Fiber Probe Enhances Plasma Spectroscopy

MUNICH, Germany -- Monitoring the exhaust plumes from industrial activities such as power generation may soon be faster and more accurate. Researchers at Munich Technical University are testing a rugged fiber optic probe that can be placed inside an exhaust stack and used to perform laser-induced plasma spectroscopy.
Laser-induced plasma spectroscopy is an attractive technique because it requires little or no sample preparation, yet can analyze multielement solids, liquids and gases, explained researcher Ulrich Panne. The availability of rugged, low-cost lasers and detectors and the potential for automation make the method suitable for environmental monitoring, industrial hygiene and various production processes, he said. "In many cases, [it] can go where no [other] instruments can go."
The technique strips electrons from a sample with a laser, creating a plasma. Using time-gated electronics, researchers then determine the elemental composition of the sample from the characteristic ionic and atomic emissions of the plasma.
They used 550-µm quartz fiber, which is the middle ground between smaller-core fibers that would not transmit enough laser energy to generate the plasma and larger fibers that are too stiff to be practical. They used an Nd:YAG laser from Spectron Laser Systems of Rugby, UK, operating at either the fundamental 1064-nm wavelength or the frequency-doubled wavelength, 532 nm. They tested both single- and dual-fiber systems.
The biggest challenge they faced, Panne said, was to create a simple but robust laser-fiber interface that can be used in the field without realignment but that still permits high-power pulse transmission. "In our application, the use of fiber optics permits an in-stack probe that is robust enough to withstand the harsh conditions of the exhaust," he said.
The researchers plan to investigate a system that can perform measurements at several sites using a single, centralized laser. Panne also said they will look into the long-term transmission performance of fibers up to 100 m long.
A report describing the fiber optic probe and details of its design appear in Applied Spectroscopy, Vol. 54, No. 6.