Patent Publication Number: US-2022221435-A1

Title: Method and device for identifying volatile compounds

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to scent recorders and volatile compounds (VCs) sensing devices. More particularly, the present invention relates to a method and a device for identifying VCs. 
     BACKGROUND OF THE INVENTION 
     Detection of VCs can be done using a variety of devices, also known as, scent recorders, scent readers, artificial noses, VC detectors etc. Some example for such devices may include: chemiresistor sensors, photoionization detectors (PIDs), ultraviolet differential optical absorption spectrometer (UV-DOAS), metal oxide sensors (MOS) and the like. Some of devises are very sensitive to repeatability of the production process, thus may vary in performance between two supposedly identical devises. For example, physical attributes, such as thickness, width, density, distribution of particles and the like. One type of such a device, the chemiresistor sensor is known to be sensitive to diversity the physical attributes of the sensor. 
     Chemiresistor sensors are sensors that can detect the presence of volatile compound (VCs). A chemiresistor sensor includes a material or structure that changes its electrical resistance in response to changes in the nearby chemical environment, for example, due to the presence of VCs. Commercial chemiresistor sensors for sensing VCs include a sensing element made from one of: carbon nanotubes, graphene, carbon nanoparticles and the like. Another optional sensor includes a sensing element made form metallic nanoparticles cores coated with organic ligands. The organic ligands are bonded to the surface of the metallic core at one end and are configured to be weakly bonded (e.g., interact) to a VC at the other end. The most suitable and widely used cores are nanoparticles of: Au, Pt, Pd Ag and further also alloys consisting of Ni, Co, Cu, Al, Au/Ag, Au/Cu, Au/Ag/Cu, Au/Pt, Au/Pd, Au/Ag/Cu/Pd, Pt/Rh, Ni/Co, and Pt/Ni/Fe. 
     The most common type of organic ligands that can bond with the surface of a metallic particle having one of the above listed metallic cores is thiol (sulfides). Thiols can be bonded with the metallic cores via functional groups such as: alkylthiols with C3-C24 chains, ω-functionalized alkanethiolates, arenethiolate, (γ-mercaptopropyl) tri-methyloxysilane, dialkyl disulfides, xanthates, oligonucleotides, polynucleotides, peptides, proteins, enzymes, polysaccharides, and phospholipids. 
     Each particle in a chemiresistor sensor may interact with more than one types of VC. However, the specific chemical—physical nature of each particle, in particular the functional groups in the organic ligands makes each of them capable of forming larger number of weak bonds with a major (e.g., favorable) type of VC. However, other VCs may also be weakly bonded to the ligands. The VC may interact or absorb in several sites of the nanoparticle, for example, core surface, bonding group, the functional groups and the like. Sensing elements known in the art usually includes a single type of particle. In such case each sensing element may be sensitive to or detect a single type of VC or may detect two or more types of VC, but without the ability to simultaneously and in real time separate between the two types of particles. 
     Chemiresistor sensors may be printed using an ink-jet printer or may be deposited using any other known method. The deposited “ink” may include one or more types of particles. Usually more than one sensor is printed to form a single scent recorder. These production methods may yield non-identical sensors within one scent recorder. For example, the sensors although having substantially the same chemical composition may vary in the thickness, width, length, distribution of particles or in any other physical attribute due to, for example, inaccuracy in the printing process. These differences affect the performance of each sensor. Some examples for such performance, may include decline or increase in measurement attributes, such as, the base resistance, base conductivity, electrical noise, base current and based voltage. Such measurement attributes may be affected from differences in the thickness, coverage, number of deposited layers of the sensors and the like. 
     Accordingly, there is a need for a method (e.g., computerized method) that can overcome the none-repeatability of the sensors and even take advantage of the none-repeatability. 
     SUMMARY OF THE INVENTION 
     Some aspects of the invention may be related to volatile compounds (VCs) sensing device, comprising: one or more scent recorders, each scent recorder comprising: a plurality of sensors from which at least two have substantially the same chemical composition and differ in at least one known physical attribute; a controller; and electrodes for connecting the one or more scent recorders to the controller. In some embodiments, the at least one known physical attribute may be selected from: the sensor&#39;s thickness, the sensor&#39;s layer coverage, Layer centering, layer morphology, the sensor&#39;s porosity, the sensor&#39;s tortuosity, the sensor&#39;s particles size, the sensor&#39;s particles distribution, thickness uniformity, organic ligands coating to conductive particle, electrode dimensions, gap between electrodes and a water contact angle of the sensors&#39; surface. In some embodiments, each sensor may be composed from a plurality of conductive particles each being covered by organic ligands. 
     In some embodiments, the controller may be configured to: receive measured attribute values from of the at least two sensors; receive signals from the at least two sensors, in response to exposure to at least one type of VC; extract values of a feature from the signals; and find mathematical correlation between the extracted values and a corresponding measured attribute value. In some embodiments, the received signals are from at least one known VC and the controller may further be configured to associate the mathematical correlation with the at least one known VC and store in a database. In some embodiments, the received signals are from at least one unknown VC and the controller may further be configured to: identify the at least one unknown VC based on the stored mathematical correlation. In some embodiments, identifying the at least one unknown VC comprises at least one of: identifying the type of the VC and the concentration of the VC. 
     In some embodiments, the measured attribute values are selected from: base resistance (e.g., background resistance), base conductivity, electrical noise, base current, based voltage, base frequency and base amplitude. 
     In some embodiments, the extracted feature values are selected from: the maximal subtracted resistance, the difference between maximum and minimum values, the average value, the maximum value, the minimum value, the first time derivative, the second time derivative, signal to noise ratio, incline gradient, decline gradient, rise time, overshooting value relative to steady state value, oscillation decay in time and oscillation frequency. 
     In some embodiments, the mathematical correlation is one of: a linear correlation, a parabolic correlation, a polynomial correlation, logarithmic correlation, exponential correlation and power correlation. 
