the relationship for CO₂ to change in slope, possibly due to dissolved ions, and related non-ideality. For ethanol, standard calibration results were fairly consistent with in-process calibration results. The main limitation with in-process calibration is the potential for a lack of variability in target concentration. This could be addressed by spiking the targeted compound at the end of the experiment. Regardless, MIMS is an ideal instrument for analysing fermentation experiments, due to its ability to measure targeted compounds semi-continuously, and due to a lack of drift over long periods.     

Calibration transfer strategy to compensate for instrumental drift in portable quadrupole mass spectrometers

We report the use of a calibration transfer strategy to correct for drift in the quantitative sensitivity of a portable quadrupole mass spectrometer (QMS) aimed at process monitoring applications. Gas mixtures of CH₄/Ar/C₂H₆/CO₂ were studied with calibration phase measurements made of the pure gases for a univariate analysis and of 40 multi-component mixtures for a multivariate approach. To evaluate calibrations, test set spectra of a CH₄/Ar/C₂H₆/CO₂ gas mixture were recorded bi-weekly over a period of 12 months. As part of the strategy a standard of pure argon was measured during both calibration and test phases so that correction factors could be calculated for each measurement day. It was shown that in the absence of a calibration transfer strategy quantifications of test set spectra could be inaccurate by more than an order of magnitude over 12 months. Furthermore, due to the effects of drift in the sensitivity over the 6 days required to record the training set in the calibration phase it was found that the multivariate analysis quantified test spectra less accurately than the univariate analysis. However, by applying the calibration transfer strategy across all measurements (both calibration and test phases) it was shown that the errors in prediction using the multivariate analysis previously seen after 2 weeks were not observed until approximately 12 months later.     

Preparing Accurate Standard Gas Mixtures of Volatile Substances at Low Concentration Levels

A simple approach for preparing standard gas mixtures of environmentally important volatile organic substances for gas chromatographic calibration is described. A liquid solution of the volatile substance of interest in a suitable solvent is prepared in a flask at known low concentration. Then, an easily measurable volume of this solution containing a very minute amount of the analyte is injected into a fixed-volume vessel (e.g., a glass sampling bulb) that has been flushed with a dilutent gas (e.g., air, N₂, He). The generated gas-phase concentration of the substance after evaporation is easily calculated. This method allows students to prepare standard gas-phase mixtures at very low concentrations by direct injection of extremely small amounts into solution using a reasonable size microsyringe.     

ICP-AES and Colorimetry as Analytical Tools for the Determination of Phosphorus Containing Compounds,Including Phosphine

Abstract

The direct quantitative determination of phosphine in gaseous mixtures or in air samples is associated with a number of difficulties. the paper describes an alternative technique based on the oxidation of phosphine to phosphate. Phosphate can be analysed either by inductively coupled plasma-atomic emission spectrometry (ICP-AES) or by a colometric method.

Because the problem of a proper (quantitative) sampling of phosphine in the air of working environments and/or in gaseous mixtures is still unsolved, standard solutions of KH₂PO₄ have been used as analytes (“artifical samples”) in the ICP-AES method.

The statistics concerning the sample variances, obtained from the 2 methods mentioned above, indicate no differences.     

Determination of total gas content and its composition in Indian PFBR blanket pellets

Total gas content and its composition are important specifications for sintered nuclear fuel pellets particularly in the case of fast breeder reactor fuels. Most commonly, total gas content and its composition is determined by hot vacuum extraction-quadrupole mass spectrometry (HVE-QMS). A number of parameters in this methodology such as temperature, duration of heating for quantitative extraction of evolved gases, total volume of the system, gas analysis conditions etc. need to be optimized for reliable measurements. In addition, sensitivity factors for various gases like H₂, CH₄, N₂, CO, O₂ and CO₂ in quadrupole mass spectrometry required for quantification of results have been determined and validated employing reference gas mixtures of known composition. Employing these optimized conditions total gas content and its composition in blanket pellets (uranium oxide pellets) of Indian prototype fast breeder reactor was determined employing HVE-QMS. The relative expanded uncertainty (at a coverage factor k = 2) in the measurement of total gas content excluding hydrogen was estimated as per ISO guidelines and it was found to be 9.2 %.     

