Primary Standard Mixtures Research Articles

International comparison CCQM-K117 ammonia

Main textAt the highest metrological level, ammonia gas standards are commonly prepared gravimetrically as Primary Standard Mixtures or using a dynamic standard. This international key comparison addressed the measurement of the amount fraction ammonia in nitrogen, to support calibration and measurement services for mainly environmental applications. The nominal amount fraction was 14 μmol mol-1. The results in this Track C key comparison on the composition of biogas are generally good.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 https://www.bipm.org/kcdb/.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).

International comparison CCQM-K112 biogas

At the highest metrological level, biogas standards are commonly prepared gravimetrically as PSMs (primary standard mixtures). This international key comparison addresses the composition of biogas, to support calibration and measurement services for this renewable energy gas. The mixtures contain nitrogen, carbon dioxide, methane, ethane, propane, hydrogen and oxygen and represent the composition of biogas from landfills. The results in this Track C key comparison on the composition of biogas are generally good. Some of the datasets, especially that of oxygen, showed substantial extra dispersion, that could not be explained by the stated uncertainties.Main textTo 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).

Preparation of primary standard mixtures for atmospheric oxygen measurements with less than 1 µmol mol−1 uncertainty for oxygen molar fractions

Abstract. Precise monitoring of changes in atmospheric O2 levels was implemented by preparing primary standard mixtures with less than 1 µmol mol−1 standard uncertainty for O2 molar fractions. In this study, these mixtures were crafted in 10 L high-pressure aluminium alloy cylinders using a gravimetric method in which unknown uncertainty factors were theoretically determined and subsequently reduced. Molar fractions of the constituents (CO2, Ar, O2, and N2) in the primary standard mixtures were mainly resolved using masses of the respective source gases (CO2, Ar, O2, and N2) that had been filled into the cylinders. To precisely determine the masses of the source gases, the difference in mass of the cylinder before and after filling the respective source gas was calculated by comparison with an almost identical reference cylinder. Although the masses of the cylinders filled with the source gas with respect to the reference cylinder tended to deviate in relation to temperature differences between the source-gas-filled cylinder and surrounding air, the degree of the deviation could be efficiently reduced by measuring the two cylinders at the exact same temperature. The standard uncertainty for the cylinder mass obtained in our weighing system was determined to be 0.82 mg. The standard uncertainties for the O2 molar fractions in the primary standard mixtures ranged from 0.7 to 0.8 µmol mol−1. Based on the primary standard mixtures, the annual average molar fractions of atmospheric O2 and Ar in 2015 at Hateruma island, Japan, were found to be 209339.1±1.1 and 9334.4±0.7 µmol mol−1, respectively. The molar fraction for atmospheric Ar was in agreement with previous reports.

Accurate determination of formaldehyde amount fraction in cylinders using mixtures of primary standards of formaldehyde in nitrogen prepared by a gravimetric permeation method

A reliable method for the quantification of formaldehyde (HCHO) amount fractions is required, because the results of comparisons between individual techniques used for the quantification of HCHO amount fractions often disagree owing to unknown causes. One reliable method comprises the quantification of HCHO using working gas standards in which the HCHO amount fraction is linked to the International System of Units (SI). In this work, the SI traceability of HCHO amount fractions in mixtures of commercial HCHO in nitrogen in cylinders used as working gas standards was established by calibrating them against primary gas standard mixtures (mixtures of HCHO in N2), which were prepared by a permeation method and were traceable to SI. The expanded relative uncertainties (k = 2) of the amount fractions in the primary gas standard mixtures were less than 0.6 %. The quantitative values of the amount fractions in the working gas standards had relative expanded uncertainties (k = 2) of 0.7 %–0.9 %.

Development of a Northern Continental Air Standard Reference Material.

