September 1, 2005

Handheld gas, liquid sensors

Tiny liquid phase sensors at strategic points in a city's water mains can detect a chemical as it passes and tell a computer to close down the affected pipes.

While the current technology is too cumbersome for this kind of rapid detection and response, new advances in liquid and gas phase chemical sensing may lead to the development of palm-sized sensing tools that can provide the instant detection needed.

Using small quantum cascade lasers, researchers at Georgia Institute of Technology, along with colleagues from Tel-Aviv University and OmniGuide Communications, have built and demonstrated a prototype handheld gas phase chemical sensing device and a liquid phase sensing device.

The quantum cascade laser is the key to scaling down mid-infrared chemical sensing tools to fit in the palm of the hand, said Boris Mizaikoff, associate professor in the School of Chemistry and Biochemistry at Georgia Tech.

“This diode laser light source emits mid-infrared frequencies, operates at room temperature, and is small, roughly the same size as the laser you use in a laser pointer or CD player,” said Mizaikoff.

Almost every organic molecule has a very distinctive absorption pattern in the mid-infrared range (roughly between three and 20 microns). Illuminating molecules with a laser tuned to its fingerprint frequency will cause the molecules to vibrate as they absorb radiation at that frequency.

Detecting a chemical is as simple as illuminating a small volume of gas or liquid with a laser.

For the gas sensing modules, Mizaikoff and graduate student Christy Charlton use a photonic band gap hollow waveguide, essentially a hollow, flexible tube, to contain very small amounts of air and assist in sensing. The waveguide can propagate only one wavelength of light very well. So, when the laser illuminates the gas molecules inside the waveguide, the waveguide will propagate only the selected fingerprint frequency for detecting a specific molecule.

“In our paper, we've shown that if we take only one meter of photonic band gap hollow waveguide with an inner diameter of 700 microns coupled to a frequency-matched quantum cascade laser, we've been able to detect levels down to 30 parts-per-billion (ppb) of ethyl chloride,” said Mizaikoff. “In our opinion, it's among the most sensitive measurement that's been demonstrated in gas phase sensing in a hollow wave guide to date.”

Gas sensing done this way requires a sample of only one milliliter of gas, compared to few hundreds of milliliters for other techniques using regular multi-pass gas cells, he added.

For the liquid phase device, researchers use a planar silver halide waveguide to transmit the radiation. As with the gas devices, the quantum cascade lasers vastly increase the sensitivity of liquid phase chemical detection at the surface of this waveguide.

“This might be the road to sensors that can continuously measure at ppb levels, with molecular selectivity and instantaneously,” said Mizaikoff. “We believe this technology will be the inroad to single digit ppb water quality measurement.”