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Developing air-quality measurements for the world on TLC plates with transparent backs. After pregnated with a particular reagent,” DasIn the U.S., a local weather report might exposure to air, an LED light source illumigupta says. The paper is spooled through a forecast a hot, dry day and also advise that tomorrow’s ozone level will be moderate. In reading area and exposed to the ambient air, nated the immobilized reagent, which turned brown on exposure to NO2. It the developing world, however, finding air-quality information, even in the worked flawlessly in the lab with known form of raw data intended for scientists, NO2 concentrations, but as soon as is almost impossible. A recent AC pathey started working with ambient air, per by Purnendu Dasgupta and colproblems arose. “When we started leagues at the University of Texas Arcomparing the data to a commercial lington (DOI 10.1021/ac801756z) instrument,” Dasgupta says, “it was describes an advance that promises to very clear that our machine was always help promote atmospheric measurereading higher, often much higher.” ments in developing parts of the world, The difficulty turned into a very where quality data are perhaps most fortuitous surprise. “We just happened needed as industrial growth escalates. to also be measuring ozone,” Dasgupta One measure of air quality in urban continues. “We looked at the data and areas is the concentration of NO2. said, ‘You know, the disagreements are the greatest when the ozone peaks are High temperatures oxidize otherwise high.’ So we thought, ‘Let’s just go inert atmospheric nitrogen into NO ahead and bubble some ozone through (primarily), which is then further oxithe reagent,’ and when we tried it, it dized to NO2 under typical atmoturned bright pink.” spheric conditions. Small quantities of This completely unexpected reacNO2 are thus found naturally, but antion of O3 with ANS “has no precethropogenic combustion is the source of elevated NO2 levels in urban air. dent,” says Dasgupta. Two separate LEDs (442 and 525 nanometers) are The concentrations of NO and centered under the two peaks. The NO2 in air are related to each other as absorbances overlap, so NO2 and O3 well as to sunlight, O3, and unburned Field test of the ANS-based sensor for NO2 and O3 in urban reactive hydrocarbons (among other concentrations are obtained from a pair things) in the atmosphere. The primary Arlington, Texas, over the course of a typical winter day. The of simultaneous equations. A third new, inexpensive sensor system (data points in blue) shows means of analyzing either NO or NO2 LED (850 nanometers) serves as a refexcellent correlation with benchmark instruments (data erence and also measures relative in the rarefied concentrations found in points in red). humidity. air, typically 10⫺200 parts per billion “If you ask me about air quality in by volume (ppbv), involves the chemiand a light source reflects a beam off the Houston or Dallas,” Dasgupta says, “I can luminescent reaction between NO and O3. paper. A colorimetric reaction gives a regive you five or six different locations right (O3 here is a reagent, not to be confused now on the web, and you can go and look with the pollutant O3, an analyte.) The reac- sponse that is specific for various gases. The researchers began with the up the ozone data for 11 o’clock last night.” tion requires a cooled photomultiplier tube Griess⫺Saltzman reagent, which undergoes The same is not true for Hyderabad, India, for sufficient sensitivity. As a result, measurthe typical chemistry used for solution-phase which is even more populous. Dasgupta’s ing NO2 in air by this method is expensive analysis of NO and nitrite: namely, diazotiteam hopes that reducing the complexity, and requires an abundance of instrumental zation of an aniline functionality and cousize, power requirements, and cost of the effort: chemical conversion of NO2 to NO, pling to an aromatic ring to give colored required instruments will promote more and generation of O3 as a reagent, and a powerbetter data collection elsewhere. “We have hungry means of cooling a reaction chamber products. “It turned out it was too slow,” Dasgupta says. “Then I remembered, when done a lot of work for developing counbelow 0 °C. I first got started as an assistant professor, I tries,” Dasgupta adds. “I grew up in such an To develop a simpler, less expensive senworked with a compound that readily diazo- environment, and that’s where this would be sor, Dasgupta and colleagues took as their tizes with nitrite and then couples with itthe most useful.” cue common filter tape samplers, which are self.” This paper appears in the June 1, 2009, often used in workplaces where people are This reagent is 8-amino-1-naphthol-5issue of AC. concerned about gas leaks. “You have a sulfonic acid (ANS), which was impregnated —Steven Powell 1-inch-wide roll of filter paper, which is im5110
ANALYTICAL CHEMISTRY /
JULY 1, 2009
10.1021/AC900977S 2009 AMERICAN CHEMICAL SOCIETY
Published on Web 05/28/2009
