实验利用电子鼻分析挥发性有机化合物(挥发性有机化合物)

Abstract - An electronic nose is an instrument intended to mimic the human sense of smell. Electronic noses (e-nose) employ an array of chemical gas sensors, a sample handling system and a pattern recognition system. Pattern recognition provides a higher degree of selectivity and reversibility to the system leading to an extensive range of applications. These ranges from the food and medical industries to environmental monitoring and process control. Many other types of different gas sensors available. These include conducting polymers (CP), metal oxide semiconductors (MOS), piezoelectric, optical fluorescence, quartz crystal microbalance (QCM) and Amperometric gas sensors. The ideal gas sensor would exhibit reliability, robustness, sensitivity, selectivity and reversibility. High selectivity with high reversibility is difficult to attain. After signal processing and feature extraction the output of the sensors provide a unique “smellprint” for that substances which can be used to classify, measure concentration, or verify quality. The present paper illustrates the function of electronic nose, its application and investigates the effective use of e-nose in detecting gases that have some smell developed by the volatile organic compounds (VOC) like ethanol, acetone and benzene at different concentrations. The response and characteristics prove that the Electronic nose is a reliable instrument which can be used for environment control (air quality, pollutants, and gas emission levels), medical science (urine, skin and breath odour etc.), food industry (coffee, milk, soft drink fish, meat etc.), pharmaceutics, chemical industry, Defence and security industries (detecting humanitarian land mines etc.) and semiconductor industrial processes.
Key words: Electronic noses, chemical sensors, sensor array, pattern recognition, volatile organic compound, smell print.
I.I NTRODUCTION
The human nose has been used as the judge for food and perfumes for years [1].The best way to know how fresh the food is to give it a sniff. If the smell of food is well, it is fresh [2].But how do we smell? After few hours of smelling, even the best inspector’s nose can come up a bit short. And what if we have caught a cold? That’s where Electronic noses come in handy. An electronic nose automatically detects and recognized odours, vapour and gases. These are not limited by human factors such as fatigue, exposure to toxins and inability to detect come compounds [1]. Instrumental methods of determining volatiles, such as gas chromatography – mass spectrometry (GC/MS), are expensive and require trained personnel. As a result there has been a drive to establish a device for rapid, inexpensive analysis of volatile organic compounds (VOC) that do not require specialist technicians. As a result there has been a drive to establish a device for rapid inexpensive analysis of volatile organic compounds (VOC), which doesn’t require specialist technicians [3].
Persaud and Dodd first reported the design of an electronic nose (E-nose) using chemical sensors and pattern recognition in 1982. An e-nose is an instrument consisting of an array of reversible but only semi-selective gas sensors coupled to a pattern recognition algorithm. The selectivity of the instrument is achieved through the application of pattern recognition techniques to the responses from the senor array [3].
The possibility of using electronic nose for applications in medical field has garnered increased research attention as of late. Several studies indicate that when people are afflicted with ailments such as diabetes, lung cancer, urinary tract infections, biological samples collected from them produce a discernable pattern of volatile organic compounds (VOCs) [1]. This forms, in essence, a “smell signature” for the disease that can be used to diagnose the condition with reasonable accuracy.
II.E LECTRONIC N OSE
The basic electronic circuit is shown in fig.1 for chemical identification using an array of sensors where each sensor is designed to respond to a specific chemical, the number of unique sensors must be at least as great as the number of chemicals begin monitored. The output characteristics of each sensor must be related with each other.
The commercial Electronic Noses use sensors like conducting polymer sensors (CP), piezoelectric– surface acoustic wave (SAW), thickness shear mode (TSM),metal oxide semiconductor (MOS), metal oxide semiconductor field effect transistor (MOSFET), electrochemical (EC), Pellistor and optical sensor arrays are reviewed by Albert et al. [4] and James et al. [5].