The National Institute of Standards and Technology (NIST) recently began to develop standard mixtures of greenhouse gases as part of a broad program mandated by the 2009 United States Congress to support research in climate change. To this end, NIST developed suites of gravimetrically assigned primary standard mixtures (PSMs) comprising carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in a dry-natural air balance at ambient mole fraction levels. In parallel, the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado, charged 30 aluminum gas cylinders with northern hemisphere air at Niwot Ridge, Colorado. These mixtures, which constitute NIST Standard Reference Material (SRM) 1720 Northern Continental Air, were certified by NIST for ambient mole fractions of CO2, CH4, and N2O relative to NIST PSMs. NOAA-assigned values are also provided as information in support of the World Meteorological Organization (WMO) Global Atmosphere Watch (GAW) Program for CO2, CH4, and N2O, since NOAA serves as the WMO Central Calibration Laboratory (CCL) for CO2, CH4, and N2O. Relative expanded uncertainties at the 95% confidence interval are <±0.06% of the certified values for CO2 and N2O and <0.2% for CH4, which represents the smallest relative uncertainties specified to date for a gaseous SRM produced by NIST. Agreement between the NOAA (WMO/GAW) and NIST values based on their respective calibration standards suites is within 0.05%, 0.13%, and 0.06% for CO2, CH4, and N2O, respectively. This collaborative development effort also represents the first of its kind for a gaseous SRM developed by NIST.

Development of a southern oceanic air standard reference material.

In 2009, the United States Congress charged the National Institute of Standards and Technology (NIST) with supporting climate change research. As part of this effort, the Gas Sensing Metrology Group at NIST began developing new gas standard mixtures for greenhouse gas mixtures relevant to atmospheric measurements. Suites of gravimetrically prepared primary standard mixtures (PSMs) were prepared at ambient concentration levels for carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in a dry-air balance. In parallel, 30 gas cylinders were filled, by the National Institute of Water and Atmospheric Research (NIWA) in Wellington, New Zealand, to high pressure from pristine southern oceanic air at Baring Head, New Zealand, and shipped to NIST. Using spectroscopic instrumentation, NIST analyzed the 30 cylinder samples for mole fractions of CO2, CH4, and N2O. Certified values were assigned to these mixtures by calibrating the instrumentation with the PSM suites that were recently developed at NIST. These mixtures became NIST Standard Reference Material (SRM) 1721 Southern Oceanic Air and are certified for ambient mole fraction, the first of their kind for NIST. The relative expanded uncertainties corresponding to coverage intervals with 95% probability are no larger than 0.06% of the certified values, representing the smallest uncertainties to date ever assigned to an NIST gas SRM.

Investigating adsorption/desorption of carbon dioxide in aluminum compressed gas cylinders.

Between June 2010 and June 2011, the National Institute of Standards and Technology (NIST) gravimetrically prepared a suite of 20 carbon dioxide (CO2) in air primary standard mixtures (PSMs). Ambient mole fraction levels were obtained through six levels of dilution beginning with pure (99.999%) CO2. The sixth level covered the ambient range from 355 to 404 μmol/mol. This level will be used to certify cylinder mixtures of compressed dry whole air from both the northern and southern hemispheres as NIST standard reference materials (SRMs). The first five levels of PSMs were verified against existing PSMs in a balance of air or nitrogen with excellent agreement observed (the average percent difference between the calculated and analyzed values was 0.002%). After the preparation of a new suite of PSMs at ambient level, they were compared to an existing suite of PSMs. It was observed that the analyzed concentration of the new PSMs was less than the calculated gravimetric concentration by as much as 0.3% relative. The existing PSMs had been used in a Consultative Committee for Amount of Substance-Metrology in Chemistry Key Comparison (K-52) in which there was excellent agreement (the NIST-analyzed value was -0.09% different from the calculated value, while the average of the difference for all 18 participants was -0.10%) with those of other National Metrology Institutes and World Meteorological Organization designated laboratories. In order to determine the magnitude of these losses at the ambient level, a series of "daughter/mother" tests were initiated and conducted in which the gas mixture containing CO2 from a "mother" cylinder was transferred into an evacuated "daughter" cylinder. These cylinder pairs were then compared using cavity ring-down spectroscopy under high reproducibility conditions (the average percent relative standard deviation of sample response was 0.02). A ratio of the daughter instrument response to the mother response was calculated, with the resultant deviation from unity being a measure of the CO2 loss or gain. Cylinders from three specialty gas vendors were tested to find the appropriate cylinder in which to prepare the new PSMs. All cylinders tested showed a loss of CO2, presumably to the walls of the cylinder. The vendor cylinders exhibiting the least loss of CO2 were then purchased to be used to gravimetrically prepare the PSMs, adjusting the calculated mole fraction for the loss bias and an uncertainty calculated from this work.

