EDITORS' COLUMN
β\τ
T H E SAME TIME as the Spring
National ACS Meeting in Boston, the Optical Society of America held their Spring meeting in New York City, April 11 to 13. Of interest to analytical chemists and environ mentalists at this meeting was the presentation on Thursday, April 13, of two papers dealing with the use of lasers in pollution detection. At the Optical Society Meeting, E. D . Hinkley of the Lincoln Laboratory of M I T described some of the work carried on there in the last two years in developing laser techniques for detection and monitoring of air pollutant gases. I n addition, grad uate student, Helge Kildal, of Nor way and applied physics professor, Robert L. Byer, of Stanford Uni versity suggested t h a t a laser moni tor scanning the air over a city could eliminate fixed monitoring stations now using more conven tional monitoring methods. Dr. Hinkley described the use of some tunable semiconductor diode lasers for in situ monitoring of sul fur dioxide from smokestacks. These tiny lasers have also been used to measure ethylene in auto mobile exhaust and other impor t a n t pollutants such as nitric oxide, carbon monoxide, and ammonia. A larger, more complete and higher powered spin-flip R a m a n laser has been used with an opto-acoustic cell to monitor nitric oxide in auto mobile exhaust and near highways. An effective program in pollution monitoring would require detection of source emissions from a distance, ambient air quality measurements, in situ source monitoring, and pointsampling for further chemical anal ysis. With remote heterodyne de tection, tunable lasers can be used to provide a completely passive, single-ended system for the detec tion of pollutant gases from station ary or mobile sources. The Stanford investigators, Kil dal and Byer, reported results of calculations for three different laser
methods: R a m a n backscattering, resonance backscattering, and res onance absorption. T h e R a m a n technique requires a high-power, single-wavelength laser. I t permits precise measurement of the concen tration and depth of pollution with the laser and telescope detector lo cated in one place b u t loses sensi tivity over long distances. Reso nance backscattering and resonance absorption methods require tunable lasers t h a t can be adjusted to the absorption frequency of a given pol lutant. I n backscattering the laser beam excites the atoms or molecules of each pollutant selectively, caus ing them to emit fluorescence which identifies the pollutants and their relative concentrations. Both laser and telescopic detectors are at the same location. Both R a m a n and resonance backscatter schemes for infrared molecular transitions re quire sophisticated optical systems and are not sensitive enough to de tect dispersed pollutants and simul taneously provide depth resolution. However, they can detect highly concentrated pollutant sources such as smokestacks. Resonance absorption must have a remote detector or reflective target for the laser beam where the char acteristic spectral absorption lines of the pollutant can be observed and identified. The absorption method measures the total amount of pollutants in the path of the laser beam but does not offer the depth perception needed to locate them. On the other hand, this method is very sensitive, the equip ment required is simpler and less expensive, and a high-power laser is not necessary. Resonance absorp tion methods can use semiconductor lasers as sources in the infrared re gion. Resonance absorption and resonance backscatter schemes can provide complementary informa tion—resonance backscattering m a y locate the pollution, but the more sensitive absorption scheme m a y determine the integrated concentra tion. With the addition of a remote reflecting mirror, a system for reso nance backscattering is also useful for resonance absorption. J. P.
DOES ALL LABORATORY CLEANUP CHORES BEST! Sorg's disposable LABORA TORY towels not only go after lab spills with a relentless thirst, they're also great for every other lab cleanup chore. Big—131/2 by 18 inches—they're highly absorbent, have great wet strength. They're ultra-soft, lint-free and ideal for cleaning and polishing any glass surface, instrument or container. Use optical equipment in the lab? These towels are unexcelled for lenses and all delicate surfaces — for drying wet film too. The perfect answer for all glassware "shine u p " operations. Write Sorg for free samples—and the name of the distributor nearest you.
SORG THE SORG PAPER COMPANY · Middletown, Ohio 45042
CIRCLE 1 8 4 O N READER SERVICE CARD
ANALYTICAL CHEMISTRY, VOL. 4 4 , NO. 7, JUNE 1972 ·
63 A
