Patent Application
20100180667
The present document relates to the field of odor detection and measurement. In particular, it relates to methods and apparatuses for detecting and/or measuring odors. It also relates to a method for reducing losses of sensitivity of gas sensors.
Within the human genome, there is 1 gene for hearing, 3 genes for vision, 12 genes for tasting, and 1,000 genes for smelling. The human nose contains approximately fifty million neuro-receptors connected to ten thousand primary neurons. The latter are in contact with a second layer of neurons linked with the olfactory bulb in the cerebral cortex, which is where odors are recognized. In electronic noses, the neuro-receptors are replaced by a sensor matrix. The interactions between the different gas molecules and the sensors alter certain physical properties of the latter. The overall set of sensor matrix signals yields the “olfactory signature” or “odor pattern” characteristic of a given odor and odor concentration. In the case of the electronic noses, the two neuron layers and the cerebral cortex are replaced by an algorithmic odor recognition and quantification element. The network of artificial neurons is a common solution of this mathematical problem. It is the resemblance of the device with the human olfactory system that led to its being named an “electronic nose”.
An odor is a quality of at least one chemical compound that stimulates the olfactory organ resulting in a sensation. Odor can be defined or quantified by various metrics such as the odor concentration, the odor intensity, the odor character, the odor persistence or the odor hedonic tone.
Odor concentration at the perception threshold is by definition 1 o.u./m3 (odor unit per cubic meter). Odor concentration is expressed as multiples of the perception threshold. By definition [2], the odor unit is the quantity of odorous substance that, evaporated in 1 m3 of odorless neutral gas (CNTP), triggers a physiological odor detection response in 50% of the population. The odor concentration of an odorous gas sample is determined by presenting that sample to a human panel, causing the concentration to vary due to dilution with a neutral gas in order to determine the dilution factor at the perception threshold of 50% of the panel. At that level of dilution the odor concentration, by definition, is 1 o.u./m3. The EN 13725 standard enables, among other things, the determination of the concentration of an odor by means of dynamic olfactometry; since the samples presented to the panelists are not to undergo any pre-treatment, no method for drying the odorous air is used, and the dilution air itself is dry.
The passage from an olfactory signature (the set of sensor matrix responses to an odor of known composition and concentration) to the characterization (recognition and quantification) of the odor is affected by means of a mathematical model. After prior training, the mathematical model will thus correlate an odor (nature and concentration) with its olfactory signature. The mathematical model may take into account parameters other than the sensor responses; for instance, humidity, temperature, air flow or measurement chamber pressure.
There are today various electronic nose (or electronic sensor) technologies (see an example in
Different sensor technologies are used for electronic noses, such as MOS (Metal-Oxide Semiconductor), QMB (Quartz Microbalance), IRS (Infra-Red Sensor), CPS (Conducting Polymer Sensor), SAW (Surface Acoustic Wave), OFS (Optical Fiber Sensor), and others. These sensor types have different sensitivity, selectivity, robustness and service life characteristics. The choice and combination of technologies depends primarily on the type of application [1].
As previously indicated, there are several major sensor families that can be used in electronic noses. Odorous molecule recognition and quantification is made indirectly by measuring changes in some physical properties of the sensors, such as electrical conductivity and the resonance frequency.
The MOS (Metal-Oxide Semiconductor) sensor family is widely used for reasons of low cost, sensitivity, broad detection spectrum and ease of use. The metal oxides used for this type of sensor (Metal-Oxide Semiconductor) are primarily tin, zinc or iron oxides, all of them are n-type intrinsic semiconductors. When heated to temperatures between 200 and 400 degrees Celsius, these semiconductors react primarily to Volatile Organic Compounds (VOCs), hydrocarbons and sulphur and nitrogen by increasing the electrical conductivity of the conducting band. The reference electrical conductivity is dictated by the adsorption of oxygen molecules on the surface coated with metal oxide. The change in electrical conductivity at the sensor surface is therefore caused by a gain or loss of electrons according to the number of oxygen molecules reacting with the gas present. In the case of tin oxide (SnO2) sensors, there will be a gain of electrons (reducing gas) or a loss of electrons (oxidizing gas) in the conducting band. This shows that in the presence of an oxidant gas, such as NO2, the conducting band of an n-type conductor will tend to diminish, while in the presence of a reducing gas, such as methane, the conducting band will tend to increase.
