04-10-2012, 04:50 PM
ELECTRONIC Noses
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INTRODUCTION
ELECTRONIC Noses (EN), in the broadest meaning, are instruments that analyze gaseous mixtures for discriminating between different (but similar) mixtures and, in the case of simple mixtures, quantify the concentration of the constituents. ENs consists of a sampling system (for a reproducible collection of the mixture), an array of chemical sensors, Electronic circuitry and data analysis software. Chemical sensors, which are the heart of the system, can be divided into three categories according to the type of sensitive material used: inorganic crystalline materials (e.g. semiconductors, as in MOSFET structures, and metal oxides); organic materials and polymers; biologically derived materials.
The use of ENs for food quality analysis tasks is twofold. ENs is normally used to discriminate different classes of similar odour-emitting products. In particular ENs already served to distinguish between different coffee blends and between different coffee roasting levels. On the other hand, ENs can also be used to predict sensorial descriptors of food quality as determined by a panel (often one generically speaks of correlating EN and sensory data). ENs can therefore represent a valid help for routine food analysis.
The combination of gas chromatography and mass spectroscopy (GC-MS) is by far the most popular technique for the identification of volatile compounds in foods and beverages. This is because the separation achieved by the gas chromatographic technique is complemented by the high sensitivity of mass spectroscopy and its ability to identify the molecules eluting from the column on the basis of their fragmentation patterns. Detection limits as low as 1 ppb (parts per billion) are frequently reached. The main drawbacks of the approach are, however, the cost and complexity of the instrumentation and the time required to fully analyze each sample (around one hour for a complete chromatogram). Comparatively, ENs are simpler, cheaper devices. They recognize a fingerprint, that is global information, of the samples to be classified. For food products, the sensory characteristics determined by a panel are important for quality assessment. While man still is the most efficient instrument for sensorial evaluation, the formation of a panel of trained judges involves considerable expenses.
Commercial coffees are blends, which, for economic reasons, contain (monovarietal) coffees of various origins. For the producers the availability of analysis and control techniques is of great importance. There exists a rich literature on the characterization of coffee using the chemical profile of one of its fractions, such as the headspace of green or roasted beans or the phenolic fraction. In the literature up to 700 diverse molecules have been identified in the headspace. Their relative abundance depends on the type, provenance and manufacturing of the coffee. It is to be noticed that none of these molecules can alone be identified as a marker. On the contrary one has to consider the whole spectrum, as for instance the gas chromatographic profile.
COMPARISION OF ELECTRONIC NOSE WITH BIOLOGICAL NOSE
Each and every part of the electronic nose is similar to human nose. The function of inhaling is done by the pump which leads the gas to the sensors. The gas inhaled by the pump is filtered which in the human is the mucus membrane. Next comes the sensing of the filtered gas, which will be done by the sensors i.e., olfactory epithelium in human nose. Now in electronic nose the chemical retain occurs which in human body is enzymal reaction. After this the cell membrane gets depolarised which is similar to the electric signals in the electronic nose. This gets transferred as nerve impulse through neurons i.e., neural network and electronic circuitries.
DIFFERENT TYPES OF SENSORS
There are different types of electronic noses which can be selected according to requirements. Some of the sensors available are calorimetric, conducting, piezoelectric etc. Conducting type sensors can again be sub divided into metal oxide and polymers. In this type of sensors the functioning is according to the change in resistance. The sensor absorbs the gas emitted from the test element and this results in the change of resistance correspondingly. According to the Resistance-Voltage relation V=I*R. Here ‘V’ is the voltage drop, ‘R’ is the resistance of the sensor and ‘I’ is the current through it. By this relation as resistance changes the voltage drop across the sensor also change. This voltage is measured and is given to the circuit for further processes. The voltage range for using metal oxide sensor in from 200˚C to 400˚C. The working principle of polymer sensor is same as that of metal oxide sensor The only change is in the temperature range i.e., the room temperature.
Piezoelectric sensors are sub-divided into quartz crystal microbalances and surface acoustic wave. In quartz crystal the surface absorbs the gas molecules. This results in the change of mass, which causes a change in the resonant frequency of the quartz crystal. This change in frequency is proportional to the concentration of the test material. The change in frequency also results a change in the phase. In surface acoustic wave we measure the change in phase of the resonant frequency.
Calorimetric sensors are preferable only for combustible species of test materials. Here the sensors measure the concentration of combustibles species by detecting the temperature rise resulting from the oxidation process on a catalytic element.
EXPERIMENTAL SET-UP
The Pico-1 Electronic nose
Five semiconductors, SnO2 based thin films sensors were utilised. Two are pure SnO2 sensors; one is catalysed with gold, one with palladium and one with platinum. They were grown by sputtering with the RGTO technique. RGTO technique is a technique for growing SnO2 thin films with high surface area. The surface of the film after thermal oxidation step of the RGTO technique presents porous, nano-sized agglomerates which are known to be well suited for gas absorption. A thin layer of noble metals was deposited as catalyst on three sensors to improve sensitivity and selectivity. Thin film sensor produced by sputtering is comparatively stable and sensitive. Furthermore, since the growing conditions are controllable, they can be taylored towards the particular application. Even if catalysed the sensors are not selective and therefore sensor arrays together with multivariate pattern recognition techniques are used.