Signal processing of GC-MS data of complex environmental samples: characterization of homologous series

Identification and characterization of homologous series by GC-MS analysis provide very relevant information on organic compounds in complex mixtures. A chemometric approach, based on the study of the autocovariance function, EACVF_tot, is described as a suitable tool for extracting molecular-structural information from the GC signal, in particular for identifying the presence of homologous series and quantifying the number of their terms. A data pre-processing procedure is introduced to transform the time axis in order to display a strictly homogenous retention pattern: n-alkanes are used as external standard to stretch or shrink the original chromatogram in order to build up a linear GC retention scale. This addition can be regarded as a further step in the direction of a signal processing procedure for achieving a systematic characterization of complex mixture from experimental chromatograms. The EACVF_tot was computed on the linearized chromatogram: if the sample presents terms of homologous series, the EACVF_tot plot shows well-defined deterministic peaks at repeated constant interdistances. By comparison with standard references, the presence of such peaks is diagnostic for the presence of the ordered series, their position can be related to the chemical structure of the compounds, their height is the basis for estimating the number of terms in the series. The power of the procedure can be magnified by studying SIM chromatograms acquired at specific m/z values characteristic of the compounds of interest: the EACVF_tot on these selective signals makes it possible to confirm the results obtained from an unknown mixture and check their reliability.The procedure was validated on standard mixtures of known composition and applied to an unknown gas oil sample. In particular, the paper focuses on the study of two specific classes of compounds: n-alkanes and oxygen-containing compounds, since their identification provides information useful for characterizing the chemical composition of many samples of different origin. The robustness of the method was tested in experimental chromatograms obtained under unfavorable conditions: chromatograms acquired in non-optimal temperature program conditions and chromatographic data affected by signal noise.     

Preparation of calibration materials for microanalysis of Ti minerals by direct fusion of synthetic and natural materials: Experience with LA–ICP–MS analysis of some important minor and trace elements in ilmenite and rutile

Calibration materials for microanalysis of Ti minerals have been prepared by direct fusion of synthetic and natural materials by resistance heating in high-purity graphite electrodes. Synthetic materials were FeTiO₃ and TiO₂ reagents doped with minor and trace elements; CRMs for ilmenite, rutile, and a Ti-rich magnetite were used as natural materials. Problems occurred during fusion of Fe₂O₃-rich materials, because at atmospheric pressure Fe₂O₃ decomposes into Fe₃O₄ and O₂ at 1462?°C. An alternative fusion technique under pressure was tested, but the resulting materials were characterized by extensive segregation and development of separate phases. Fe₂O₃-rich materials were therefore fused below this temperature, resulting in a form of sintering, without conversion of the materials into amorphous glasses. The fused materials were studied by optical microscopy and EPMA, and tested as calibration materials by inductively coupled plasma mass spectrometry, equipped with laser ablation for sample introduction (LA–ICP–MS). It was demonstrated that calibration curves based on materials of rutile composition, within normal analytical uncertainty, generally coincide with calibration curves based on materials of ilmenite composition. It is, therefore, concluded that LA–ICP–MS analysis of Ti minerals can with advantage be based exclusively on calibration materials prepared for rutile, thereby avoiding the special fusion problems related to oxide mixtures of ilmenite composition. It is documented that sintered materials were in good overall agreement with homogeneous glass materials, an observation that indicates that in other situations also sintered mineral concentrates might be a useful alternative for instrument calibration, e.g. as alternative to pressed powders.     

Steady-state electrode potentials of solid-electrolyte cells in reducing chemically nonequilibrium gas mixtures

The paper studies the steady-state potential response of the electrodes of solid-electrolyte cells with ZrO₂ + 9 mol % Y₂O₃ in reducing nonequilibrium gas mixtures of N₂ + H₂ + H₂O + O₂ with a low content of O₂ and in the mixtures of CH₄ + CO₂ + CO with a low content of CO, in the temperature range of 450–900°C. The dependences of potentials of the electrodes of different materials on the temperature and composition of gas mixtures were measured. The results were compared with the values of potentials in corresponding mixtures after chemical equilibrium was established. Characteristic temperature were determined, above which the electrodes feature a potential corresponding to chemically equilibrium mixtures.     

Hydrogen Reduction of MoF6 and Molybdenum Carbide Formation in RF Inductively Coupled Low-Pressure Discharge: Experiment and Equilibrium Thermodynamics Consideration

The physical plasma parameters, temperature and electron number density, are studied in the RF-IC (RF inductively coupled) discharge at a reduced pressure of 3 Torr in mixtures of MoF₆ with Ar, H₂ and CH₄. The emission spectra of mixtures are investigated. It is shown that in the presence of argon, the concentration of free electrons in plasma and dissociation rate of MoF₆ increase. A main role of molecular hydrogen is the generation of atomic hydrogen that binds atomic fluorine and leads to the formation of gaseous and solid products. Exhaust gas mixtures exiting the reactor are analyzed by mass spectrometry. It is shown that for all cases, the conversion of MoF₆ into reaction products is close to 100%. A thermodynamic analysis of the equilibrium composition of MoF₆ systems with Ar, H₂ and CH₄ was carried out and the obtained results are in good agreement with experimentally observed composition of the solid and gas phases. Analysis of solid deposits from mixture MoF₆/H₂/Ar revealed the presence of molybdenum powder and large amount of amorphous MoF_x. The deposit obtained from mixtures with methane, MoF₆/H₂/Ar/CH₄, contained crystalline molybdenum carbide, Mo₃C₂.