Experimental Use of Electronic Nose for Analysis of Volatile Organic Compound (VOC)
Syed Hasan Saeed1 Zia Abbas2 Prof. Bal Gopal 3
1. Deptt. of Electronics & Comm. Engg. Integral University, Lucknow:s.saeed@
2. Deptt. of Electronics & Comm. Engg. Integral University, Lucknow:ziaabbas@
3.Deptt. of Electronics & Comm. Engg. Integral University, Lucknow:prof.balgopal@
Fig.1 Basic measuring circuit of an E-Nose (TGS-822)
The sensing element of sensors is a tin oxide (SnO2) semiconductor which has low conductivity in clean air. In the presence of a detectable gas, the sensor’s conductivity increases depending on the concentration of gas in the air. A simple electrical circuit can convert the change in conductivity to an output signal which corresponds to the gas concentration.
The conducting polymer sensors used in electronic noses actually designed with thin films of insulating polymers loaded with carbon black as a conductive medium, to form polymer carbon composite. The polymers are selected by statistical analysis of responses of these films to a subset of the target compounds used.
Since the resistance in most of the polymer- carbon composite films is sensitive to changes in temperature, heaters are included on back of the ceramic substrate to provide a constant temperature at the sensors. This constant temperature, 28, 32 or 360C, depending Nose (TGS-822) of Figaro Company of Japan has been used.
III.D ETECTION P ROCESS
The detection process of an Electronic Nose is shown in fig.2. This process is divided into five groups namely, raw measurement, pre-processing, feature extraction, pattern recognition, classification and decision making. The initial block in the figure represents the electronic nose hardware, which typically consists of a gas sensor array. Preprocessing compensate for sensor drift, compress the transient response of the sensor array, and reduces sample to sample variations.
Fig.2 Odour Detection Process
Feature extraction has two purposes: to reduce the dimensionality of the measurement space to a lower dimensional space in order to avoid problems associated with high dimensional space datasets, and to extract the information relevant for pattern recognition. The resulting low dimensional feature vector is then used to solve a given prediction problem, typically classification, regression, or clustering. Classification task address the problem of identifying an unknown sample as one from a set of previously learned odorants. In regression task, the goal is to predict a set of properties (e.g. concentration, quality etc.) for an analyte. Finally, in clustering tasks the goal is to learn the structural relationships among different odorants. A final step is the selection of models and parameter settings and the estimation of the true error rates for a trained model by means of validation techniques.
IV.A PPLICATION FIELDS AND COMMERCIAL SYSTEMS The application areas for electronic nose are growing on the basis of the results so far achieved on behalf of numerous research groups and more and more companies on the market. They are the following:
1.Environmental control (air quality, pollutants, gas
emission levels of factories, chemical plant monitoring
etc.)
2.Medical applications such as urine skin and breathe
odour analysis, ulcer monitoring etc.
3.Food industry (coffee, soft drinks, fish, meat, wine
aroma control, fermentation process, identification of
bacterial organism etc.)
4.cosmetics/ perfumes and aroma
5.Defence and security industries ( detecting
humanitarian land mines etc)
6.Pharmaceutics, chemical industry (measuring odour,
quality control of pharmaceutical compounds etc.)
7.Semiconductor industrial process and others.
VI.R ESULT
The characteristic graphs for different volatile organic compounds with different concentrations using smell sensor TGS-822 are shown in fig.3. The compounds used are ethanol, acetone and benzene with different concentration. Other chemical compounds can be used in
VI.C ONCLUSION
The characteristic of three volatile organic compounds shows that the electronic nose can be used for other gases which are harmful. Thus an electronic nose is a reliable instrument for the detection of harmful gases. Many methods of pattern recognition are available which can be used in electronic nose. For accuracy, sensitivity, and selectivity of an electronic nose a comparative analysis of different pattern recognition techniques is required. On larger scale an array of such sensors can be employed to construct an electronic nose and different techniques can be used for the analysis of a mixture of various volatile organic compounds.
Rs-Fixed Resistance, Vc- Circuit Voltage, VL- Voltage across load, VH- heater Voltage, Concentration in ppm
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