To monitor the amount or detect the presence of pollutants in food, air, and water, scientists are developing techniques and instruments that deliver reliable and most importantly fast results. One of the most interesting analytical technologies are based on so called electronic nose concept.
An electronic nose (e-nose) is a device which imitates the way human nose senses and recognizes different smells. The instrument consists of three components. The first one is for sampling of volatile odours. The second is the sensor zone (with 6-32 sensors) where testing is done. The third processing component is where sensor responses are analysed.
How does an e-nose work? It uses specialised sensors for chemical detection. Generally, there are three types of sensors for this application. The first works on the principle of frequency shifts. Frequency shifts can be caused by mass changes on sensor’s surface. It is measured on a Quartz Microbalance (QMB) sensor or by temperature changes detected by a Surface Acoustic Wave (SAW) sensor.
The second type of sensor uses conducting polymers. This type of sensor measures the change in resistance caused by the association of gas molecules to its graphite loaded surface.
The third type uses the semiconductor property of a metal oxide at higher temperatures. It measures the change in resistance when a redox reaction occurs with the analyte on its surface. The sensor consists of a heating element and sintered metal oxide which is separated by a thin membrane.
After sample testing using the sensors, e-nose devices use pattern recognition techniques with a large prewritten database as a reference for the analysis of sensor responses. Information obtained from sensors is matched to ones stored in the database. This is because analytes can only be recognised after they were introduced to the database by the process of calibration. Finally, the result is obtained with the use of pattern recognition techniques.
Two of the most important factors in using e-nose devices are data analysis and software control. By using the right dynamics in software for temperature and measurements, the sensors’ sensitivity can be significantly enhanced.
So far, e-nose is widely used in food spoilage detection. It is also widely used in microorganism detection and identification. There are also some data about its use in environmental analysis.
One possible use is for the detection of heavy metals such as arsenic, cadmium, lead, and zinc in water. For example, in 2003 scientists at Cranfield University, Bedfordshire, UK managed to detect some differences between samples of clean water and water in which there are added heavy metals in the concentration of 0.5 ppm. They concluded that there is a potential for low concentration detection of heavy metals in different types of water using the e-nose technique.
Another possible use of e-nose in environmental pollution monitoring is for the detection of gaseous elemental mercury (a well known global pollutant). Scientists at CRIM Laboratory-Polo Sant’AnnaValdera, Pontedera, Italy developed a device that can achieve 1 µg/m3 detection limit for mercury.
Other scientists have already developed devices with the size of a mobile phone that can obtain results in 6 minutes. These can also give information whether the applied sample passes some regulations or not. Future developments might lead to small devices which can be incorporated in mobile phones. This way, the analyst can download the calibration pattern database and readily use the device for analysis.
The video below talks about basic principle of e-nose technology with application to sensing explosive compounds.