     

HovaCAL®—a generator for multi-component humid calibration gases

Process control, breath analysis for medical purpose or the investigation of biological samples are new applications of ion mobility spectrometry or differential mobility spectrometry coupled with rapid gas-chromatographic pre-separation. Especially if pre-concentration steps should be avoided, they require a realistic and flexible multi-compound calibration down to the ppt_V range including relative humidity values up to 100% for identification of analytes using mobility and retention time as well as for their quantification using the signal intensity as a measure. With HovaCAL® 3834SP-VOC, a novel calibration gas generator is presented that fulfils those requirements. The performance of HovaCAL® was validated for various compounds and mixtures with varying humidity comparing 3 particular equipments. Excellent results have been obtained with standard deviations of the provided concentrations of <8% and of <0.7% for the relative humidity range of 0–100%. Furthermore, standard deviation of the provided concentrations was <3% for varying experimental conditions. The long term stability of the provided concentrations for different instrumental parameters was proofed, standard deviations of <3% have been obtained. HovaCAL® enables for the first time a reliable calibration with complex humid mixtures down to the ppt_V range and—compared to permeation sources—a flexible and rapid change of compounds.     

Calibration of the X‐Ray Photoelectron Spectroscopy Binding Energy Scale for the Characterization of Heterogeneous Catalysts: Is Everything Really under Control?

Investigations of X‐ray photoelectron spectra from solid samples need corrections for the surface charging effect. For powder samples such as heterogeneous catalysts and their supports, the C ? (C,H) component of the C 1s peak is often used as an internal standard for the calibration of the binding energy scale. Although this method is widely recognized as suitable for the study of heterogeneous catalysts, we show that a significant calibration bias can be encountered upon comparing samples with different bulk composition. In this paper, a series of SiO₂–Al₂O₃ supports and Pd/SiO₂–Al₂O₃ catalysts with various Si/Al ratios were studied. The spectra issued from these samples were processed with the classical calibration method on the basis of the carbon peak. Important discrepancies in the relative position of the photoelectron peaks were noticed. After systematically discarding instrument‐related issues, a true chemical influence of the bulk matrix on the analyzed surface species was evidenced. The extent of this chemical effect was dependent on the composition of the sample and more precisely on its ionicity. Two possible mechanisms for this chemical effect were proposed and discussed. Finally, an alternative calibration method was offered.     

Improving the Selectivity of Semiconductor Gas Sensors Using the Pulse Adsorption Mode

A procedure is proposed for improving the selectivity of semiconductor gas sensors by the pulsed supply of the test gas mixture, with the use of transient spectroscopy for the separation and assessment of component contributions to the response of an electrical conductivity detector. The experimental verification of the procedure showed that the selective determination of several compounds and their mixtures can be performed using the same SnO₂ detector. The simulation of the detector response as the sum of simplest relaxations revealed three types of surface sites on SnO₂ films, with energy levels of 0.97, 1.38, and 1.50 eV. This means that the number of gas components that can be selectively determined with a single detector is limited to three.     

Calibration of a planar differential CO2 probe

The working mechanism of the differential CO₂ sensor inherently contains several calibration problems. These difficulties are highlighted and some possibilities for eliminating them are discussed. Finally, a simple error analysis is shown for optimizing the composition of the standard solutions to be used for the calibration of the differential CO₂ probe in blood electrolyte analyzers.     

Studies on Gas Purification by a Pulsed Microwave Discharge at 2.46 GHz in Mixtures of N2/NO/O2 at Atmospheric Pressure

Pulsed microwave discharges operated at atmospheric pressure in gas mixtures containing N₂, O₂, and NO are investigated experimentally and theoretically for various gas mixture constituents and operating conditions with respect to the ability of exhaust gas purification. The rotational gas temperature and the vibrational temperature of N₂ are derived from CARS measurements. The composition of the exhaust gas after treatment is monitored using FTIR spectroscopy. The processes of the chemical, electronic, and vibrational kinetics are described by a model that has been developed to calculate the species densities. The results obtained show that in N₂/NO gas mixtures an overall reduction of NO_x takes place. In the case of N₂/O₂/NO gas mixtures, no net reduction of NO_x is achieved for a pulsed microwave power below 3600 W, a pulse length of 50 s, and a typical repetition frequency of 2 kHz.