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Introduction
There is a continuing interest in new analytical instruments to quantify and identify the constituents of gas samples obtained from automobile exhausts, factory emissions, and process streams. Ideally, the instrument should offer high sensitivity, accurate quantification, and unambiguous species identification, and be compact, inexpensive, and easy to use. Moreover, the ability to analyze the sample directly in near real time is often critical, to obtain rapid feedback on the gas chemistry. Since instruments presently available, such as gas chromatography, infrared spectrometers, and mass spectrometers, are deficient in several of these areas, efforts are continually underway to develop new gas analyzers. One such instrument suggested to have significant potential for gas analysis is the Fourier-transform microwave (FTMW) spectrometer, which uses microwave radiation and the time-domain techniques of nuclear-magnetic-resonance (NMR) spectroscopy to record the rotational spectrum of a molecule in the microwave spectral region, i.e., between approximately 6 GHz and 40 GHz. FTMW spectroscopy was developed approximately 20 years ago to measure the rotational spectra of molecules and exotic chemical species such, as dimers, hydrogen-bonded complexes, and free radicals, to extract fundamental molecular properties. The technique operates by injecting a pulse of a gas sample into a high-Q microwave cavity situated in a vacuum chamber, where it is interrogated by microwave radiation. 
The principle advantages that FTMW offers for chemical analysis is 100% unambiguous species identification, high sensitivity, and the ability
to detect nearly every dipolar molecule with less than approximately 12
to 15 nonhydrogenic atoms, provided that it has sufficient vapor pressure
to be entrained in a gas sample. Moreover, when properly equipped with an
appropriate gas extraction system, FTMW offers near real time gas detection.
It suffers from its rather large size of approximately 0.6 m x 1.5 m x
1.5 m in its present configuration, with a potential for an
approximately
25% size reduction. Its accuracy, sensitivity, and utility
for quantification
of gas composition are presently being assessed,
using the measurement
of oxygenated hydrocarbon mixtures involving aldehydes,
ketones, ethers,
and alcohols, such as found in automobile exhaust, as a test
case.
To meet regulatory demands, the automobile manufacturers have a significant interest in new technologies for the measurement of automobile emissions. Their interest in this area is illustrated by their involvement in the American Industry/Government Emissions Research (AIGER) Cooperative Research and Development Agreement (CRADA) begun in 1992 and expected to continue into 2002. Members of the CRADA include governments laboratories, such as NIST, and the Californian Air Resources Board (CARB), the Environmental Protection Agency (EPA) and the United States Council for Automobile Research (USCAR). The goal of this CRADA is "to identify, encourage, evaluate and develop the instruments and techniques to accurately and efficiently measure emissions from motor vehicles as required by the Federal Clean Air Act and the California Health and Safety Code." Of particular interest to the automobile industry is near real-time quantification of various oxygenates (aldehydes, ketones, ethers, and alcohols) in the exhaust during the first few minutes after the engine is started, when significant pollutants are released. Based on laboratory measurements of ideal samples, FTMW has the ability to detect, and potentially quantify, the oxygenates listed below which are of interest to the automobile industry and expected to be present in automobile exhaust.
Our primary laboratory effort undertaken in collaboration with the Analytical Chemistry Division will determine the sensitivity, accuracy, and time response of the analytical determinations using gas samples prepared using the sample preparation techniques established by the Analytical Chemistry Division at NIST for their Standard Reference Gas Mixtures. References Lovas, F.J., Pereyra, W., Suenram, R.D., Fraser, G.T., Grabow, J.U., and Hight Walker, A.R., "Using Fourier transform microwave spectroscopy to detect hazardous air pollutants," in Proc. 1994 U.S. EPA/A&WMA Intern. Symp. - Optical Sensors for Environmental and Chemical Process Monitoring, (1994).
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