     Some additional aspects of the invention may be directed to method of finding mathematical correlation between extracted feature values and corresponding measured attribute values, comprising: receiving at least one measured attribute from at least two sensors having substantially the same chemical composition and differ in at least one physical attribute; receiving signals from the at least two sensors, in response to exposure to at least one VC; extracting values of a feature from at least some of the plurality of signals; finding mathematical correlation between the extracted values and a corresponding measured attribute values. 
     In some embodiments, the exposure is to at least one known and the method may further include associating the mathematical correlation with the at least one known VC and store in a database. 
     In some embodiments, the method may further include: receiving signals from the at least two sensors, in response to exposure to at least one unknown VC; and identifying the at least one unknown VC based on the stored mathematical correlation. 
     In some embodiments, the method may further include training a machine learning (ML) module to identify the known VC based on the mathematical correlation. In some embodiments, the method may further include: receiving signals from the at least two sensors, in response to exposure to at least one unknown VC; and identifying the at least one unknown VC based on the stored mathematical correlation, using the trained ML module. 
     In some embodiments, the measured attribute value may be selected from: base resistance (e.g., background resistance), base conductivity, electrical noise, base current, based voltage, base frequency and base amplitude. In some embodiments, the extracted feature values are selected from: the maximal subtracted resistance, the difference between maximum and minimum values, the average value, the maximum value, the minimum value, the first time derivative, the second time derivative, Signal to noise ratio, incline gradient, decline gradient, rise time, overshooting value relative to steady state value, oscillation decay in time and oscillation frequency. In some embodiments, the mathematical correlation is one of: a linear correlation, a parabolic correlation, a polynomial correlation, logarithmic correlation, exponential correlation, and power correlation. 
     In some embodiments, the at least one physical attribute is selected from: the sensor&#39;s thickness, the sensor&#39;s layer coverage, the sensor&#39;s porosity, the sensor&#39;s tortuosity layer centering, layer morphology, the sensor&#39;s particles size, the sensor&#39;s particles distribution, thickness uniformity, organic ligands coating to conductive particle, electrode dimensions, gap between electrodes and a water contact angle of the sensors&#39; surface. 
     In some embodiments, each sensor may be composed from a plurality of conductive particles each being covered by organic ligands. 
     Some additional aspects of the invention may be directed to a method of calibrating sensors included in a scent recorder. In some embodiments, the method may include: receiving at least one measured attribute from a plurality of sensors included in the scent recorder from which at least two sensors have substantially the same chemical composition and vary in at least one physical attribute, and wherein values of the at least one physical attribute are unknown; receiving signals from the at least two sensors, in response to exposure to at least one type of VC; extracting values of a feature from at least some of the signals; finding mathematical correlation between the extracted values and a corresponding measured attribute values; and calibrating values extracted from new signals received from the at least two sensors using the mathematical correlation. 
     In some embodiments, the measured attribute values are selected from: base resistance, base conductivity, electrical noise, base current, based voltage, base frequency and base amplitude. In some embodiments, the extracted feature values are selected from: the maximal subtracted resistance, the difference between maximum and minimum values, the average value, the maximum value, the minimum value, the first time derivative, the second time derivative, Signal to noise ratio, incline gradient, decline gradient, rise time, overshooting value relative to steady state value, oscillation decay in time and oscillation frequency. 
     In some embodiments, the mathematical correlation is one of: a linear correlation, a parabolic correlation, a polynomial correlation, logarithmic correlation, exponential correlation and power correlation. In some embodiments, the at least one physical attribute is selected from: the sensor&#39;s thickness, the sensor&#39;s layer coverage, the sensor&#39;s porosity, the sensor&#39;s tortuosity layer centering, layer morphology, the sensor&#39;s particles size, the sensor&#39;s particles distribution, thickness uniformity, organic ligands coating to conductive particle, electrode dimensions, gap between electrodes and a water contact angle of the sensors&#39; surface. In some embodiments, each sensor is composed from a plurality of conductive particles each being covered by organic ligands. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which: 
         FIG. 1A , is an illustrations of scent recorder according to some embodiments of the invention; 
         FIG. 1B  is an illustration of a chemiresistor sensor according to some embodiments of the invention; 
         FIG. 1C  is an illustration of a particle in a chemiresistor sensor according to some embodiments of the invention; 
         FIG. 2  is a block diagram of a VC sensing device according to some embodiments of the invention; 
         FIG. 3  is a flowchart of a method of finding mathematical correlation between extracted value and a measured attribute value according to some embodiments of the invention; 
         FIG. 4A  is a flowchart of a method of identifying a VC according to some embodiments of the invention; 
         FIG. 4B  is a flowchart of a method of calibrating a scent recorder according to some embodiments of the invention; 
         FIG. 5  presents a graph showing the relationship between a physical attribute (the number of deposited layers) and a measured attribute value (the base resistance), and some micrographs of sensors having different numbers of deposited layers, according to some embodiments of the invention; 
         FIG. 6  presents a graph showing the relationship between another physical attribute (the coverage (e.g., area) of the deposited layers) and a measured attribute value (the base resistance), and some micrographs of sensors having different covered areas, according to some embodiments of the invention; 
         FIG. 7  presents a graph showing an extracted feature, according to some embodiments of the invention; 
         FIG. 8  presents a graph showing linear correlation between an extracted feature and measured attribute value according to some embodiments of the invention; 
         FIG. 9  presents a graph showing power correlation between another extracted feature and measured attribute value according to some embodiments of the invention; 
         FIGS. 10A-10D  presents graphs demonstrating the various steps included in the method of  FIG. 4C  according to some embodiments of the invention; 
         FIG. 11  presents graph showing an influence of accelerated degradation in the physical attributes of a sensor on the calculated mathematical correlation, according to some embodiments of the invention. 
     
