Stability assessment of gas mixtures containing terpenes at nominal 5 nmol/mol contained in treated aluminum gas cylinders

Studies of climate change increasingly recognize the diverse influences exerted by terpenes in the atmosphere, including roles in particulates, ozone formation, and their oxidizing potential. Measurements of key terpenes suggest atmospheric concentrations ranging from low pmol/mol (parts per trillion) to nmol/mol (parts per billion), depending on location and compound. To accurately establish concentration trends, assess the role of terpenes in atmospheric chemistry, and relate measurement records from many laboratories and researchers, it is essential to have good calibration standards. The feasibility of preparing well-characterized, stable gas cylinder standards for terpenes at the nmol/mol level is not yet well established. Several of the world’s National Metrology Institutes (NMIs) are researching the feasibility of developing primary and secondary reference gas standards at the nmol/mol level for terpenes. The US NMI, the National Institute of Standards and Technology, has prepared several nmol/mol mixtures, in treated aluminum gas cylinders, containing terpenes in dry nitrogen at nominal 5 nmol/mol for stability studies. Overall, 11 terpenes were studied for stability. An initial gas mixture containing nine terpenes, one oxygenate, and six aromatic compounds, including benzene as an internal standard, was prepared. Results for four of the nine terpenes in this initial mixture indicate stability in these treated aluminum gas cylinders for over 6 months and project long term (years) stability. Interesting results were seen for β-pinene, which when using a linear equation rate decline predicts that it will reach a zero concentration level at day 416. At the same time, increases in α-pinene, d-limonene (R-(+)-limonene), and p-cymene were observed, including camphene, a terpene not prepared in the gas mixture, indicating a chemical transformation of β-pinene to these species. Additional mixtures containing combination of either α-pinene, camphor, α-terpinene, and benzene indicate a second-order quadratic rate decline for the α-pinene and α-terpinene, a linear rate decline for camphor, and a second-order quadratic rate increase of camphene.     

A Bayesian approach to the evaluation of comparisons of individually value-assigned reference materials

Several recent international comparison studies used a relatively novel experimental design to evaluate the measurement capabilities of participating organizations. These studies compared the values assigned by each participant to one or more qualitatively similar materials with measurements made on all of the materials by one laboratory under repeatability conditions. A statistical model was then established relating the values to the repeatability measurements; the extent of agreement between the assigned value(s) and the consensus model reflected the participants’ measurement capabilities. Since each participant used their own supplies, equipment, and methods to produce and value-assign their material(s), the agreement between the assigned value(s) and the model was a fairer reflection of their intrinsic capabilities than provided by studies that directly compared time- and material-constrained measurements on unknown samples prepared elsewhere. A new statistical procedure is presented for the analysis of such data. The procedure incorporates several novel concepts, most importantly a leave-one-out strategy for the estimation of the consensus value of the measurand, model fitting via Bayesian posterior probabilities, and posterior coverage probability calculation for the assigned 95% uncertainty intervals. The benefits of the new procedure are illustrated using data from the CCQM-K54 comparison of eight cylinders of n-hexane in methane.     

Cross-linked aggregates of (R)-oxynitrilase: a stable, recyclable biocatalyst for enantioselective hydrocyanation

[Reaction: see text] The (R)-oxynitrilase from almonds was immobilized as a cross-linked enzyme aggregate (CLEA) via precipitation with 1,2-dimethoxyethane and subsequent cross-linking using glutaraldehyde. The resulting preparation was a highly effective hydrocyanation catalyst under microaqueous conditions, which suppress the nonenzymatic background reaction. The beneficial effect of these latter conditions on the hydrocyanation of slow-reacting aldehydes is demonstrated. The oxynitrilase CLEA was recycled 10 times without loss of activity.     

Differences between propane in nitrogen versus air matrix analyzed using gas chromatography with flame-ionization detection

New US Federal low-level automobile emission requirements, such as Zero Level Emission Vehicle (ZLEV), for hydrocarbons and other species have resulted in manufacturers need for new certified reference materials. The new emission requirement for hydrocarbons requires the use, by automobile manufacturing testing facilities, of 100 nmol/mol (ppb) propane in air gas standard. Emission measurement instruments are required, by Federal law, to be calibrated with the US National Institute of Standards and Technology (NIST) traceable reference materials. A NIST Standard Reference Material (SRM) containing 100 nmol/mol propane has been developed. During the development of this SRM a critical question arose as to the matrix of the primary propane standards. The automobile companies make their measurements using total hydrocarbon analyzers with flame-ionization detectors which integrate all hydrocarbons in a sample. NIST uses gas chromatography/flame-ionization detection (GC/FID) with a column to separate all components. Since the SRM mixtures were in air, the question as to the effect of oxygen on the detector arose. To investigate this effect, two suites of propane primary standards were developed: one in air and the other in nitrogen. The two suites of primary standards were analyzed using NIST methods, and the concentration of propane in an air mixture was determined. The results show that there was a difference of 0.63% in the propane concentration determined versus air and nitrogen suites.     