However, one of the main drawbacks [1] of the majority of chemical sensors used for measuring odor is their sensitivity to water molecules. The effect of humidity on SnO2-type sensor response is not yet fully understood. It would seem that there are hydroxyl groups formed at the oxide surface and that they are at equilibrium with the water vapor, in accordance to the various vapour pressures. This effect would tend to alter the response of sensors by reducing their sensitivity. Various solutions have been proposed so far in order to deal with this problem. For example, the addition of various doping additives has been used [6].
According to one aspect there is provided a method for detecting at least one odor in a gas sample, the method comprising:
According to another aspect there is provided a method for reducing losses of sensitivity of at least one gas sensor adapted to detect and/or measure at least one odor in a gas sample, the method comprising at least partially reducing an amount of water present in the gas sample before contacting the sample with the at least one sensor.
According to another aspect there is provided an apparatus for detecting and/or measuring odors in a gas sample, the apparatus comprising:
According to another aspect there is provided in an apparatus for detecting and/or measuring at least one odor in a gas sample comprising at least one gas sensor, the improvement wherein the apparatus comprises means for at least partially reducing an amount of water present in the gas sample, disposed upstream of the at least one gas sensor.
According to another aspect there is provided in an apparatus for detecting and/or measuring at least one odor in a gas sample comprising at least one metal oxide semiconductor gas sensor, the improvement wherein the apparatus comprises means for at least partially reducing an amount of water present in the gas sample, disposed upstream of the at least one gas sensor.
Water can be mainly present in the gas sample as water vapor. For example, at least 10%, 20%, 30%, 40%, 50%, 55%, 60%, 70%, or 75% of water present in the gas sample can be removed by using the previously mentioned methods and apparatuses. Alternatively, about 10 to about 75% of water can be removed. Water can be at least partially removed from the gas sample by means of a membrane adapted to be at least substantially permeable to water and at least substantially impermeable to the at least one odor. The membrane can be a hollow fiber membrane comprising at least one hollow fiber into which the gas sample is passed through. The gas sample can be passed through the membrane so as to least partially reduce the amount of water present therein so as to obtain a gas sample having a reduced content of water as compared to the gas sample before passing through the membrane. The gas sample having a reduced content of water is then contacted with the at least one gas sensor so as to detect the presence or absence of at least one odor. A purge gas is contacted with an exterior wall of the at least one hollow fiber so as to cause water to exit the membrane. Alternatively, the gas sample can be passed through the membrane so as to least partially reduce the amount of water present therein so as to obtain a gas sample having a reduced content of water as compared to the gas sample before passing through the membrane. The gas sample having a reduced content of water is then contacted with at least one gas sensor so as to detect the presence or absence of the at least one odor, and the gas sample having a reduced content of water is then contacted with an exterior wall of the at least one hollow fiber so as to cause water to exit the membrane.
The membrane can comprise a plurality of hollow fibers and the sample having a reduced content of water can then be contacted with at least one exterior wall of one of the hollow fibers. The hollow fiber membrane can comprise a cartridge comprising the hollow fibers. The cartridge can comprise an inlet for receiving the gas sample and an outlet for exiting the gas sample having a reduced content of water. The inlet and the outlet are in fluid flow communication with interior walls of the hollow fibers and disposed at each extremities of the hollow fibers. The cartridge can further comprise a purge inlet adapted to receive the gas sample having a reduced content of water. The gas purge inlet can be disposed downstream of the at least one gas sensor and being in fluid flow communication with the at least one gas sensor and with the exterior walls of the hollow fibers. The cartridge can also comprise a purge outlet which is in fluid flow communication with the exterior walls of the hollow fibers and the purge inlet, the purge outlet being adapted to exit water from the cartridge.
