CA2947079A1 - Apparatus for volatile organic compound (voc) detection - Google Patents

Abstract

Provided is an apparatus for the detection of volatile organic compounds (VOCs) for biological analysis, environmental testing and analytical testing. The gas detection apparatus includes: a channel having an inner surface and having at least one opening, such that the channel is optionally in fluid communication with a sample gas, the inner surface having a coating comprising: a first layer comprising a non-reactive metal or non-reactive metalloid compound; a second layer comprising a moisture barrier with high porosity; and a gas sensor disposed within the channel. Embodiments described herein provide low cost and highly selective gas detectors.

Description

APPARATUS FOR VOLATILE ORGANIC COMPOUND (VOC) DETECTION
[0001] TECHNICAL FIELD

[0002] The present invention provides an apparatus for detecting and differentiating volatile organic compounds (VOC). In particular, this invention relates to gas detection apparatus having a coated channel and a gas sensor.

[0003] BACKGROUND

[0004] There is a need for rapid, sensitive and high precision detectors of volatile organic compound (VOC) gases for different applications including beverage and food quality assessment [1], analytical chemistry [2], biological diagnosis [3-5], and safety and environmental monitoring [6]. Numerous approaches have been developed for detection of VOCs. Gas chromatography (GC) [7] and mass spectrometry (MS) [8] are the most commonly used methods which provide high sensitivity and selectivity. However, miniaturization of these methods which is required for numerous emerging applications [9-10] is challenging due to the complexity of their fabrication, calibration and sample extraction processes.
Moreover, their high cost and long processing time hinder the implementation of these techniques to applications which require disposable and rapid detection methods [11].

[0005] More recently, electronic noses (e-nose) have been used as an alternative method of gas detection. E-nose systems are based on sensor arrays coupled with pattern recognition systems. In an e-nose system, the gas sensor array provides a fingerprint response to a given odor; then, a pattern recognition software tool is used to perform odor identification and discrimination [12-13]. Despite the general success of electronic noses, there are practical challenges in adaptation of this technology: in essence, the inevitable multidimensional drifts of the components of the sensor array result in frequent replacement of the expensive parts and cumbersome recalibrations [14]. Moreover, since general-purpose gas sensors are not selective against different gases, the sensor array used in e-noses is required to have a specific sensor for detecting each target gas. This makes the drift compensation and sensor recalibration even more complicated [15-16].

[0006] Recently, microfluidic-based gas detectors with high selectivity and sensitivity features of both traditional methods (GC and MS) and e-noses have been introduced [17-21].
These systems function based on analyzing the kinetic response of diffused gases in micro-channels using a single general purpose gas sensor [18-21]. As each gas has different diffusion and physical adsorption rates, microfluidic-based gas detectors successfully differentiate among the components of a mixture (and even binary mixtures of different isomers) [20]. Although these devices are selective to different gases, they cannot differentiate among components of complex mixtures at low concentrations. Moreover, due to the slow process of gas diffusion in the microchannels and also chemical adsorption of gas molecules to the channel walls, the recovery process of fabricated sensors takes relatively long time (up to 10 minutes) [20]. It has been recognized that the diffusion constants of a target gas depends on the temperature of the diffusion medium [29] and clearance of a channel may be accomplished by providing flow of air or a pure gas in the opposite direction of the diffusion process [29].
However, the design of microfluidic-based gas detectors must be further optimized to improve their performance.

[0007] SUMMARY

[0008] The present invention is based in part on the discovery that different channel coating materials can have a beneficial effect the performance of the microfluidic-based gas detectors.
In particular, 11 different coating combinations for the channel were compared. Moreover, the geometry of the channel was optimized to study the effect of channel dimensions on the selectivity and recovery time of the device. To show the diagnostic power of the developed miniaturized gas detector, in terms of differentiating small concentrations (ppm level) of different volatile organic compounds (VOCs), a range of different target gases including alcohol and ketone vapors; methanol and tetrahydrocannabanol (THC) were tested and successfully differentiated. As described herein, the selectivity of microfluidic gas detectors may be significantly enhanced by optimizing the micro-channel geometry and surface treatment.
Moreover, the sensor recovery time may be reduced to 150 seconds, which is significantly faster than the recovery time reported in previous studies [20]. Furthermore, the integration of heaters along the micro-channels to enhance the diffusion rate of the THC
molecules in the channel and decreasing the sensor response and recovery time to below 200 s.
Accordingly, the improvements described herein may advance the state-of-the-art gas analysis methods, but especially for applications [22] requiring real-time sensing.