    
    
     It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. 
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components, modules, units and/or circuits have not been described in detail so as not to obscure the invention. Some features or elements described with respect to one embodiment may be combined with features or elements described with respect to other embodiments. For the sake of clarity, discussion of same or similar features or elements may not be repeated. 
     Although embodiments of the invention are not limited in this regard, discussions utilizing terms such as, for example, “processing,” “computing,” “calculating,” “determining,” “establishing”, “analyzing”, “checking”, “synchronizing” or the like, may refer to operation(s) and/or process(es) of a computer, a computing platform, a computing system, or other electronic computing device (e.g., a wearable electronic computing device), that manipulates and/or transforms data represented as physical (e.g., electronic) quantities within the computer&#39;s registers and/or memories into other data similarly represented as physical quantities within the computer&#39;s registers and/or memories or other information non-transitory storage medium that may store instructions to perform operations and/or processes. Although embodiments of the invention are not limited in this regard, the terms “plurality” and “a plurality” as used herein may include, for example, “multiple” or “two or more”. The terms “plurality” or “a plurality” may be used throughout the specification to describe two or more components, devices, elements, units, parameters, or the like. The term set when used herein may include one or more items. Unless explicitly stated, the method embodiments described herein are not constrained to a particular order or sequence. Additionally, some of the described method embodiments or elements thereof can occur or be performed simultaneously, at the same point in time, or concurrently. 
     As used herein the term “volatile compound” (VC) may be related to any volatile material, either organic or inorganic compound. Some examples for VCs may include: Cr 2 , O 2 , H 2 , CH 4 , NH 3 , CH 3 OH, SO x , NO x  BTEX, Benzene, Toluene, Ortho-Xylene, Para-Xylene, Meta-Xylene and Ethylbenzene and the like. 
     The term “chemical composition” of a sensor, in the context of this application, refers to the chemical formula(s) of compound. Accordingly, two sensors having substantially the same chemical composition may include having similar components having the same chemical formulas. For example, sensors having substantially the same chemical composition may include sensors having the same conductive cores (e.g., same alloy, same conductive oxide coating and the like) covered by the same type of organic ligands. 
     As used herein, the term “physical attribute” of a sensor refers to any physical property of a sensor that does not affect the chemical composition of each of the components/particles included in a sensor. For example, physical attribute may be at least one of: the sensor&#39;s thickness, the number of deposited sensing layers, the sensor&#39;s layer coverage, sensor&#39;s layer centering, sensor&#39;s layer morphology, the sensor&#39;s particles size, the sensor&#39;s particles distribution, sensor&#39;s layer thickness uniformity, organic ligands coating to conductive particle (e.g., a metal, conductive oxide, graphite, etc.) ratio, electrode dimensions, gap between electrodes, a water contact angle of the sensors&#39; surface, deposited layer porosity, deposited layer tortuosity and the like. 
     As used herein, the term “measured attribute value” of a scent recorder refers to any measured property that can be measured by a scent recorder that was either controllably exposed to a specific VC (e.g., a calibration gas) or not exposed to any VC. In such case, the scent recorder may measure the base/background concentrations of VC in the air. For example, a measured attribute value may be or may include at least one of: base resistance (e.g., background resistance), base conductivity, electrical noise, base current, based voltage, base frequency, base amplitude, and the like. In some embodiments, the base resistance may be measured at at least one of the following conditions: under pure gas such as N 2 , in indoor or outdoor air, during flow of any of the gasses, at room temperature or at any other temperature of interest, at room pressure or at any other pressure of interest, under vacuum and the like. 
     The term “extracted feature values”, as used herein, refers to any mathematical property/attribute that may be derived from measured values from a signal received from a sensor. For example, extracted feature values may include one or more of: the difference between maximum and minimum values, the average value, the maximum value, the minimum value, the first time derivative, the second time derivative, signal to noise ratio, incline gradient, decline gradient, rise time, overshooting value relative to steady state value, oscillation decay in time, oscillation frequency and the like. 
     As used herein, the term “mathematical correlation” may include any correlation between two values (e.g., numbers). for example, mathematical correlation may be selected from: a linear correlation, a parabolic correlation, a polynomial correlation, logarithmic correlation, exponential correlation, and power correlation. Mathematical correlation according to some embodiments of the invention may include correlations between the extracted feature values and a measured attribute values, for example, the measured maximum value of the signal and the signal noise. 
