A real-time gas- and water-quality monitoring system that consists of a miniature sensor array packaged in a weatherproof housing developed by the Department of Energy’s Sandia National Laboratories may become one tool in the effort to protect America’s water supply.
‘The electronic sniffer is a unique monitor that can be put directly underground – in groundwater or soils where the humidity reaches nearly 100 percent – and detect toxic chemicals at the site [in-situ] without taking samples to the lab,’ said researcher Cliff Ho. ‘It has the capability of detecting in real time undesirable chemicals being pumped into the water supply accidentally or intentionally.’
Traditional monitoring methods for contaminated sites often involve physically collecting water, gas, or soil specimens at the location and taking them to a laboratory for analysis. This can become extremely expensive, with each sample analysis costing between $100 to $1,000. In addition, the integrity of off-site analysis can be compromised during sample collection, transport, and storage.
The monitoring system developed by Ho and colleague Bob Hughes is designed to be left at the site. It would send back information in real time on solvents present and their concentrations to a data collection station where information would be downloaded and analysed.
Telemetry methods can also transmit data wirelessly from remote stations to a computer that would upload information to an interactive web site, providing immediate access to authorised individuals anywhere in the country.
The heart of the device is an array of differing miniature sensors that can detect volatile organic compounds (VOCs). Dubbed a chemiresistor, each polymer-absorption sensor is fabricated by mixing a commercial polymer dissolved in a solvent with conductive carbon particles.
The ink-like fluid is deposited and dried on wire-like electrodes on a specially designed microfabricated circuit. When VOCs are present, the chemicals absorb into the polymers, causing them to swell. The swelling changes the electrical resistance that can then be measured and recorded. The amount of swelling corresponds to the concentration of the chemical vapour in contact with the polymers.
The polymers will shrink once the chemical is removed, reverting the resistance to its original state.
‘By using four different kinds of polymers – one for each sensor – we think we can detect all solvents of interest,’ says Hughes, who developed the sensors for the project.
The array of differing sensors can be used to identify different VOCs by comparing the resulting chemical signatures with those of known samples.
But it’s the way the sensors are packaged that allows the device to be placed in water or underground. The protective package surrounding the chemiresistor chip designed by Ho and other team members is small and is designed to be rugged and waterproof while allowing the chemiresistors to be exposed to VOCs in both aqueous and gas phases.
‘The package is modular like a water-tight flashlight and is fitted with o-rings,’ Ho says. ‘It can be unscrewed, allowing for easy exchange of components.’
Another reportedly unique feature of the packaging is that it has a small ‘window’ covered by a GORE-TEX membrane to maintain a waterproof seal. Like clothing made of GORE-TEX it repels liquid water but ‘breathes,’ allowing vapours to diffuse across the membrane.
If the device is immersed in contaminated water, VOCs dissolved in the water will partition across the membrane into the gas phase, where they are detected by the chemiresistors.
Inside the packaging the chemiresistors are placed on a 16-pin dual inline package connected to a long weatherproof cable. The cable then can be connected to any data logger. Since only DC measurements are being made, the cable can be almost any length.