[0009] In accordance with a first embodiment, there is provided a gas detection apparatus, the apparatus including: (a) a channel having an inner surface and having at least one opening, such that the channel may be in fluid communication with a sample gas through the opening, the inner surface having a coating including: (i) a first layer comprising a non-reactive metal or non-reactive metalloid compound; (ii) a second layer comprising a moisture barrier; and (b) a gas sensor disposed within the channel.

[0010] In accordance with a further embodiment, there is provided a gas detection apparatus, the apparatus including: (a) a channel having an inner surface and having at least one opening, such that the channel may be optionally in fluid communication with a sample gas when the opening is in an open position and optionally not in fluid communication when the opening is in a closed position, the inner surface may have a coating including: (i) a first layer comprising a non-reactive metal or non-reactive metalloid compound; (ii) a second layer comprising a moisture barrier; and (b) a gas sensor disposed within the channel.

[0011] In accordance with a further embodiment, there is provided a gas detection apparatus, the apparatus including: (a) a channel having an inner surface and having at least one opening, such that the channel may be optionally in fluid communication with a sample gas when the opening is in an open position and an optional closed position, the inner surface may have a coating including: (i) a first layer comprising a non-reactive metal or non-reactive metalloid compound; (ii) a second layer comprising a moisture barrier; and (b) a gas sensor disposed within the channel.

[0012] In accordance with a further embodiment, there is provided an apparatus comprising the gas detection apparatus described herein for use in a Tetrahydrocannabinol (THC) breathalyzer.

[0013] In accordance with a further embodiment, there is provided an apparatus comprising the gas detection apparatus described herein for use in natural gas leakage detection.

[0014] In accordance with a further embodiment, there is provided an apparatus comprising the gas detection apparatus described herein for use in nuisance sewer gas detection.

[0015] The second layer may include a moisture barrier has a gas permeability sufficient to absorb the gas particles being sampled. The non-reactive metal may be selected from one or more of the following: copper; chromium; ruthenium; rhodium; palladium; gold;
silver;
osmium; iridium; platinum; titanium; niobium; tantalum; bismuth; tungsten;
tin; nickel; cobalt;
manganese; and zinc; or (ii) may be metalloid compound is SiO2. The moisture barrier with high porosity may be Parylene or Polydimethylsiloxane (PDMS). The Parylene may be selected from Parylene C, Parylene N or Parylene D. The Parylene may be Parylene C. The non-reactive metal may be selected from one or more of the following: copper; chromium;
ruthenium;
rhodium; palladium; gold; silver; iridium; platinum; titanium; niobium; and tantalum. The coating may be chromium, gold and Parylene C. The channel may further include a heater. The heater may be operable to increase the channel temperature to at least 8o C.
The gas sensor may be a Metal Oxide Semiconductor (MOS). The gas sensor may be a tin oxide-based chemoresistive gas sensor. There may be more than one gas sensor in the channel. There may be a pluralitiy of channels with one sensor per channel. There may be a pluralitiy of channels with more than one gas sensor in the channel. The channel length to channel depth ration may be 15o:i. The channel width to channel depth ration may be 3:1. The channel length may be 3mm wide, 30 mm long and 200 pLM deep. The first layer may include chromium and gold.
The chromium may be applied to the channel prior to the gold. The second layer may include Parylene C. The first layer may include Si02. The second layer may include Parylene C. The opening may further include a closed position. The opening may further include a open position. The opening may include an open and a closed position. The apparatus may further include a second opening. The second opening may have both an open and closed position.

