'Hearing' the telltale sounds of dangerous chemicals
15 August 2012
To warn of chemical attacks it is vital to determine the presence of trace levels of potentially deadly chemicals, such as the nerve gas sarin and other odourless, colourless agents.
Working towards this goal, US Army researchers have developed a new chemical sensor that can simultaneously identify a potentially limitless numbers of agents - in real time. The new system is based on a phenomenon known as the photoacoustic effect, in which the absorption of light by materials generates characteristic acoustic waves.
By using a laser and very sensitive microphones - in a technique called laser photoacoustic spectroscopy (LPAS) - vanishingly low concentrations of gases, at parts per billion or even parts per trillion levels, can be detected. The drawback of traditional LPAS systems, however, is that they can identify only one chemical at a time.
"Photoacoustics is an excellent analytic tool, but is somewhat limited in the sense that one traditionally only measures one absorption parameter at a time," says Kristan Gurton, an experimental physicist at the US Army Research Laboratory (ARL) in Adelphi, Maryland. "As I started looking into the chemical/biological detection problem, it became apparent that multiple LPAS absorption measurements - representing an 'absorption spectrum' - might provide the added information required in any detection and identification scheme."
To create such a multi-wavelength LPAS system, Gurton, along with co-authors Melvin Felton and Richard Tober of the ARL, designed a sensor known as a photoacoustic cell. This hollow, cylindrical device holds the gas being sampled and contains microphones that can listen for the characteristic signal when light is applied to the sample.
In this experiment, the researchers used a specialized cell that allows different gases to flow through the device for testing. As the vapour of five nerve agent mimics was flowed in, three laser beams, each modulated at a different frequency in the acoustic range, were propagated through the cell.
"A portion of the laser power is absorbed, usually via molecular transitions, and this absorption results in localised heating of the gas," Gurton explains. Molecular transitions occur when the electrons in a molecule are excited from one energy level to a higher energy level. Since gas dissipates thermal energy fairly quickly, the modulated laser results in a rapid heat/cooling cycle that produces a faint acoustic wave, which is picked up by the microphone. Each laser in the system will produce a single tone; so, for example, six laser sources have six possible tones. "Different agents will affect the relative 'loudness' of each tone," he says, "so for one gas, some tones will be louder than others, and it is these differences that allow for species identification."
The signals produced by each laser were separated using multiple "lock-in" amplifiers, which can extract signals from noisy environments, each tuned for a specific laser frequency. Then, by comparing the results to a database of absorption information for a range of chemical species, the system identified each of the five gases.
Because it is optically based, the method allows for instant identification of agents, as long as the signal-to-noise ratio, which depends on both laser power and the concentration of the compound being measured, is sufficiently high, and the material in question is in the database.
Before a device based on the technique could be used in the field, Gurton says, a quantum cascade (QC) laser array with at least six "well-chosen" mid-infrared (MidIR) laser wavelengths would need to be available.
"There are groups of researchers producing QC laser arrays that will operate with sufficient power, and will house as many as ten or more lasers at different frequencies in the spectroscopically rich region of the MidIR," he says.
A sufficiently rugged device for in-the-field use, could be about the size of a milk carton. "A photoacoustic cell is surprisingly simple and inexpensive to produce, with all of the cost and size driven primarily by the packaging of the quantum cascade laser array," he adds. In theory, the method could be used to identify an unlimited number of chemical agents.
A paper describing the system is published in the Optical Society's journal, Optics Letters.