Concept of an electronic nose
The concept of an electronic nose originated in the seventies. Up to then, analytical chemistry had been pre-occupied with developing highly specific sensors and methods, aimed at identifying unique
The new availability of personal computing made it possible to apply pattern recognition techniques to complex measurement data.An important consequence of the concept is that a substance, or mixtures of substances, can only be recognized after a calibration phase: in order to match a pattern, it must be known beforehand (‘seen’ before) substances.
The proposal was to have a general, broadly responsive sensor system generating complex multi-dimensional measurement data and use pattern recognition techniques to match measured ‘patterns’ to previously ‘seen’ patterns.This is analogous to how we smell, hence the name ‘electronic nose’. This is illustrated in the figure.
Electronic nose sensor types
The basic concept of an electronic nose, or machine olfaction, is a measurement unit that generates complex multi-dimensional data for each measurement combined with a pattern recognition technique that interprets the complex data and relates it to a target value or class.
In academic literature systems based on (for example) a mass-spectrometer in combination with pattern recognition are sometimes presented as an ‘electronic nose’ application or artificial olfaction. However, in this section only relatively low-cost sensor technologies are discussed which are in principle suitable for bench-top or portable devices are discussed.
The requirement that a multi-dimensional measurement signal is generated excludes single detection elements used for example PID meters. This is often overcome by using an array of broadly sensitive elements with different sensitivities to important chemical compounds. As an electronic nose device is frequently exposed to volatile chemicals arrays of potentiometric sensors are not useable.
The latter type are aimed at leak detection because they have a limited amount of reactive chemicals (the working principle is similar to a chemical battery) which is depleted when exposed to the target substances.
The technologies which are feasible for application are QMB/SAW, conducting polymers and metal-oxide sensors.
A QMB is a quartz crystal with a chemically active surface, usually a polymer. When gas molecules adsorb to the surface, the mass changes and the resonant frequency of the crystal shifts. These minute shifts need to be measured with high frequency electronics (complex, expensive).
Small temperature variations result in similar frequency shifts thus dictating strict environmental temperature control. A variation of a QMB is a SAW (surface acoustic wave) sensor which also works on the principle of frequency shifts.
Conducting polymers are polymers which are either intrisically conducting or non-conducting types which have been ‘loaded’ with graphite. In the former type the conductivity may alter when exposed to volatiles. In the latter case the graphite provides an electrical resistance path which can be measured very easily. When gas molecules associate with the polymer, it will swell thus breaking contact points between graphite particles and thus changes the resistance.
In this case also, temperature changes will result in expansion/contraction and thus in resistance changes, therefore it is also advisable to apply strict environmental temperature control for this sensor type.
Although the possible variations in polymers is enormous (and thus the variations in arrays also), they are chemically not very stable. Strong oxidizers such as chlorine and ozone can fairly easily disrupt a polymer.
Metal Oxide sensors
The basic choice of sensor is a so-called micro hotplate Metal-oxide sensor (MOS).
Certain metal-oxides behave as semiconductors at higher temperatures. Sensors based on this are designed as having a heater element and an sensor element (sintered metal-oxide with or without catalyst). Both elements are separated by a very thin isolating membrane.
Redox-reactions occurring at the sensor surface result in changes in resistance which can be measured. These redox-reactions depend on the nature of the metal-oxide/catalyst, the reacting gas(es), and the temperature. A minimum of 0.1% of ambient oxygen is required for normal operation.
Depending on sensor type and temperature, a very broad range of substance will give a redox reaction. Notable exceptions are N2, CO2 (will not oxidize further) and noble gases such as Helium and Argon.