[0016] The apparatus may further include a liquid trap positioned in fluid communication with the at least one opening. The apparatus may further include a humidity filter positioned in fluid communication with the at least one opening. The apparatus may further include may further include a pump which may optionally be in fluid communication with the second opening. The apparatus may further include a compressed air source which may optionally be in fluid communication with the channel. The apparatus may further include a compressed gas source which is optionally in fluid communication with the channel. The apparatus may further include a pentane plume which may optionally be in fluid communication with the channel. The apparatus may further include a compressed 02 source or N2 source or separate 02 and N2 sources which may optionally be in fluid communication with the channel. The apparatus may further include a cleaning solution which may optionally be in fluid communication with the channel. The compressed gas source may be selected from one or more of the following: air;
pentane; CO2; 02; or N2. The more than one compressed gas source, may be selected from the following: air; pentane; CO2; 02; or N2*
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 shows a schematic of a chemo-resistor (MOS gas sensor) and it's bias circuit, where Vb is the bias voltage for the sensor and VII is the voltage across the heater (A); (B) shows the equivalent electrical circuit of the sensor in a DC bias; and (C) shows a typical response of a sensor exposed to a certain concentration of a certain gas, wherein i/Rair and i/Rgas are the conductances of the sensor in clean air and after exposure to a gas, respectively.

[0017] FIGURE 2 shows a schematic of a MOS gas sensor and its bias circuit exposed to two different gases in (A); typical transient responses of the sensor to two different gases (. and A) are almost the same in (B); a schematic of a MOS gas sensor integrated with a micro-channel and its bias circuit exposed to two different gases in (C); typical transient responses of the microfluidic-based gas sensor to two different gases (= and A) are distinct in (D); and a schematic of the gas sensor integrated with a micro-channel is shown in (E), wherein analyte molecules diffuse into the channel, and some of the molecules get adsorbed while some of the adsorbed molecules get desorbed.

[0018] FIGURE 3 shows a schematic of the experimental setup in (A), wherein the sensor is mounted on a chamber, while three different positions (i.e. Bl; B2; and B3) are overlaid on a typical normalized transient response of the sensor to a concentration of a gas, with Bi showing the analyte injection position; B2 showing the exposure position; and B3 showing the recovery position.

[0019] FIGURE 4 shows a schematic diagram of an embodiment of a 3D-printed gas detector, having a channel coated with chromium (Cr), gold (Au), and Parylene C, wherein the Cr forms a part of the first layer with Au and the Parylene C forms the second layer of the channel.

[0020] FIGURE 5 shows normalized responses from six sensors with six different coating material combinations deposited on the channel to 2000 ppm Ethanol (coatings are as follows:
(1) Si02 and Parylene C; (2) Parylene C alone; (3) Copper and Parylene C; (4) chromium/gold and Parylene C; (5) chromium and gold; and (6) chromium/gold and Cytonix).

[0021] FIGURE 6 shows normalized responses for three different analytes (i.e. ethanol (-);
methanol (0); and acetone( A)) with four different channel coatings, as follows: (A) Si02 and Parylene C, (B) Cr and Au, (C) Cu and Parylene C, (D) Cr and Au and Parylene C.

[0022] FIGURE 7 shows typical normalized responses for three different analytes (i.e.
ethanol (-); methanol (0); and acetone( A)), wherein the separation factor is defined to show the differentiation power of the sensor.

[0023] FIGURE 8 shows a feature space for the sensor with the coating combination of Cr and Au and Parylene C, which had the best performance for three VOCs (Acetone:
V, Ethanol: X, and Methanol: 0) in terms of selectively and recovery time.

[0024] FIGURE 9 shows normalized responses for three different analytes (i.e. ethanol (-);
methanol (0); and acetone( A)) for four different channel dimensions, as follows: 1=20 MM;
d=500 1AM (A); 1=30 mm, d=500 pm (B); 1=40 mm, d=500 im (C); and1=30 mm, d=200 pm (D).

[0025] FIGURE 10 shows recorded transient responses for 8 different concentrations (250 ppm-4000 ppm) for 6 different targets, including three alcohols: 2-Pentanol (A), Methanol (B), Ethanol (C), and three ketone: Acetone (D), 2-butanone (E), 2-pentanone (F).

(0026] FIGURE ii shows a feature space presentation for all the responses shown in FIGURE 10.

[0027] FIGURE 12 shows a schematic of a breath-analyzer prototype.

[0028] FIGURE 13 shows normalized responses of the sensor to (A) THC-methanol, and (B) pure methanol at different temperatures, wherein the 3D feature space is shown for (C) THC-methanol and (D) pure Methanol, and features Fi. and F2 are the points in time at which the normalized response level reaches 5% and 95% of the maximum level, respectively, and F3 is the magnitude of the normalized response at the final read out, wherein (25 C:
V, 40 C: X, and 80 C: 0).

