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Ozone monitoring for the masses Too much ozone in the air is a health hazard, reducing lung capacity and aggravating asthma, but it’s been impossible to measure exposure levels in individuals as they go about normal daily activities. Until now. A study published in AC (DOI 10.1021/ac1013578) describes the development of an ozone monitor small enough for personal use. “There’s a need for small instruments so people can carry them around,” says John Birks, who developed the portable monitor with colleagues at 2B Technologies, Inc., in Boulder, Colorado. “We don’t know what people’s exposures actually are.” The most common method for ozone detection is UV absorbance. “What [Birks] has done is taken what everybody uses today as the best method of measuring ozone and miniaturized it,” says Donald Stedman of the University of Denver. The new palm-sized monitor weighs 0.3 kg and is 18⫻ smaller and 7⫻ lighter than its predecessor. “We used much more modern electronics,” relative to conventional ozone monitors, explains Birks, “We were able to use a miniature lamp and a photodiode in place of the photomultiplier tube.” They also incorporated a rechargeable battery that provides power for 15.5 h and a data logger that stores 2.8 days worth of measurements taken every 10 s. A challenge to UV absorbance miniaturization is the positive relationship between path length and sensitivity. Therefore, a 15 cm u-shaped cell was constructed for the monitor with mirrors positioned at each bend to propagate light. “I like the way they folded the light path so they still have the long path length in a conveniently sized instrument,” says Charles Weschler of the University of Medicine & Dentistry of New Jersey. Measuring ozone using UV absorbance requires the selective removal of 7864
ANALYTICAL CHEMISTRY /
ozone from air to generate a “blank”. This was accomplished by alternately pumping air through a tube containing hopcalite, an “ozone scrubber” that de-
An exploded view of the personal ozone monitor showing the casing (top and bottom), UV absorbance cell (upper middle), and circuit board with components affixed (lower middle).
stroys ozone at concentrations up to 1 ppm by volume. Calculating ozone levels from the UV absorbance of scrubbed and unscrubbed air with the Beer-Lambert Law gives solutions in molecules per cubic centimeter. The monitor was outfitted with pressure and temperature sensors so that ozone concentrations could then be converted to parts per billion by volume (ppbv) using the ideal gas law. Another issue the researchers addressed was humidity. “As you go smaller, you have an increased surfaceto-volume ratio,” says Birks, which is a problem because moisture adsorbs to surfaces and can change the reflectivity of light as it passes by, distorting measurements. The ozone scrubber can also
OCTOBER 1, 2010
create moisture irregularities because it removes water from air. To overcome the water problems, Birks and colleagues used multiple strategies. A Nafion tube was positioned after the ozone scrubbing chamber to equalize moisture between samples of scrubbed and unscrubbed air. “[Nafion] is a material that equilibrates with water very quickly, so we could match the water concentrations very rapidly,” says Birks. Initially, aluminum was selected for the cell’s material because of its excellent optical throughput for 254 nm light, the region of peak ozone absorbance. But the researchers soon noticed data inconsistencies and realized the aluminum surface was adsorbing too much water. By placing quartz tubes inside the cell, the relative humidity could vary 0⫺100% without affecting measurements. “They’ve done a good job of stepping around water interference,” says Weschler. To assess the sensitivity and precision of the miniature monitor, its measurements of known ozone concentrations were compared against those from a calibrated commercial ozone monitor. The precision was determined to be 1.5 ppbv and the limit of detection 4.5 ppbv. According to Weschler, “We can make an enormous number of useful measurements with a detection limit of 4.5 ppbv.” It’s not yet clear what particular experiments will be done with personal ozone monitoring. But, Stedman says, “if you invent a device that measures something in a way that’s never been measured before, it will be very educational. —Erika Gebel
10.1021/AC102172Q 2010 AMERICAN CHEMICAL SOCIETY
Published on Web 08/25/2010
