When you draw a breath of air, you inhale more than the oxygen needed to sustain your life. Ever-present airborne particles are inhaled, as well. The smallest particles, those less than 0.3 micrometers, are usually exhaled as easily as they are inhaled, so they hardly interact at all with your lungs. Particles greater than 10 micrometers (µm), such as pollen, are usually caught in nasal and bronchial passages, sometimes producing allergic reactions. Particles between 0.3 and 2.5 µm in diameter are thought to be small enough to sneak past the body’s filters, land in the lungs, and remain there long enough to cause respiratory health problems. Potentially hazardous particles in this size range are tobacco smoke particles, coal dust that causes black lung disease, asbestos fibers, and particles that cause silicosis and berylliosis.
There is a growing concern that particulate matter in this intermediate size range may be a greater health hazard than previously thought. The concern stems from research findings based on either gross particle concentrations (sorted by size, independent of composition) or bulk chemical analyses of particles collected on filters.
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Fig. 1. Peter Reilly, Mo Yang, and Rainer Gieray (left to right) with the airborne particle mass spectrometer. |
To determine the precise chemical nature of individual airborne microparticles in real time, researchers Bill Whitten, Pete Reilly, Rainer Gieray, Mo Yang, and Mike Ramsey—all of ORNL’s Chemical and Analytical Sciences Division—have developed an instrument incorporating a laser and an ion trap mass spectrometer, thanks to funding from DOE’s Office of Research and Development (see Fig. 1). The instrument, called an airborne particle mass spectrometer, enables us to determine with high sensitivity which particles bear small quantities of harmful substances in the presence of a large number of normal background particles.
Air containing the particles—whether dust, pollen, microorganisms, or spores—is sucked into a chamber where the individual particles are detected and analyzed using laser ablation mass spectrometry. Once set up, the apparatus can operate unattended to serve as a real-time monitor of air quality. It has even greater importance, however, as an analytical instrument, exploiting the power of ion trap mass spectrometry for the characterization of particles from many sources.
To chemically analyze airborne particles, we use this instrument to vaporize a portion of an incoming particle into its atomic and molecular constituents and to convert some of these into ions. In laser ablation mass spectrometry, an energetic laser pulse both evaporates and ionizes the particle. The trick is to time the laser pulse so that it intercepts the particle, traveling at a speed of about 300 m/s (600 miles/h), when it arrives at the center of the ion trap. The ions that are produced are then stored and their masses determined by the techniques described in the main article here.
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We are investigating the use of laser ablation and ion trap mass spectrometry to monitor background levels of airborne bacteria and to help with environmental monitoring near nuclear facilities. We have shown, for example, that uranium can be detected in a particle of picogram mass containing fewer than 3000 uranium atoms. We are also studying carcinogenic polycyclic aromatic hydrocarbon compounds in soot and diesel engine exhaust particles (see Fig. 2) and chemical substances that make up urban smog. By slightly modifying the apparatus, we can characterize waterborne particles individually, permitting high-sensitivity analysis of groundwater and sediments for substances of low solubility. Because of its high sensitivity, our new application of ion trap mass spectrometry could be a breath of fresh air for the field of environmental monitoring.—Bill Whitten, research chemist in ORNL’s Chemical and Analytical Sciences Division. |
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