[0029] FIGURE 14 shows normalized responses for two different analytes A (.);
and B (s);
the selectivity factor is defined to examine the differentiation power of the sensor is shown in (A) and the sensor response time and selectivity factor between binary mixture of THC-methanol and methanol vs. channel temperature is shown in (B).

[0030] FIGURE 15 shows a scanning electron micrograph (SEM) to demonstrate pore size of a typical the channel surface coated with parylene C (i.e. pore size is about 50 nm, with a range of between 36 nm and 84 nm).

[0031] FIGURE 16 shows a typical transient response of the sensor to a concentration of a gas in (A); and the feature extraction method used for identification of the concentration of the analyte is presented in (B), wherein the three selected features are the maximum level of the transient response (Fl), the response level at the final readout (F2), and the area under the transient response curve (F3).

[0032] FIGURE 17 shows the transient response of the sensor to three different concentrations of ethanol, i.e., moo ppm (X), 2000 ppm (0), and 3000 ppm (V) in (A); and the feature space (using the method described in FIGURE 16) is presented for identification of the concentration of the analyte in (B).

[0033] FIGURE 18 shows the regression model used for characterization of the concentration of the analyte (C) with respect to the area underneath the transient response curve (A), wherein the relation between the concentration and the average of the area underneath the curves is linear and each square marker is the average of 5 points and the error bars present the deviation from the average.

[0034] FIGURE 19A shows a schematic of an embodiment for a pentane detector using a single microfluidic sensor, wherein the sensor uses solenoid valves to expose the sensor to the pentane plume prior to recovery with compressed on-board gas and the purging air exits through the exhaust valve.

[0035] FIGURE 19B shows a schematic of an embodiment for a UAV-mountable detector for NG leakage monitoring, having a valve network and sensor array ensures rapid sampling of surrounding air for fugitive NG, with compressed air or another source of clean air (uncontaminated with target gases) to recover the sensor.

[0036] FIGURE 20 shows a schematic of an embodiment for nuisance sewer gas detector including the supporting systems and sensing unit.
DETAILED DESCRIPTION

[0037] Any terms not directly defined herein shall be understood to have the meanings commonly associated with them as understood within the present field of art.
Certain terms are discussed below, or elsewhere in the specification, to provide additional guidance to the practitioner in describing the compositions, devices, methods and the like of embodiments, and how to make or use them. It will be appreciated that the same thing may be said in more than one way. Consequently, alternative language and synonyms may be used for any one or more of the terms discussed herein. No significance is to be placed upon whether or not a term is elaborated or discussed herein. Some synonyms or substitutable methods, materials and the like are provided. Recital of one or a few synonyms or equivalents does not exclude use of other synonyms or equivalents, unless it is explicitly stated. Use of examples in the specification, including examples of terms, is for illustrative purposes only and does not limit the scope and meaning of the embodiments described herein.

[0038] The most widely-used type of gas sensors is Metal Oxide Semiconductor (MOS) gas sensors [23]. In the basic configuration of MOS sensors, which is shown in FIGURE IA, a chemo-resistor is made by deposition of a thick film metal oxide sensing pallet and a thick film thermo-resistor micro-heater on the opposite surfaces of a millimeter-scale ceramic substrate [23].

[0039] The electrical behavior of a MOS sensor in a DC bias can be modeled as a variable resistance Rs (see FIGURE 1B). The value of this resistance depends on the type of the gas molecule, the gas concentration, and the temperature of the sensing pallet.
The resistance of the sensor in the clean air is called baseline resistance (Rail.). The sensitivity (S) of such a sensor is defined by Rair = - , (1) Rgas where Rair and Rgas are the resistances of the sensing pallet measured in the clean air and target gas, respectively (see FIGURE iC). The selectivity of a sensor between two gases (i, j) is defined by s =
Sel(i, j) = , (2) S =
where Si and Sj are the sensitivity of the gas sensor to gas i and j, respectively.

