TCE from initial concentrations of more than 3000 ug/L d o w n to 116 ug/L.
MEASUREMENTS A new analytical method for measuring gas-phase H 2 0 2 , hydroxymefhyl hydroperoxide, and methyl hydroperoxide in the field is described in this m o n t h ' s ES&T (p. 1180). The approach, developed by researchers at Brookhaven National Laboratory (Upton, NY) and the State University of N e w York at Old Westbury, is based on the hydroxylation of benzoic acid by Fenton reagent to form fluorescent hydroxybenzoic acid. The authors have incorporated this technique into a 3-channel sampling device, a n d report on potential interferences a n d field studies with their apparatus. The development of a radically new type of solid-state laser with potential applications for environmental monitoring has been announced by AT&T Bell Laboratories (Murray Hill, NJ). The quantum cascade (QC) semiconductor laser can be tailored to emit light at specific wavelengths over a wide range of the mid- to far-IR spectrum by simply varying the thickness of the layers. Development of the QC laser culminates a 30-year effort, say AT&T
physicists. Compared with current semiconductor lasers, the n e w QC lasers should be less sensitive to temperature variation, p r o d u c e a narrower linewidth, a n d display a different frequency response. In conventional diode lasers, the photon's wavelength is determined by the bandgap or energy difference between the electron in a conduction b a n d and the " h o l e " or positive charge in a valence band. Thus, the wavelength is a fundamental property of the semiconductor material; to significantly change the wavelength requires a different composition. QC lasers d e p e n d on electrons jumping between two energy levels in the conduction b a n d of the active layers (or q u a n t u m wells). However, that energy difference is controlled by the thickness of the active layer. The AT&T QC laser emits 4.25-pm light at temperatures u p to 125 K w i t h 5 m W of power.
typical 10-year-old unit. " U p until now, there hasn't been a compressor that could operate with a chlorine-free refrigerant," said Carrier North American president John Lord. "The n e w compressor works just as efficiently—and potentially better—with a chlorinefree refrigerant than what is presently available." With the impending phase-out of CFCs and, eventually, hydrochlorofluorocarbons, manufacturers are under the gun to produce commercial products that run on acceptable alternatives.
BUSINESS What is claimed to be the world's first chlorine-free, non-ozonedepleting residential air conditioner w a s unveiled by Carrier Corporation in April. The WeatherMaker 134a operates w i t h hydrofluorocarbon 134a (CF 3 CH 2 F) and, says the manufacturer, is 50% more energy efficient than a
Some Washington, DC, taxicabs took on a new look in March after Green Seal (Washington, DC) slapped its label on a fleet of "environmentally friendly" cabs. The Clean Air Cab c o m p a n y operates a small fleet of automobiles in the District that run on compressed natural gas, a lower polluting fuel.
Microwaves for detecting t r a c e gases? BY A L A N
A
NEWMAN
lthough not usually considered a routine analytical tool, Fourier transform (FT) microwave spectrometry is being developed by researchers at the National Institute of Standards a n d Technology (NIST, Gaithersburg, MD) for detecting and quantifying species in the gas phase (2). According to NIST chemist Richard Suenram, "basically, we can measure d o w n to 10 to 100 p p b for any volatile organic or inorganic c o m p o u n d with a permanent dipole." Such a system could be used for monitoring automotive exhaust emissions, smokestack fumes, or indoor air quality. Unlike its better k n o w n cousin FT-infrared spectrometry (see May ESS-T, p . 224A), FT-microwave spectrometry can reliably identify analytes on the basis of a
single transition which, typically, has linewidths of < 10 kHz. Moreover, the NIST instrument uses a specially designed nozzle that cools the sample to about 1 K, leaving only the lowest rotational levels populated. T h u s , say the NIST researchers, peak overlap problems are virtually eliminated a n d even large molecules can be detected. (NIST also has collected rotational spectra into a database that could form the core of a computer library). The NIST scientists estimate that they can detect acrolein d o w n to about 3 p p b using an average of 100 repetitive scans. However, this result is based on studies of acrolein diluted in neon. Nitrogen or air can work as a carrier gas, but w i t h a significant loss of signal-to-noise ratio caused by the less efficient cooling of diatomic gases. Suenram estimates that the cur-
rent NIST instrument costs about $100,000. The NIST FT-microwave spectrometer pulses the sample into an evacuated chamber containing two movable alum i n u m microwave mirrors that form a Fabry—Perot cavity. The mirrors are tuned to the exact frequency of interest, a n d the sample is excited with a weak microwave pulse of about 1 m W for several microseconds. The microwave pulse generates Fourier components that p u m p the rotational transition to the higher energy, and the emission signal is detected, amplified, a n d eventually Fourier transformed. Samples enter the cavity through a sample inlet nozzle set in one of the microwave mirrors. The gas stream can be pulsed into the chamber at an 8-Hz repetition rate, limited only by the vacuum p u m p i n g speed. The pulsed nozzle also serves to cool the gas by Environ. Sci. Technol., Vol. 28, No. 6, 1994
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