Frequency-stabilized cavity ring-down absorption spectrometry (FS-CRDS) with single-mode excitation using a tunable continuous-wave diode laser is being developed to help support the delivery of reference gas concentration standards. This paper describes initial efforts to compare FS-CRDS measurements with National Institute of Standards and Technology (NIST) methane-in-air standard reference materials to demonstrate the potential of this method to deliver standards-grade measurements with uncertainties of 1 % or lower. The current work demonstrates measurements with residual standard deviations of approximately 1 % for methane sample mole fractions of 50 μmol mol-1 and above. The results for lower mole fraction samples are poorer due to the poor signal-to-noise ratios and the higher pressures required for the measurements. The current results are potentially limited by the Voigt line shape which was used to model the data.
Quantitative, high resolution (0.1 cm-1) infrared spectra have been acquired for a number of pressure broadened (101.3 KPa N2), vapor phase chemicals including: Sarin (GB), Soman (GD), Tabun (GA), Cyclosarin (GF), VX, nitrogen mustard (HN3), sulfur mustard (HD) and Lewisite (L). The spectra are acquired using a heated, flow-through White cell of 5.6 m optical path length. Each reported spectrum represents a statistical fit to Beer's law, which allows for a rigorous calculation of uncertainty in the absorption coefficients. As part of an ongoing collaboration with the National Institute of Standards and Technology (NIST), cross-laboratory validation is a critical aspect of this work. In order to identify possible errors in the Dugway flow-through system, quantitative spectra of isopropyl alcohol from both NIST and Pacific Northwest National Laboratory (PNNL) are compared to similar data taken at the Dugway Proving Ground (DPG).
The National Institute of Standards and Technology (NIST) and the Pacific Northwest National Laboratory (PNNL) are independently creating quantitative, approximately 0.10 cm-1 resolution, infrared spectral libraries of vapor phase compounds. The NIST library will consist of approximately 100 vapor phase spectra of volatile hazardous air pollutants (HAPs) and suspected greenhouse gases. The PNNL library will consist of approximately 400 vapor phase spectra associated with DOE's remediation mission. A critical part of creating and validating any quantitative work involves independent verification based on inter-laboratory comparisons. The two laboratories use significantly different sample preparation and handling techniques. NIST uses gravimetric dilution and a continuous flowing sample while PNNL uses partial pressure dilution and a static sample. Agreement is generally found to be within the statistical uncertainties of the Beer's law fit and less than 3 percent of the total integrated band areas for the 4 chemicals used in this comparison. There does appear to be a small systematic difference between the PNNL and NIST data, however. Possible sources of the systematic difference will be discussed as well as technical details concerning the sample preparation and the procedures for overcoming instrumental artifacts.
There is a continuing need for improved analytical techniques to measure the concentration of trace gases for monitoring hazardous air pollutants, industrial emissions, chemical-warfare agent release, etc. Methods of analysis that can conclusively identify several analytes in a mixture are particularly desired. Towards this end, the use of Fourier-transform microwave (FTMW) spectroscopy as a quantitative analytical technique has been proposed. The high spectral resolution of FTMW provides a quick and unambiguous method for identifying multiple analytes in the gas phase. A small-scale FTMW spectrometer has recently been constructed for use in quantitative analysis. Prior to the present investigation, however, the use of this spectrometer in quantitative work has not been rigorously evaluated. This work summarizes efforts to identify and categorize sources of signal instability in the FTMW spectrometer. Methods employed to minimize these effects will also be discussed.
Fourier-transform infrared (FT-IR) spectrometry has become a useful real-time in situ analytical technique for quantitative gas phase measurements. In fact, the U.S. Environmental Protection Agency (EPA) has recently approved open-path FT-IR monitoring for the determination of hazardous air pollutants (HAP) 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 standard samples. The procedures developed to ensure the quantitative accuracy of the reference data are discussed, including sample preparation, residual sample contaminants, data processing considerations, and estimates of error.
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