     Aspects of the invention may be directed to provide more reliable, stable VC measurements that are not sensitive to: chemical degradation, drift (e.g., accumulation of VCs), wear and fatigue processes within VC sensors or scent recorders. A system and method according to some embodiments of the invention may include finding mathematical correlation(s) between values measured by the sensors or scent recorders. These mathematical correlations may not be sensitive to degradation processes of the sensors over time. These degradation processes may cause a drift in values measured by the sensors. In some embodiments, the degradation processes may result in variations in at least some of the physical attributes between two sensors included in a single scent recorder. 
     Some aspects of the invention may be related to deliberately causing differences in the physical attributes between two or more sensors included in a single scent recorder in order to determine the mathematical correlation(s) for the scent recorder. In some embodiments, the mathematical correlation may correspond to the type of physical attribute. For example, for sensors having the same chemical composition (e.g., the same conductive cores covered by the same type of organic ligands) that may be printed in various thicknesses, the mathematical correlation between the average measured value and the base resistance may be the same regardless of the sensor&#39;s thicknesses. Meaning that any change in a physical attribute (either as part of a degradation process over time or deliberately conducted) may result in corresponding changes in both the measured attribute values and extracted feature values, but the correlation between the two may stay the same, as presented in the nonlimiting example given with respect to  FIG. 11 . 
     Reference is now made to  FIG. 1A  which is an illustration of a scent recorder  120  according to some embodiments of the invention. Scent recorder  120  may include one or more sensors  122  and  123 , a communication unit (e.g., wireless or wired unit)  124  for communicating with a controller  130  (illustrated in  FIG. 2 ) and a processor  126 . Processor  126  (e.g., a chip) may collect readings from one or more sensors  122  and  123  and send them as a signal to controller  130  via communication unit  124 . 
     Sensors  122  may be any sensor that may detect the presence of one or more volatile compounds (VCs). For example, sensor  122  may include a material or structure that changes its electrical resistance in response to changes in the nearby chemical environment, for example, due to the presence of VCs. In some embodiments, sensor  122  may be a chemiresistor sensor for sensing VCs that may include a sensing element made from one of: carbon nanotubes, graphene, carbon nanoparticles, conductive polymers and the like. These chemiresistor sensors are sensitive to cleaning and regeneration cycles which are required after each measurement, due to the nonuniform nature of the sensor&#39;s material. Another optional chemiresistor sensor  122  may include conductive nanoparticles cores coated with organic ligands, as illustrated and discussed with respect to  FIGS. 1B and 1C . In some embodiments, at least two sensors  122  may have substantially the same chemical composition and may differ in at least one physical attribute. In some embodiments, one or more sensors  123  may have a different chemical composition than sensors  122 . It should be appreciated that scent recorder  120  may include an array of sensors all having the same chemical composition, such as sensors  122  or may include an array with two or more types of sensors, e.g., having different chemical composition. According to some embodiments, scent recorder  120  may include at least two sensors from each type of sensors in the array of sensors  122 ,  123 . 
     Reference is now made to  FIG. 1B  which is an illustration of a chemiresistor sensor according to some embodiments of the invention. A chemiresistor sensor  122  may include: two electrodes  110  and  115  and a sensing element  114 . Electrodes  110  and  115  may be electrically connected to electrical connectors (not insulated). Sensing element  114  may include a plurality of particles  10  (illustrated and discussed with respect to  FIG. 1C ). Particles  10  may be deposited between or on electrodes  110  and  115 , using for example, ink jet printing. In some embodiments, the thickness H of sensing element  114  may vary between at least some of deposited sensors  122  included in scent recorder  120 . In some embodiments, the width W or the length (not illustrated) may vary between at least some of deposited sensors  122  included in scent recorder  120 . In some embodiments, the distribution or density of particles  10  may vary between at least some of deposited sensors  122  included in scent recorder  120 . For example, some of sensors  122  may include sensing elements  114  having a first density or a second density or a combination the first and second densities, as illustrated. Any other number of different densities may appear in different sensors  122 . 
     Reference is now made to  FIG. 1C  which is an illustration of a particle for a chemiresistor sensor according to some embodiments of the invention. In some embodiments, particle  10  may include a nanoparticle core  12 . Nanoparticle core  12  may be made of a conductive material selected from a group consisting of: Jr, Jr-alloy, IrO x , Ru, Ru-alloy, RuO x  Au, Pt, Pd, Ag, Ni, Co, Cu, Al, Au/Ag, Au/Cu, Au/Ag/Cu, Au/Pt, Au/Pd, Au/Ag/Cu/Pd, Pt/Rh, Ni/Co, Pt/Ni/Fe and any combination thereof. In some embodiments, nanoparticle core  12  may be made from any conductive metallic oxide. In some embodiments, the average diameter of nanoparticle core  12  may be of at most 100 nm, for example, 50 nm, 20 nm, 10 nm, 5 nm and 1 nm. 