[0040] Current off-the-shelf gas sensors are inexpensive and durable, however, they are either made to be evenly sensitive to different gases or fabricated for detecting a single specific target. Hence, differentiating among different gases or gas mixtures using a single sensor is very challenging, as the transient responses of the sensor to two different gases are almost the same.
The schematic of a MOS gas sensor and its bias circuit and responses of the sensor to two different gases are depicted in FIGURES 2A and 2B. To enhance the selectivity of the gas sensor, it can be integrated into a microfluidic channel. The schematic of a MOS gas sensor equipped with a channel and its bias circuit is shown in FIGURE 2C. The microfluidic-based gas sensor can provide distinct kinetic responses for different gases (see FIGURE 2D). The response of such a sensor is dependent on (a) the analyte diffusivity in the surrounding media (air), and (b) the physical adsorption/desorption rate of the gas molecules to/from the channel walls (see FIGURE 2E).

[0041] The analyte concentration, C(x, t), changes along the channel over time as a result of diffusion of the gas molecules into the channel. The gas concentration can mathematically be predicted by the solving the diffusion¨ physical adsorption (physisorption) equation [213] of (1 2Ca a )ac(x,t) D 02c(x,t) (3) d (i+a c(x,t))) at a X2

[0042] where Ca is the number of the surface adsorption sites available per unit volume of the channel, a is a modified Langmuir constant, d is the effective microfluidic channel depth, and D is the analyte diffusion coefficient (diffusivity) in air [24].

[0043] As used herein "gas permeability" refers to the rate at which a gas or vapor passes through the channel coating. The gas permeability process includes absorption of the gas or gases into the channel coating and subsequent desorption of the of the gas or gases from the channel coating. The second layer may include a moisture barrier having a gas permeability sufficient to absorb and desorb the gas particles being sampled. Accordingly, the coatings may be optimized for the testing of a particular sample. Factors which may affect permeability of a polymer include the following: chain packing; side group complexity; polarity;
crystallinity, orientation; fillers; humidity; and plasticization. Furthermore, the non-reactive metals and non-reactive metalloid compounds used are non-porous and have very low permeability as compared to parylene C, which will stop the gas from going down and reaching to the substrate or the channel and facilitate desorption of the VOC.

[0044] Gas permeability is significant, since sufficient permeability is needed to adsorb and desorb the gas molecules. The molecular dimensions of most VOCs are couple of angstroms so they can diffuse into the voids of Parylene C (which are on average about 50 nm, see FIGURE
15) and reach to the first layer of the channel.

[0045] TABLE 1: Properties of Parylene N, C and D
Parylene Barrier Parylene N Parylene C Parylene D
Properties Nitrogen Gas 7.7 0.95 4.5 Permeability (cm3-mil/loo in2-24hr-atm (23 C)) Oxygen Gas 30 7.1 32 Permeability (cm3-mil/ioo in2-24hr-atm (23 C)) Carbon Dioxide Gas 214 7.7 13 Permeability (cm3-mil/loo in2-24hr-atm (23 C)) Hydrogen Sulfide Gas 795 13 1.45 Permeability (cm3-mil/100 in2-24hr-atm (23 C)) Sulphur Dioxide Gas 1,890 11 4.75 Permeability (cm3-milhoo in2-24hr-atm (23 C)) Chlorine Gas 74 0.35 0.55 Permeability (cm3-mil/loo in2-24hr-atm (23 C)) Moisture Vapor 1.50 0.14 0.25 Transmission (g-mil/loo in2 -24hr, 37 C, 90% RH) Data from Para Tech Parylene Property Data Sheet and gathered following appropriate ASTM methods

[0046] As used herein "porosity" refers to the "void fraction" which is a measure of the void or empty spaces in a material, and is calculated as a fraction of the volume of voids over the total volume of the material (i.e. between o and 1, or as a percentage between o and l00%). The porosity may be measured with a BET (Brunauer¨Emmett¨Teller) measurement device or other surface analysis device. As used herein "porosity" may be a measure of the "accessible void"
(i.e. the total amount of void space accessible from the surface) or "total void" as known in the art. Accordingly, "porosity" may be used as an alternative measure for determining the suitability of a particular coating to make up the second layer which includes a moisture barrier.