Tunable infrared generation in KTA and applications

Noncollinear difference frequency mixing of dye laser and Nd:YAG second harmonic (fundamental) radiation from a commercial laser system is employed for the generation of 2.7–5.3 μm (1.6–1.7 μm) radiations in a flux-grown KTiOAsO₁ crystal. The generated radiation is used to scan the methane absorption in the fundamental (v ₃) and its first overtone (2v ₃) band at pressure 90 torr in a laboratory made single pass gas cell of length 33 cm.     

Feasibility of preparing and analyzing gas standards containing heavy hydrocarbons (C10–C16)

State and federal agencies are beginning to monitor ambient air for compounds related to diesel exhaust. The National Institute of Standards and Technology (NIST) was asked to develop standards containing heavy (C10-C16) alkanes which could then be used in air monitoring and exhaust studies. Several primary gravimetric standards, containing heavy (C10-C16) alkanes in nitrogen, were developed and analyzed by gas chromatography (GC) with flame-ionization detection (FID). The results of this research indicate that accurate gas standards containing these hydrocarbons can be prepared. However, the analytical results show that the temperature of the transfer system from the gas cylinder to the GC column (including the gas-sample valve) must be heated in order to prevent adsorption of these compounds within the analytical system. The results indicate that even at elevated temperatures these compounds are being absorbed within the system. The results show that quantitative results cannot be obtained by using one compound such as hexane, as an internal standard to determine the concentration of other hydrocarbons. Quantitative and accurate results are best obtained if standards for each hydrocarbon of interest are used to determine concentrations of unknowns for the respective hydrocarbon.     

Development of perfluorocarbon (PFC) primary standards for monitoring of emissions from aluminum production

An EPA Voluntary Aluminum Industrial Partnership (VAIP) program has been formed to help US primary producers focus on reducing the emissions of two perfluorocarbons (PFCs), tetrafluoromethane (CF₄) and hexafluoroethane (C₂F₆), during the production of aluminum. To ensure comparability of measurements over space and time, traceability to national sources was desirable. Hence, the EPA approached the NIST to develop a suite of primary standards to cover a mole fraction (concentration) range of 0.1 to 1400 μmol mol^–1 for CF₄ and 0.01 to 150 μmol mol^–1 of C₂F₆. A total of eight gravimetric PFC gas standards were prepared with relative expanded uncertainties of ≤ 0.52% (≈95% confidence level). These primary standards were ultimately used to assign values to a series of secondary gas standards at three mole-fraction levels with relative expanded uncertainties ranging from ± 0.7% to 5.3% (≈95% confidence level). This series of secondary standards was used within the aluminum industry to calibrate instruments used to make emission measurements. Assignment of values to the secondary standards was performed by use of gas chromatography with flame-ionization detection (GC–FID) and Fourier transform infrared spectrometry (FTIR). Real time pot-line and stack samples from a local aluminum plant were also obtained and sub-samples sent to each participating facility for analysis. The data generated from each facility were sent to NIST for analysis. The maximum difference between the NIST and individual facilities’ values for the same sub-sample was ± 26%. Received: 19 January 2001 / Revised: 27 March 2001 / Accepted: 6 April 2001     

Stability of compressed gas mixtures containing low level volatile organic compounds in aluminum cylinders

Compressed gas mixtures containing up to twenty-six volatile organic compounds (VOCs) in a balance of nitrogen have been prepared and analyzed at the National Institute of Standards and Technology (NIST). The mixtures are contained in aluminum cylinders and the hydrocarbons included are aromatic or aliphatic, both saturated and unsaturated and some containing a halogen, oxygen or nitrogen atom. The individual compounds are present at concentrations ranging from 0.1–3000 nmol/mol and the relative standard uncertainty in the concentration of each is between ±2–5%. The stability of the mixtures over various time intervals is discussed.     

A quantitative infrared spectral database of hazardous air pollutants

Open-path monitoring of the atmosphere using Fourier transform infrared spectrometry has recently become a useful real-time in situ analytical technique. The U.S. Environmental Protection Agency (EPA) is currently formulating Method TO-16, a protocol for infrared remote sensing of the hazardous air pollutants (HAPs) identified in EPA’s Clean Air Act of 1990. To support infrared based sensing technologies, the National Institute of Standards and Technology (NIST) is currently developing a standard quantitative spectral database of the HAPs based on gravimetrically prepared samples. This paper presents the protocol used to prepare the gravimetric mixtures and initial results. Received: 6 June 1997 / Revised: 4 August 1997 / Accepted: 7 August 1997