Hearing the telltale sounds of dangerous chemicals

Aug 14, 2012
A device based on selective real-time detection of gaseous nerve agent stimulants using multi-wavelength photoacoustics could look like this, in which now "multiple" lasers are used. Credit: U.S. Army Research Laboratory

To warn of chemical attacks and help save lives, it's vital to quickly determine if even trace levels of potentially deadly chemicals—such as the nerve gas sarin and other odorless, colorless agents—are present. U.S. Army researchers have developed a new chemical sensor that can simultaneously identify a potentially limitless numbers of agents, in real time. A paper describing the system has been published today in the Optical Society's journal, Optics Letters.

The new system is based on a phenomenon known as the photoacoustic effect, which was discovered by Alexander Graham Bell, 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 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 U.S. Army Research Laboratory (ARL) in Adelphi, Md. "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 vapor 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.

This laser is used to optically "heat" very small particles. Credit: U.S. Army Research Laboratory

"A portion of the laser power is absorbed, usually via molecular transitions, and this absorption results in localized 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.

Dr. Kristan Gurton, an experimental physicist in the Battlefield Environmental Division, Computational and Information Sciences Directorate, US Army Research Laboratory, conducts experiments. Credit: U.S. Army Research Laboratory.

"There are groups of researchers producing QC laser arrays that will operate with sufficient power, and will house as many as 10 -- or more -- lasers at different frequencies in the spectroscopically rich region of the MidIR," he says.

Once such laser arrays are available, the method ultimately "could be tailored for a variety of detection scenarios ranging from the obvious need to protect our soldiers during conflict to civilian applications like detecting the presence of harmful chemical gases that are difficult to detect with conventional techniques," Gurton says. A sufficiently rugged device for in-the-field use, he envisions, 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.

"In our paper we demonstrated the ability to measure as many identifying absorption features as you want," Gurton says. "You're only limited by the number of sources available." However, he notes, "at some point, as the number of species spectra increase in the database, a degree of spectral overlap would occur, which might result in erroneous identification. It just depends on how similar the spectra are to each other. You could have just two that have very similar spectra and that could cause problems, or you could have 20 to 30 species spectra that all have distinguishable features that can be identified easily."

Explore further: Optical nanoantennas set the stage for a NEMS lab-on-a-chip revolution

More information: "Selective real-time detection of gaseous nerve agent simulants using multi-wavelength photoacoustics (www.opticsinfobase.org/ol/abst… fm?uri=ol-37-16-3474)," Optics Letters, Vol. 37, Issue 16, pp. 3474-3476 (2012).

Related Stories

NRL Develops Technique To Speed Detection Process

Feb 15, 2010

(PhysOrg.com) -- Researchers at the Naval Research Laboratory are developing a device to enable rapid detection and identification of bacteria, chemicals, and explosives in the environment or on the battlefield.

Tunable quantum cascade laser

Jul 07, 2010

(PhysOrg.com) -- One of the issues associated with lasers is their tunability. In many cases, if you want to produce a particular wavelength, you have to build a laser to accomplish this. In order to get another wavelength, ...

Plastic laser detects tiny amounts of explosives

Jun 08, 2010

(PhysOrg.com) -- Detecting hidden explosives is a difficult task but now researchers in the UK have developed a completely new way of detecting them, with a laser sensor capable of detecting molecules of explosives ...

Tiny spectrometer offers precision laser calibration

May 11, 2007

A tiny device for calibrating or stabilizing precision lasers has been designed and demonstrated at the National Institute of Standards and Technology. The prototype device could replace table-top-sized instruments ...

Recommended for you

New filter could advance terahertz data transmission

7 hours ago

University of Utah engineers have discovered a new approach for designing filters capable of separating different frequencies in the terahertz spectrum, the next generation of communications bandwidth that ...

The super-resolution revolution

7 hours ago

Cambridge scientists are part of a resolution revolution. Building powerful instruments that shatter the physical limits of optical microscopy, they are beginning to watch molecular processes as they happen, ...

Precision gas sensor could fit on a chip

9 hours ago

Using their expertise in silicon optics, Cornell engineers have miniaturized a light source in the elusive mid-infrared (mid-IR) spectrum, effectively squeezing the capabilities of a large, tabletop laser onto a 1-millimeter ...

A new X-ray microscope for nanoscale imaging

10 hours ago

Delivering the capability to image nanostructures and chemical reactions down to nanometer resolution requires a new class of x-ray microscope that can perform precision microscopy experiments using ultra-bright ...

New research signals big future for quantum radar

22 hours ago

A prototype quantum radar that has the potential to detect objects which are invisible to conventional systems has been developed by an international research team led by a quantum information scientist at the University ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Aug 18, 2012
I wonder how distinct those nerve gas signatures are, though. If they are not very distinct and cannot be easily distinguished from the allowed stuff, this might lead to a lot of false positives.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.