     In some embodiments, nanoparticle core  12  may include a single metal or alloy and may have crystalline structure. In some embodiments, nanoparticle core  12  may include a single metal or alloy and may have an amorphous structure. In some embodiments, nanoparticle core  12  may include more than one metal or alloy, for example, a conductive oxide such as RuO x , IrO x  and a metal, such as, Ru alloy, Ru, Jr alloy, Ir. 
     In some embodiments, particle  10  may further include a plurality of organic ligands  16  bonded, for example, via covalent bonds, from one side to the metallic core and capable of interacting with a VC. In some embodiments, the organic ligands are selected from a group consisting of: thiols, diazoniums, silanes, carboxylic acids, tri-chloro, methoxy, ethoxy, tri hydroxide, di-chloro, chloro and the like. In some embodiments, more than one type of organic ligand may be bonded with one of core  10 . For example, a core  10  that includes crystalline RuO x  may be coated with organic ligands  16  of methoxy silane and trichloro silane. In some embodiments, the organic ligands can bond from one side, the side that includes the bonding group, for example, the chloro-silane, group with the surface of core  10  and from the other side with a VC, for example, a specific functional group can be added to the ligand chain that is configured to target (e.g., interact with) a specific VC. In some embodiments, the VC may interact with the branch of the organic ligands, for example, via VDW bonds, hydrogen bonds, Pi-Pi orbital interaction (for aromatics), dipole-dipole interaction (for polar), steric interference, charge to charge interaction, coordination bond, etc. 
     In some embodiments, nanoparticle core  10  may at least be partially covered with a conductive oxide layer  18  including at least one of: IrO x  and RuO x  or any other conductive oxides. In some embodiments, when nanoparticle cores  10  include crystalline or amorphous Jr, Jr-alloy, Ru or Ru-alloy, a thin oxidation layer may form on at least a portion of the surface of nanoparticle core  10 . The thin oxidation layer may be formed due to the exposure of the nanoparticle cores to air or oxygen. The thickness of the oxidation layer may be of few nanometers, for example, a native oxide layer of 1-2 nm. In some embodiments, such an oxidation layer may allow stronger bonding between organic ligand  16  and the surface of cores  10 . 
     Reference is now made to  FIG. 2  which is a block diagram of a VC sensing device according to some embodiments of the invention. A device  100  may include one or more scent recorders  120 , for example, an array  110  of at least 2 scent recorders  120 . In some embodiments, devise  100  may further include a controller  130 . 
     In some embodiments, controller  130  may include a processor  132 , a memory  134  and an input/output unit  136 . Processor  132  may be a central processing unit (CPU) processor, a chip, PCB or any suitable computing or computational device. Memory  134  may be or may include, for example, a Random Access Memory (RAM), a read only memory (ROM), a Dynamic RAM (DRAM), a Synchronous DRAM (SD-RAM), a double data rate (DDR) memory chip, a Flash memory, a volatile memory, a non-volatile memory, a cache memory, a buffer, a short term memory unit, a long term memory unit, or other suitable memory units or storage units. Memory  134  may be or may include a plurality of, possibly different memory units. Memory  134  may be a computer or processor non-transitory readable medium, or a computer non-transitory storage medium, e.g., a RAM. In one embodiment, a non-transitory storage medium such as memory  134 , a hard disk drive, another storage device, etc. may store instructions or code which when executed by a processor may cause the processor to carry out methods as described herein. 
     In some embodiments, memory  134  may store an operation system that may include any code segment designed and/or configured to perform tasks involving coordination, scheduling, arbitration, supervising, controlling or otherwise managing operation of processor  132 . In some embodiments, memory  134  may store a code or instructions for identifying a VC and or calibrating a scent recorder such a scent recorder  120 . 
     In some embodiments, memory may store a Machine Learning (ML) module  135 . ML module  135  may, for example, use any appropriate type of a neural network (NN) based ML model for identifying VCs. ML module  134  may be trained to identify VCs. 
     Input/output unit  136  may include any unit/units that allows controller  130  to send and/or receive information from external devices, for example, scent recorders  120 ,  120 A and  120 B and user devices (e.g., laptops, tables, smartphones and the like). Input/output unit  136  may be or may include any suitable input devices, components or systems, e.g., a detachable keyboard or keypad, a mouse and the like. Input/output unit  136  may include one or more (possibly detachable) displays or monitors, speakers and/or any other suitable output devices. Any applicable input/output (I/O) devices may be connected to controller  130 , for example, a wired or wireless network interface card (NIC), a universal serial bus (USB) device or external hard drive. 
     Reference is now made to  FIG. 3  which is a flowchart of a method of finding mathematical correlations between set of extracted feature values and the corresponding measured attribute values according to some embodiments of the invention. The method of  FIG. 3  may be conducted by processor  126  or controller  130  or by any other suitable computing device. 