For example, the volume flow rate of the gas contacting the exterior wall of the at least one hollow fiber can be at least 2 times greater or about 2 to 3 times than the volume flow rate of the gas sample passed through the membrane so as to least partially reduce the amount of water present therein. For example, the volume flow rate of gas entering the purge inlet of the cartridge can be at least 2 times greater or about 2 to 3 times greater than the volume flow rate of gas entering the inlet of the cartridge.
The at least one gas sensor can be for example chosen from MOS (Metal Oxide Semiconductor) gas sensors, QMB (Quartz Microbalance) gas sensors, IRS (Infra-Red Sensor) gas sensors, CPS (Conducting Polymer Sensor) gas sensors, SAW (Surface Acoustic Wave) gas sensor, and OFS (Optical Fiber Sensor) gas sensors. For example, the at least one gas sensor can be a metal oxide semiconductor sensor.
In the method and apparatuses for detecting odor, the odor detection can be carried out for example in a continuous manner. The gas samples of a predetermined volume can be provided and analyzed in a continuous manner. For example, the method can be carried out in a continuous manner so as to analyze a plurality of gas samples one after the other, each gas sample of a predetermined volume being passed through the membrane so as to reduce the content of water present therein, contacted with the at least one gas sensor, and used to purge water out of the membrane. Alternatively, the method can be carried out in a non-continuous manner. Detection of the at least one odor can further comprise measuring the concentration of the at least one odor in the gas sample.
The means for at least partially reducing an amount of water present in the gas sample can comprise a membrane adapted to be at least substantially permeable to water and at least substantially impermeable to the at least one odor. The membrane can be a hollow fiber membrane comprising at least one hollow fiber into which the gas sample is passed through. The membrane can be a hollow fiber membrane comprising a plurality of hollow fibers.
The apparatuses can further comprise means for controlling the pressure of the gas sample. For example, the means for controlling the pressure of the gas sample can comprise a vacuum pump, a flow controller and a pressure gauge. The means for at least partially reducing the amount of water present in the gas sample can comprise a membrane adapted to be at least substantially permeable to water and at least substantially impermeable to the at least one odor. The hollow fiber membrane can comprise a cartridge comprising the hollow fibers. The cartridge can comprise an inlet for receiving the gas sample and an outlet for exiting the gas sample. The inlet and the outlet are in fluid flow communication with interior walls of the hollow fibers and disposed at each extremities of the hollow fibers. The outlet is in fluid flow communication with the at least one gas sensor. The cartridge can further comprise a gas purge inlet adapted to receive a purge gas. The gas purge inlet can be disposed downstream of the at least one gas sensor and being in fluid flow communication with the at least one gas sensor and with the exterior walls of the hollow fibers. The cartridge can also comprise a gas purge outlet which is in fluid flow communication with the exterior walls of the hollow fibers and the gas purge inlet. The gas purge outlet can be adapted to exit water from the cartridge. The apparatus can comprise a flow controller disposed between the at least one gas sensor and the gas purge inlet. The apparatus can comprise a vacuum pump disposed downstream of the gas purge outlet. The apparatus can also comprise a pressure gauge disposed between the vacuum pump and the gas purge outlet.
Further features and advantages will become more readily apparent from the following description of various embodiments as illustrated by way of examples in the appended drawings wherein:
The following examples represent in a non-limitative manner, various specific embodiments.