The National Institute of Standards and Technology ambient level methane in air Standard Reference Material historical record

The National Institute of Standards and Technology (NIST) has been certifying lots, or series, of Standard Reference Materials (SRMs) containing ambient level methane in air for over 40 years. The historical record contains six traditional series of SRM 1658 (1 μmol mol(-1)), five of SRM 1660 (4 μmol mol(-1)), and seven of SRM 1659 (10 μmol mol(-1)) methane in air. All series of any one particular SRM can be linked to each other through the historical suites of gravimetric primary standard mixtures (PSMs) developed at NIST. One gas mixture cylinder from a series is chosen as the lot standard (LS), retained and held at NIST, and periodically compared to the PSMs to assure its stability. Recently, 6 of the original 18 LS still in service in the Gas Metrology Group inventory, and cylinder samples held at NIST from 6 other SRM lots, were analyzed against a newly prepared suite of PSMs using cavity ring-down spectroscopy. Data were analyzed using a generalized least squares linear regression. The results indicate that, within the original 95% confidence intervals, the methane concentration has remained the same for all the SRM LS and lot samples. The current predicted concentrations of the LS and samples for SRMs 1659 and 1660 are within 0.002 to 0.051 μmol mol(-1), or ≤0.5%, relative of the original certificate value. SRM 1658 LS and samples are within 0.0001 to 0.0023 μmol mol(-1), or ≤0.2% relative. These results illustrate the consistency, repeatability, and stability of these methane in air SRMs over the historical 35+-year record. It also demonstrates that the historical gravimetric primary methane in air suites have remained accurate and consistent over time.

International comparison CCQM K23b: Natural gas type II

At the highest metrological level, natural gas standards are commonly prepared gravimetrically as PSMs (Primary Standard Mixtures). This international key comparison is a repeat of CCQM-K1e-g. The mixtures concerned contain nitrogen, carbon dioxide and the alkanes up to butane. The only difference from CCQM-K1e-g is the addition of iso-butane to the list. The agreement of the results in this key comparison is very good. For all parameters, with a few exceptions, the results agree within 0.5% (or better) with the key comparison reference value. For methane, the results are generally within 0.1% (or better) of the KCRV.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).

International comparison CCQM-K54: Primary standard gas mixtures of hexane in methane

The aim of this comparison is to evaluate the gravimetry and purity verification of gas mixtures prepared by introducing a liquid into a gas cylinder. This type of preparation is well known from, e.g., ethanol in nitrogen/air (CCQM-K4), and n-hexane in synthetic natural gas (CCQM-K16). The choice for n-hexane in methane is based on the relevance of natural gas primary standard mixtures and higher hydrocarbons in synthetic natural gas (for, e.g., dew point calibrations). Furthermore, n-hexane is relatively straightforward to analyse using a GC/FID (gas chromatograph equipped with a flame ionisation detector) with good repeatability.Eight laboratories participated. After the data evaluation, the result of one laboratory was eliminated to arrive at a consistent subset. The quality of the fitted line was not overly good, leading to small discrepancies in the degrees-of-equivalence calculated for three other laboratories. Only four laboratories are consistent with the KCRV.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).