     In step  310 , at least one measured attribute value may be received from a plurality of sensors included in the scent recorder (e.g., sensors  122  and  123 ). In some embodiments, at least two sensors (e.g., sensors  122 ) may have substantially the same chemical composition and may vary in at least one physical attribute. For example, two or more sensors  122  included in a single scent recorder  120  may differ in at least one of: the sensor&#39;s thickness, the number of deposited layers, the sensor&#39;s layer coverage (e.g., how much electrode area is covered by the sensor), layer centering, layer morphology (e.g., porosity level, tortuosity level, etc.) the sensor&#39;s particles size, the sensor&#39;s particles distribution, the sensor&#39;s thickness uniformity, ratio between organic ligands coating to conductive particle, electrode dimensions, gap between electrodes and a water contact angle of the sensors&#39; surface (e.g., hydrophilicity or hydrophobicity). Two nonlimiting examples for the connection between measured attribute values and physical attributes are given in  FIGS. 5 and 6 . 
     In some embodiments, the difference in the physical attribute may be a result of variation in the scent recorder production process. In such case similar variations in the physical attribute may be included in all sensors  122  included in scent recorders  120  fabricated in the same fabrication line. For example, all scent recorders  120  fabricated in the same fabrication line may each include the same number of sensors  122  that may vary from one another at least in the particles&#39; distribution. In some embodiments, the difference is physical attribute may be a result of degradation over time (e.g., due to wear and fatigue processes) of various sensors  122  in scent recorder  120 . In some embodiments, the difference in physical attributes may be deliberately caused during the production process. For example, scent recorders  120  may be fabricated with two or more of sensors  122  having different thicknesses. 
     In some embodiments, the received measured attribute values may be selected from: base resistance (e.g., background resistance) base conductivity, electrical noise, base current, based voltage, base frequency and base amplitude. In some embodiments, the measured attribute values may be measured and recorded (e.g., in memory  134 ) in one of the following conditions: under pure gas such as N 2 , under indoor or outdoor air, during flow of any of type of known gasses, at room temperature or at any other temperature of interest, at room pressure or at any other pressure of interest, under vacuum and the like. 
     In step  320 , signals may be received from the of two or more sensors, in response to exposure to at least one type of VC. The exposure may change the resistance of each sensor. In some embodiments, the at least one type of VC may be a known VC, for example, the type (e.g., chemical composition) and/or the concentration of the VC may be known. Such known VCs may be used to associate the mathematical correlation with the at least one known VC and store the association in a database (e.g., memory  134 ). In some embodiments the VCs may be a unknown VCs and system  100  may be configured to identify the unknown VC, as discussed with respect to method of  FIG. 4A . 
     In step  330 , values of a feature may be extracted from at least some of the signals. The extracted values may be selected from: maximal subtracted values (e.g., the difference between the maximal resistance and the base resistance), the difference between maximum and minimum values, the average value, the maximum value, the minimum value, the first time derivative, the second time derivative, signal to noise ratio, incline gradient, decline gradient, rise time, overshooting value relative to steady state value, oscillation decay in time and oscillation frequency. A nonlimiting example, for an extracted feature value as a function of time is given in  FIG. 7 . 
     In step  340 , mathematical correlation may be found between the extracted values and a corresponding measured attribute values. In some embodiments, the mathematical correlation may be selected from a linear correlation, a parabolic correlation, a polynomial correlation, logarithmic correlation, exponential correlation, power correlation and the like. For example, a polynomial correlation may be found between the base resistance and the first time-derivative of the signal. Some nonlimiting examples for mathematical correlations are given herein below with respect to  FIGS. 8 and 9 . 
     Reference is now made to  FIG. 4A  which is a flowchart of a method of identifying volatile compounds (VCs) according to some embodiments of the invention. In some embodiments, the method of  FIG. 4A  may include steps  310 - 340  of the method of  FIG. 3 . In some embodiments, the scent recorders  120  may first be exposed to at least one known VC. For example, a scent recorder  120  having a known type of variation in at least one physical attribute (e.g., thickness) between at least two sensors  122  included in scent recorder  120  may be exposed to a variety of known types and/or known concentrations of VCs. In some embodiments, mathematical correlations may be found or calculated (e.g., by processor  123  or controller  130 ) for each type/concentration of VC. The calculated correlation may associate with the at least one known VC and store in a database (e.g., in a lookup table), in step  350 . In some embodiments, the correlation may be used to train an ML module. The ML module may receive signals from a plurality of known VCs and the corresponding calculated correlations. The ML module may identify and store (e.g., in memory  134 ) a typical calculated correlation for each type of known VCs. The typical calculated correlation may be used to later identify unknown types of VC. 