As it can be seen in
The membrane is in fluid flow communication with a measurement chamber 3 which comprises at least one gas sensor. The at least one gas sensor can be for example chosen from MOS (Metal Oxide Semiconductor) gas sensors, QMB (Quartz Microbalance) gas sensors, IRS (Infra-Red Sensor) gas sensors, CPS (Conducting Polymer Sensor) gas sensors, SAW (Surface Acoustic Wave) gas sensor, and OFS (Optical Fiber Sensor) gas sensors. According to a specific embodiment, the apparatus comprises a plurality of sensors. Each of the sensors can be adapted to detect and measure a particular odor. Each sensor is thus adapted to detect and measure several compounds associated to a particular odor. These sensors can be chosen from metal oxide semiconductor sensors.
The apparatus of
The measurement chamber 3 is connected to and in fluid flow communication with a flow controller 2. Such a flow controller permits to control the backflow or purge gas which is introduced into the membrane 1 so as to cause water to exit from the membrane. According to another embodiment, the purge gas can be different than the dehumidified gas sample. The apparatus shown in
The membrane can be, for example, a hollow fiber membrane comprising a cartridge comprising hollow fibers. Such a membrane can be a membrane as shown in
It was also observed that due to the chemical composition of the membrane used, the latter has a high selectivity (permeability) for water molecules, and more specifically for the presence of water vapor. Moreover, the membrane system is highly resistant to chemical attack, and therefore not corrodible. Chemical retention of the water molecules in vapor phase is thus effected before the measurement chamber.
The outlet 114 can is in fluid flow communication with the at least one gas sensor of the measurement chamber 3. The cartridge 100 further comprises a gas purge inlet 116 adapted to receive a purge gas, the gas purge inlet being disposed downstream of the at least one gas sensor of the measurement chamber 3 and being in fluid flow communication with the at least one gas sensor and with the exterior walls of the hollow fibers 110. The cartridge 100 also comprises a gas purge outlet 118 which is in fluid flow communication with the exterior walls of the hollow fibers 110 and the gas purge inlet 116. The gas purge outlet 118 is adapted to exit water from the cartridge 100.
As shown in
The gas sample to be analyzed is passed through the membrane 1 (see
As shown in
Moreover, the results prove that for given and maintained operating conditions (0.5 atm vacuum and 3 lpm exit flow rate, 29.0° temperature), the polymer membrane can maintain a practically constant humidity (see
Membrane stability and response time were also evaluated. With stable inlet humidity, stable outlet humidity is obtained (a plateau is reached around 15% RH (removal of more than 60% of water) in a two-hour test) (see
To reduce the water content of the gas sample, the apparatus needs some source of dry gas. There are various means for generating that dry air that can be used (air cylinder, zero air generator, filtration system, and others). The solution used in the case of
The previously mentioned methods and apparatuses play a key role in odor measurement by means of electronic noses, regardless of the sensor technology used. Their great effectiveness was clearly demonstrated.
Since humidity plays an important role in sensor response, it was found that important advantages are obtained when using methods and apparatuses as previously defined. In fact, it was observed that when using such methods and apparatuses, it was possible to considerably reduce the undesirable effects of humidity on electronic noses and it was also possible to render them substantially independent of humidity having regard to their sensitivity and their analyses carried out on gas samples.
It was also found that the methods and apparatuses previously described allowed to reduce the relative humidity in the odorous sample while minimizing the alteration of the chemical composition of the odor. In other words, the chemical composition of the odorous sample on a dry basis was not altered and therefore, the same was observed concerning the response given by the gas sensor after analysis of the sample.
These methods and apparatuses thus have several advantages: 1) sensor response independent of humidity, 2) increased sensor sensitivity (the water molecules occupy fewer sites at the sensor surface), and 3) longer sensor service life.
While a description was made with particular reference to the illustrated embodiments, it will be understood that numerous modifications thereto will appear to those skilled in the art. Accordingly, the above description and accompanying drawings should be taken as specific examples and not in a limiting sense.
| Number | Date | Country | Kind |
|---|---|---|---|
| 60940011 | May 2007 | US | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CA2008/000706 | 4/15/2008 | WO | 00 | 12/3/2009 |