Gas Standards Development in Support of NASA’s Sensor Calibration Program Around the Space Shuttle

The National Aeronautics and Space Administration (NASA) Kennedy Space Center (KSC) requires accurate gas mixtures containing argon (Ar), helium (He), hydrogen (H(2)), and oxygen (O(2)) in a balance of nitrogen (N(2)) to calibrate mass spectrometer-based sensors used around their manned and unmanned space vehicles. This also includes space shuttle monitoring around the launch area and inside the shuttle cabin. NASA was in need of these gas mixtures to ensure the safety of the shuttle cabin and the launch system. In 1993, the National Institute of Standards and Technology (NIST) was contracted by NASA to develop a suite of primary standard mixtures (PSMs) containing helium, hydrogen, argon, and oxygen in a balance gas of nitrogen. NIST proceeded to develop a suite of 20 new gravimetric primary PSMs. At the same time NIST contracted Scott Specialty Gases (Plumsteadville, PA) to prepare 18 cylinder gas mixtures which were then sent to NIST. NIST used their newly prepared PSMs to assign concentration values ranging from 100 to 10,000 micromol/mol with relative expanded uncertainties (95% confidence interval) of 0.8-10% to the 18 Scott Specialty Gases prepared mixtures. A total of 12 of the mixtures were sent to NASA as NIST traceable standards for calibration of their mass spectrometers. The remaining 6 AIRGAS mixtures were retained at NIST. In 2006, these original 12 gas standards at NASA had become low in pressure and additionally NASA needed a lower concentration level; therefore, NIST was contracted to certify three new sets of gas standards. NIST prepared a new suite of 22 PSMs with weighing uncertainties of <0.1%. These 22 PSMs were compared to some of the original 20 PSMs developed in 1993 and with the NIST valued assigned Scott Specialty Gas mixtures that NIST had retained. Results between the two suites of primary standards and the 1993 NASA mixtures agreed, verifying their stability. At the same time, NASA contracted AIRGAS (Chicago, Illinois) to prepare 45 cylinder gas mixtures which were then sent to NIST. Each of the 3 sets of standards contained 15 cylinder gas mixtures: set no. 1, He at 12,000 micromol/mol, H(2) at 600 micromol/mol, Ar at 100 micromol/mol, and O(2) at 600 micromol/mol; set no. 2, He at 15 000 micromol/mol, H(2) at 5000 micromol/mol, Ar at 1000 micromol/mol, O(2) at 5000 micromol/mol; and set no. 3, He at 50 micromol/mol, H(2), Ar, and O(2) each at 25 micromol/mol with a balance gas of N(2). NIST used their newly prepared primary standards to assign concentration values to each component in these three new mixture sets to relative expanded uncertainties of 0.5-2.2%. The NIST certified AIRGAS prepared mixtures were then sent to NASA to use as "working standards" to calibrate their mass spectrometers (MSs).

The certification of a nominal 20 μmol/mol H2S/N2 standard reference material using two independent methods

To support federal, state and local regulations on the amount of hydrogen sulfide (H(2)S) released into the atmosphere, the National Institute of Standards and Technology (NIST) has certified standard reference material (SRM) 2731 Hydrogen Sulfide in Nitrogen (nominal amount-of-substance fraction 20 micromol/mol H(2)S). Since it was first produced and certified in 1989, NIST has certified four separate lots of this SRM. In each case, the value assignment of the lot concentration was accomplished by comparison to a permeation tube-generated calibration curve. For the certification of the new lot, two independent methods were used to value-assign the concentration. In addition to the permeation tube method, a second method involving the dynamic dilution of a high-concentration gravimetrically prepared primary standard mixture containing H(2)S/N(2) was also used. Agreement between the two methods was 0.10% and the total uncertainty assigned to the lot concentration was 1.35% (relative).

Final Report on International comparison CCQM K23ac: Natural gas types I and III

At the highest metrological level, natural gas standards are commonly prepared gravimetrically as PSMs (primary standard mixtures). This international key comparison is a repeat of CCQM-K1e-g. The mixtures concerned contain nitrogen, carbon dioxide and the alkanes up to butane. The only difference with CCQM-K1e-g is the addition of iso-butane to the list. The results usually agree within 1% (or better) with the key comparison reference value. For ethane, nitrogen and carbon dioxide, the agreement is within 0.5% (or better), and for methane within 0.1% (or better) of the KCRV.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).

