Lasers Help Identify Airborne Particles in Real Time

R. Winn Hardin

RIVERSIDE, Calif. -- Around the globe, lasers are becoming one of the most accurate ways to measure wind speed and direction. Now researchers are using coherent beams to determine in real time and in situ the size and chemical composition of particles carried by those winds.
A system created by Kimberly A. Prather, her husband, Joseph Mayer, and a research group at the University of California has been hailed as an important development in environmental monitoring. It incorporates a series of lasers and a mass spectrometer, and could create a host of opportunities including the testing of combustion engines and aerosol products and the measuring of air quality in cleanrooms used for semiconductor manufacturing.
"It's basically a hybridization of time-of-flight mass spectrometry and particle sizing," said Prather, the lead developer. Conventional methods for detecting particles require passive fil-ters to collect samples, which can take hours or weeks of preparation. Then the sample has to be transported to the laboratory for analysis, raising questions about the integrity of the sample and contamination of the filter.
The system created by the University of California at Riverside group directly collects particulates from the air. A pump sends the particles through three "skimmers" that help to collimate the particle beam. They then enter into a near vacuum (10 to 18 t) and pass by the beam from an argon-ion laser from Omnichrome of Chino, Calif.

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Wavelengths >500 nm act as the "on" switch for the particulate sizing portion of the system. A photomultiplier tube from Hamamatsu Corp. of Bridgewater, N.J., detects the scattered light caused by the particles passing through the beam. As they pass through a second argon beam (<500 nm) at a known distance from the first particle-beam interaction, a second PMT detects the particle's exit; the system records the time of flight and calculates speed for up to 10 particles per second.
Because larger particles move slower through the vacuum than smaller particles, flight times are indicative of aerodynamic size, Prather said. This data can later be combined with the calibration curve created using particles of a known size to determine the exact particle size.
The speed of each particle is then determined and used to time a pulse from a frequency-quadrupled Nd: YAG laser from Continuum of Santa Clara, Calif. This laser blasts the particles in component ions, and a time-of-flight mass spectrometer from R.M. Jordan Inc. measures the ion's mass-to-charge ratio. A computer immediately displays the data.
Prather said future designs will use smaller, lower-cost diode lasers to replace the argon-ion laser, and computers will turn the raw data into chemical composition and size data as they are acquired.