[0047] As used herein "moisture barrier" refers to a water impermeable material or compound. In some embodiments, a parylene (i.e. poly(p-xylylene) polymers) may be used to form the moisture barrier, in part because the parylene polmers may be added in a thin uniform layer that is chemically inert. Some common gas permeabilities and moisture vapor transmission for Paylenes N, C and D are given in TABLE 1. There are a number of parylenes commonly used.
\
(H2C . CH2 1.---

[0048] Parylene N In

[0049] Parylene N has the highest dielectric strength of the three versions, and a dielectric constant value independent of frequency. It is able to penetrate crevices more effectively than the other two versions because of the higher level of molecular activity that occurs during deposition. Parylene N is commonly used in high frequency applications because of its low dissipation factor and dielectric constant values.
CI
--(H2C 40 CH2)

[0050] Parylene C n

[0051] Parylene C differs chemically, having a chlorine atom on the benzene ring that results in a useful combinationof electrical and physical properties including particularly low moisture and gas permeability. This version deposits on substrates faster is than Parylene N, with a consequent reduction in crevice penetration activity.
/ CI
H2c 4. CH2)7 \

[0052] Parylene D CI

[0053] Parylene D has two chlorine atoms added to the benzene ring. This gives the resulting film greater thermal stability than either Parylene N or C, but Prylene D has reduced ability to penetrate crevices as compared to Parylenes N and C.

[0054] As used herein "reactivity" refers to the tendency of a substance (i.e. an element or compound) to undergo a chemical reaction, either by itself or with other substances. However, all elements and compounds (except helium) undergo at least some chemical reactions under the proper conditions.

[0055] As used herein "non-reactive" refers to a reduced or limited tendency of a substance (i.e. an element or compound) to undergo a chemical reaction, either by itself or with other substances and not a complete absence of reactivity. Furthermore, a non-reactive element or compound will still undergo physical reactions (adsorption and desorption) with the VOCs diffusing through the channel.

[0056] A non-reactive metal may be selected from one or more of the following:
copper;
chromium; ruthenium; rhodium; palladium; gold; silver; osmium; iridium;
platinum; titanium;
niobium; tantalum; bismuth; tungsten; tin; nickel; cobalt; manganese; and zinc. The non-reative metalloid compound may be Si02.

[0057] METHODS AND MATERIALS

[0058] Gas Detector Setup

[0059] The schematic diagram of the experimental setup is shown in FIGURE 3A.
The device consists of a gas chamber, three-dimensional (3D) printed microfluidic channel and gas sensor. The sample in liquid phase is injected into the chamber through its opening using a precise Pipet-Lite XLSTM microsampler (analyte injection stage shown in FIGURE
3B1). After a few minutes, the sample is evaporated into the chamber. The sensor is rotated around the hinge and exposed to the gas inside the 1 L polymethyl methacrylate (PMMA) chamber for 40 seconds (exposure stage shown in FIGURE 3B2). The gas molecules diffuse into the micro-channel and reach the sensing pallet of the sensor, which is placed at the other end of the channel. The competition between the diffusion process and adsorption of the gas molecules to the available adsorption sites on the channel walls creates a unique response of the sensor (also known as the smell-print). The different smell-prints of gases result in selective sensing of different gases. Finally, the sensor is rotated back to its original position where it is exposed to clean air again and the gas molecules diffuse out from the channel (recovery stage shown in FIGURE 3B3). Alternatively, the channels could be flushed with clean air or gas (for example, 02 or CO,) to shorten the recovery time. The data may be collected (using a microprocessor) for 100 seconds. The device remains in this position for 150 seconds or less where the channel is flushed before the sensor becomes fully recovered and ready for the next test.
Most of the experiments were all carried out at the room temperature (25 1 C), and relative humidity of 40 5%.

[0060] Feature Extraction

[0061] The typical normalized response of the sensor to a typical gas concentration is shown in FIGURE 3. Note that the normalization process eliminates the effects of the analyte concentration and baseline variations from the responses. Using equation (2), the sensor conductance (G(t) = VR(t)) change is normalized as G G(t)¨min(G(t)) t) ,( (4) max(G(t))¨min(G(tV
where Gn(t), min(G(t)) and max(G(t)) are the normalized conductance, minimum value of the measured conductance and the maximum value of the measured conductance, respectively.
Three significant features are extracted and used from each response [20]: a) tr which is the time at which the normalized response level reaches 0.05, b) tm which is the time at which the normalized response level reaches 0.95, and c) Rf which is the magnitude of the normalized response at the final read out. A 3D feature space coordinate is defined based on tr, tm, and Rf, where each response is depicted as a point (tr, tm, Re. The regular atmosphere of the laboratory is the background media for all the experiments.