Generation of Sulfur Compound Vapor Standards Based on Permeation Vials in Series/Parallel

A reliable and accurate system to generate vaporous standards for semivolatile sulfur compounds of ethanethiol, 1-propanethiol, 1-butanethiol, methyl sulfide, and dimethyl sulfide at parts-per-million and parts-per-billion levels has been developed. Known concentrations of the sulfur compound standards are prepared by passing a stream of nitrogen gas over the permeation vials thermostated at 30 °C. The standards are generated on the basis of constant permeation rates of the liquid compounds in each vial and calibrated gravimetrically. The permeation rates of the vials are found from the linear regression plots of the weight loss of the vials against the elapsed time. The design has the advantage of preparing various concentrations of primary standard mixtures containing one or more than one component.

Quantitative analysis of synthetic mixtures of triacylglycerols with fatty acids from caprylic to stearic

Separation and quantitation of triacylglycerols tricaprylin to tristearin was achieved by high‐performance liquid chromatography with refractive index detection and an eluent composed of propionitrile and butyronitrile (80:20, vol/vol). Peak identification was based on the logarithms of retention times, and quantitation was achieved by way of theoretical relative response factors. The validity of the response factor calculations was tested by analysis of primary standard mixtures of saturated triacylglycerols and of triacylglycerols obtained by interesterification of binary mixtures of monoacid triacylglycerols.

Separation of instrumental and chemical errors in the analysis of oils by gas chromatography—A collaborative evaluation

Thirty‐five analysts studied the concept that, in the gas chromatographic (GC) analysis of fatty acid composition, errors can be separated into those caused by poor chromatograph optimization and those related to inefficient conversion of triacylglycerols (TAG) to fatty acid methyl esters (FAME). A primary standard mixture of FAME was used to determine how well the participants had optimized their chromatographs. A primary standard of the equivalent TAG was used to determine total error of analysis. “Chemistry error” was calculated as the difference between the absolute errors found for the FAME and the TAG standards. Grades of analysis were computed for the FAME and TAG results and for the chemistry errors calculated from these analyses.Only four analysts achieved grades of analysis for the FAME standard that can be considered excellent or good. These four analysts used different injector/column configurations, indicating that, when properly optimized, a GC with a flame ionization detector is an extremely accurate instrument. Conversely, it is evident that there is the potential for most analysts to improve their instrumental optimization. In agreement with published information, AOCS method Ce 2‐66 and AOAC method 969.33 gave low chemistry grades, but a number of analysts used modifications of these methods, and some achieved much better grades. It would appear that many of the standard methods that are in common use are capable of producing improved results, but that critical parameters need to be better specified to ensure minimization of error. The concept of separating errors into those of instrument origin and those caused by the chemical component of the total method would appear to be a useful concept for the validation of analytical methods.

Analysis of fatty acid methyl esters with high accuracy and reliability : III. Literature review of and investigations into the development of rapid procedures for the methoxide-catalysed methanolysis of fats and oils

The literature leading to the development of rapid procedures for the preparation of fatty acid methyl esters for gas—liquid chromatography by methoxide-catalysed methanolysis of fats and oils is reviewed. A rapid and reliable procedure utilizing the best features of the literature methods was developed and shown to give excellent quantitative results in the difficult case of a triacylglycerol primary standard mixture simulating coconut oil. Evidence is presented that brief refluxing of the reaction mixture is advantageous and that removal of methoxide by aqueous extraction at the completion of the reaction, as distinct form injecting the whole reaction mixture, is essential if good quantitative resutls are to be obtained. The method is faster and simpler than the existing international procedures, and the ester solution may be ready for injection into the chromatograph in about 2 min.