New chemical sensor set for field trials

Researchers from the US Department of Energy’s Sandia National Laboratories are about to commence field trials on a lightweight, portable chemical vapour detection system that is designed to identify chemical signatures from weapons of mass destruction.

The seven-pound, battery-powered sensor system, built into a box 10 by 8 by 4 inches, along with its notebook computer is said to be light enough to be hand-carried into the field where it can test for and identify 18 different chemicals in minutes.

Project leader, Richard Cernosek said the system’s weight and ability to test for a multitude of chemicals make it a significant step in the area of vapour detection.

‘The technique is adaptable for a variety of applications ranging from environmental monitoring and remediation to industrial process control and from personal health and safety to law enforcement and drug detection,’ Cernosek added.

The heart of the detection system is an array of seven surface acoustic wave (SAW) sensors, each with a different chemically sensitive thin film coating. The substrate of the sensors is quartz, cut at a crystalline angle that supports a surface wave. The quartz converts electric signals to surface acoustic waves that travel much like waves on water.

A thin, selectively absorbing layer is coated on the quartz surface and when specific chemical molecules sorb into or onto this layer, the acoustic waves travel more slowly. This change can be detected by the sensor microelectronics once the acoustic wave is converted back to an electric signal.

Because each sensor is coated with a distinct chemically sensitive layer, the sensors will respond differently to the same chemical, forming a recognisable pattern.

The data from all the sensors are put into a Sandia-developed pattern-recognition algorithm; a computational program called Visual Empirical Region-of-Influence (VERI), which uses a clustering technique to identify the various chemicals. The algorithms can identify 18 different volatile organic compounds over a wide concentration range with 95 percent accuracy.

The sensor array is contained in a small, sealed, temperature-controlled case with electrical connections and gas flow ports. The array itself can be removed from the case, allowing sensor arrays to be customised for particular applications.

A small diaphragm pump pulls vapour samples from the environment through the test cavity. The system uses a Windows-based notebook computer for system control, real-time display, data storage, and pattern recognition.

Sandia researchers have so far tested the system in a laboratory setting using chemicals in vials and beakers. They are refining its capabilities — the pattern recognition and sensor array — to increase performance and prepare for field testing in a chemical plume in 2001.

‘Then we’ll see this system operate in a realistic setting,’ said Cernosek. ‘It’s been several years in the development and preparation. We should be ready for this.’