     In some embodiments, the stored correlations may be used to identify unknown VCs. In some embodiment, the same scent recorders  120  or scent recorders  120  made in the same production line having the same known type of variation in at least one physical attribute may be used. Scent recorder  120  may be exposed to one or more unknown VCs. In some embodiments, the method may include steps  310 - 340  of the method of  FIG. 3 . In step  360 , at least one type of VC may be identified based on the stored mathematical correlation. In some embodiments the identification may be done using the trained ML module (e.g., module  135 ). For example, controller  130  may be configured to use ML module  135  to identify at least the type and/or concentration of the one or more unknown VCs. 
     Reference is now made to  FIG. 4B  which is a flowchart of a method of calibrating a scent recorder according to some embodiments of the invention. In some embodiments, scent recorder  120  may include two or more sensors  122  having unknown values of the at least one physical attribute. Embodiments of the method may include steps  310 - 340  of the method of  FIG. 3 . In some embodiments, the at least one unknown physical attribute may result from variation during the production process or from wear processes during the scent recorder service time. In step  360 , the mathematical correlation may be used to calibrate values extracted from futuristic signals received from the scent recorder. Some nonlimiting examples of such a calibration process are presented in  FIGS. 10A-10D . 
     Reference is now made to  FIG. 5  which presents a graph showing a nonlimiting example for relationship between a physical attribute (the number of deposited layers) and measured attribute values (the base resistance) taken form 16 sensors, and some micrographs of sensors having different numbers of deposited layers, according to some embodiments of the invention. As shown in the graph of  FIG. 5  there is a mathematical correlation between the number of deposited layers and the base resistance of each sensor. 
     Reference is now made to  FIG. 6  presents a graph showing a nonlimiting example for relationship between another physical attribute, the coverage (e.g., area) of the deposited layers, having the same number of deposited layers, and measured attribute values (the base resistance) taken form 16 sensors, and some micrographs of sensors having different covered areas, according to some embodiments of the invention. As shown in the graph of  FIG. 6  there is a mathematical correlation between the area covered by the deposited layers and the base resistance of a sensor. 
     The calculated correlations shown in  FIGS. 5 and/or 6  may be applicable to all sensors having the same chemistry, produced in the same production line under the same production conditions. 
     Therefore, since measuring the measured attribute values is a lot simpler than measuring the physical attribute, changes in measured attribute values, such as the base resistance, may be indicative to changes in one of more physical attribute. 
     Reference is now made to  FIG. 7  which shows graphs of extracted feature values from various sensors, according to some embodiments of the invention. The graphs show subtracted resistance values (the measured resistance subtracted by the base resistances) measured over time in 16 sensors having substantially the same chemical compositions. The sensors differ in the value of at least one physical attribute (e.g., the thickness of the sensor). The drop in the subtracted resistances in all sensors indicates the exposure of the sensor to a VC. The exposure was to a known VC at a known concentration. 
     Reference is now made to  FIG. 8  which presents a graph showing linear mathematical correlation between extracted feature values and measured attribute values according to some embodiments of the invention. The graph of  FIG. 8  shows linear correlation between the maximal subtracted resistance values (e.g., the difference between the maximal resistance and base resistance) extracted feature and the base resistance measurement attribute. The correlation was based on 16 sensors with different sensing layer thickness. 
     Reference is now made to  FIG. 9  which presents a graph showing power mathematical correlation between another extracted feature values and measured attribute values according to some embodiments of the invention. The graph of  FIG. 9  shows polynomial correlation between rising time extracted feature and base resistance measurement attribute. The correlation is based on 16 sensors with different sensing layer thickness. 
     Reference is now made to  FIGS. 10A, 10B, 10C and 10D  which present graphs demonstrating the various steps included in the calibration method of  FIG. 4C  according to some embodiments of the invention. 
     The graphs in  FIG. 10A  show the base resistances measured over time in 8 sensors which differ only in the sensor thickness. The base resistances of  FIG. 10A  is a nonlimiting example, for the measured attribute values, received in step  310  of  FIG. 3 . As shown hereinabove, the difference in the base resistance may be indicative of changes in the thickness between the different sensors. 
     The graphs in  FIG. 10B  show the feature subtracted resistances (the difference between the measured resistance and the base resistance) value extracted from signals received from the 8 sensors over exposure cycle (a nonlimiting example for step  320 ). The rise in the leveled resistances value in all sensors indicates the exposure of the sensor to a VC. The exposure was to a know VC at a known concentration. 
     An additional extracted feature may be calculated from the graphs of  FIG. 10B . The maximal subtracted resistance value from each signal may further be calculate (e.g., as in step  330  of the method of  FIG. 3 ). 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                   
                   