[attachment=35321]
INTRODUCTION
ELECTRONIC Noses (EN), in the broadest meaning, are instruments that analyze gaseous mixtures for discriminating between different (but similar) mixtures and, in the case of simple mixtures, quantify the concentration of the constituents. ENs consists of a sampling system (for a reproducible collection of the mixture), an array of chemical sensors, Electronic circuitry and data analysis software. Chemical sensors, which are the heart of the system, can be divided into three categories according to the type of sensitive material used: inorganic crystalline materials (e.g. semiconductors, as in MOSFET structures, and metal oxides); organic materials and polymers; biologically derived materials.
The use of ENs for food quality analysis tasks is twofold. ENs is normally used to discriminate different classes of similar odour-emitting products. In particular ENs already served to distinguish between different coffee blends and between different coffee roasting levels. On the other hand, ENs can also be used to predict sensorial descriptors of food quality as determined by a panel (often one generically speaks of correlating EN and sensory data). ENs can therefore represent a valid help for routine food analysis.
The combination of gas chromatography and mass spectroscopy (GC-MS) is by far the most popular technique for the identification of volatile compounds in foods and beverages. This is because the separation achieved by the gas chromatographic technique is complemented by the high sensitivity of mass spectroscopy and its ability to identify the molecules eluting from the column on the basis of their fragmentation patterns. Detection limits as low as 1 ppb (parts per billion) are frequently reached. The main drawbacks of the approach are, however, the cost and complexity of the instrumentation and the time required to fully analyze each sample (around one hour for a complete chromatogram). Comparatively, ENs are simpler, cheaper devices. They recognize a fingerprint, that is global information, of the samples to be classified. For food products, the sensory characteristics determined by a panel are important for quality assessment. While man still is the most efficient instrument for sensorial evaluation, the formation of a panel of trained judges involves considerable expenses.
Commercial coffees are blends, which, for economic reasons, contain (monovarietal) coffees of various origins. For the producers the availability of analysis and control techniques is of great importance. There exists a rich literature on the characterization of coffee using the chemical profile of one of its fractions, such as the headspace of green or roasted beans or the phenolic fraction. In the literature up to 700 diverse molecules have been identified in the headspace. Their relative abundance depends on the type, provenance and manufacturing of the coffee. It is to be noticed that none of these molecules can alone be identified as a marker. On the contrary one has to consider the whole spectrum, as for instance the gas chromatographic profile.
COMPARISION OF ELECTRONIC NOSE WITH BIOLOGICAL NOSE
Each and every part of the electronic nose is similar to human nose. The function of inhaling is done by the pump which leads the gas to the sensors. The gas inhaled by the pump is filtered which in the human is the mucus membrane. Next comes the sensing of the filtered gas, which will be done by the sensors i.e., olfactory epithelium in human nose. Now in electronic nose the chemical retain occurs which in human body is enzymal reaction. After this the cell membrane gets depolarised which is similar to the electric signals in the electronic nose. This gets transferred as nerve impulse through neurons i.e., neural network and electronic circuitries.
DIFFERENT TYPES OF SENSORS
There are different types of electronic noses which can be selected according to requirements. Some of the sensors available are calorimetric, conducting, piezoelectric etc. Conducting type sensors can again be sub divided into metal oxide and polymers. In this type of sensors the functioning is according to the change in resistance. The sensor absorbs the gas emitted from the test element and this results in the change of resistance correspondingly. According to the Resistance-Voltage relation V=I*R. Here ‘V’ is the voltage drop, ‘R’ is the resistance of the sensor and ‘I’ is the current through it. By this relation as resistance changes the voltage drop across the sensor also change. This voltage is measured and is given to the circuit for further processes. The voltage range for using metal oxide sensor in from 200˚C to 400˚C. The working principle of polymer sensor is same as that of metal oxide sensor The only change is in the temperature range i.e., the room temperature.
Piezoelectric sensors are sub-divided into quartz crystal microbalances and surface acoustic wave. In quartz crystal the surface absorbs the gas molecules. This results in the change of mass, which causes a change in the resonant frequency of the quartz crystal. This change in frequency is proportional to the concentration of the test material. The change in frequency also results a change in the phase. In surface acoustic wave we measure the change in phase of the resonant frequency.
Calorimetric sensors are preferable only for combustible species of test materials. Here the sensors measure the concentration of combustibles species by detecting the temperature rise resulting from the oxidation process on a catalytic element.
EXPERIMENTAL SET-UP
The Pico-1 Electronic nose
Five semiconductors, SnO2 based thin films sensors were utilised. Two are pure SnO2 sensors; one is catalysed with gold, one with palladium and one with platinum. They were grown by sputtering with the RGTO technique. RGTO technique is a technique for growing SnO2 thin films with high surface area. The surface of the film after thermal oxidation step of the RGTO technique presents porous, nano-sized agglomerates which are known to be well suited for gas absorption. A thin layer of noble metals was deposited as catalyst on three sensors to improve sensitivity and selectivity. Thin film sensor produced by sputtering is comparatively stable and sensitive. Furthermore, since the growing conditions are controllable, they can be taylored towards the particular application. Even if catalysed the sensors are not selective and therefore sensor arrays together with multivariate pattern recognition techniques are used.