                   
                 Maximal Subtracted 
               
               
                   
                   
                   
                 resistance = max 
               
               
                   
                 Maximal resistance 
                 Base resistance 
                 resistance − base 
               
               
                   
                 [kΩ] at 
                 [kΩ] at 
                 resistance 
               
               
                 Sensor # 
                 t = 82 seconds 
                 t = 81 seconds 
                 [kΩ] 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 1 
                 661.9598 
                 662 
                 10.42377 
               
               
                 2 
                 864.6587 
                 865 
                 14.44472 
               
               
                 3 
                 994.5364 
                 996 
                 16.05755 
               
               
                 4 
                 832.6055 
                 833 
                 12.54964 
               
               
                 5 
                 915.9909 
                 917 
                 14.87218 
               
               
                 6 
                 1210.547 
                 1213 
                 19.06258 
               
               
                 7 
                 1986.338 
                 1990.5 
                 31.5549 
               
               
                 8 
                 1572.592 
                 1574 
                 26.13079 
               
               
                   
               
            
           
         
       
     
     A nonlimiting example for step  340  of  FIG. 3  is given in  FIG. 10C . The graph in  FIG. 10C  shows the linear correlation between the extracted maximal subtracted resistance value as function of the base resistance. 
     The graph in  FIG. 10D  shows a nonlimiting example, for step  670  of  FIG. 4C , calibrating values extracted from a new signal received from the scent recorder using the mathematical correlation calculated in  FIG. 10C . As shown in  FIG. 10D  after calibration all the received signals presents similar resistance behavior over exposure cycle, regardless of the differences in the physical attributes (e.g., the thickness of the sensors). 
     Reference is now made to  FIG. 11  which shows a nonlimiting example of the influence of accelerated degradation in the physical attributes of a sensor on the calculated mathematical correlation, according to some embodiments of the invention. Sensors having substantially the same chemical composition were heated to elevated temperature for 3, 6 and 17 hours in order to accelerate the thickness degradation of the sensors. The base resistance was measured after each heating cycle. After each heating cycle the sensors were exposed to a known VC to which the maximal subtracted resistance was extracted from signals received from the sensors. The graph shows the linear correlation between the maximal subtracted resistance and the base resistance of all tested sensors regardless to the deviation in production processes or the degradation processes of the sensors. Accordingly, a mathematical correlation calculated for sensors having the same chemical composting, may not change in the entire service time of all sensors fabricated at the same production line. 
     While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents may occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. 
     Various embodiments have been presented. Each of these embodiments may of course include features from other embodiments presented, and embodiments not specifically described may include various features described herein.