[0062] Fabrication Process

[0063] The fabrication process for each component of the system is explained below: Gas sensor: A commercially available tin oxide-based chemoresistive gas sensor (SP3-AQ2, FIS
Inc.TM, Japan) was used in this study. The nominal operating temperature is 300 C was maintained by applying 5 V DC to the microheater. The bias circuit for the sensor is depicted in FIGURE 1.

[0064] Microchannel: The microchannels and micro-chambers were printed with a printer (ConnexTM 500), using the material VeroClear RGD81oTM (see FIGURE 4).
To study the effect of channel dimensions and channel surface treatment on the selectivity and recovery time of the sensors, different devices were printed with different channel sizes. Channels with six different dimensions including three lengths (2 cm, 3 cm, and 4 cm) and two heights (200 mm and 500 jam) were fabricated. The width of the channel, which was limited to the dimensions of the sensor chamber, was kept at 3 mm for different channel dimensions.

[0065] Channel Coating: The inner surfaces of the micro-channels were coated with single layers and multi-layer combinations of different materials including: gold (with chromium under for adhesion), copper, CytonixTM (Cytonix LLCTM, Product: PFCM 1104V), and Parylene C
(poly (p-xylylene) polymer, CAS No: 28804-46-8). The total number of ii sensors (listed in TABLE 2) were fabricated using different material combinations for the channel coating. For some of the targets (such as Au, Cr, Cu, and Si02) the channel surfaces were coated using Physical Vapor Deposition (PVD) sputtering machine (Angstrom EngineeringTM, NexdepTM
deposition system). Parylene C was coated using a Chemical Vapor Deposition (CVD) Parylene C coating machine (SCSTM. PDS 2010 LabcoaterTm), and for the CytonixTM the dip in and spin coating methods were both used. Inner surfaces of the microchannel shown in FIGURE 4 were coated with multi-layer materials including 65 nm gold (with 35 nm chromium under for adhesion) and 4 [tm Parylene C.

[0066] TABLE 2 Different Channel Coating Used for Sensor Fabrication Number Single Layer/Multilayer Coating Coating Method 1 VeroClear RGD8i0 No coating (3D printed material) 2 Copper (Cu) Sputtering 3 Chromium (Cr) & Gold (Au) Sputtering 4 Parylene C CVD
Si02 Sputtering 6 Cytonix Spin Coating 7 Cu & Cytonix Sputtering (Cu) & Spin coating (Cytonix) 8 Cr &Au & Cytonix Sputtering (Cr and Au) &
Spin coating (Cytonix) 9 Cu & Parylene C Sputtering (Cu) & CVD (Parylene C) 113 Cr & Au & Parylene C
Sputtering (Cr and Au) 8z CVD (Parylene C) 11 Si02 & Parylene C Sputtering (Si02) & CVD
(Parylene C)

[0067] Chamber: A small opening on the chamber (made of PMMA) was provided for both analyte injection and purging clean air into the chamber. An electric fan (DC
Brushess. DC24V.
1.41A. Delta ElectronicsTM) was installed in the chamber to make a uniform environment inside the gas chamber. The microchannel was attached to the chamber using a screw hinge, which allows the device to rotate on the chamber. The sensor was first exposed to the clean air.

[0068] The following methods and materials were employed with respect to the EXAMPLES
described herein.

EXAMPLES

[0069] EXAMPLE 1: Channel Coating

[0070] The analyte diffusion process was independent of the channel coating material and dependent on the analyte type. However, the adsorption and desorption processes are dependent on both gas type and the channel surface material. Therefore, it was expected that the surface treatment of the channel would results in different transient response profiles. To study the effects of channel coating on the sensor response, a set of materials, as listed in TABLE 2, were tested.

[0071] Normalized transient responses of six of the sensors (coatings number 3-4 and 8-11) to 2000 ppm ethanol are shown in FIGURE 5. The rest of the channel coatings (coatings number 1-2 and 5- 7) did not show significant responses as some of the materials hindered the diffuse-in process. As a result, these five coatings seemed to trap all the ethanol molecules stopping them from travelling along the channel and approaching the sensor. As it can be seen in FIGURE 5, the interaction of the gas molecules with different materials was different resulting in varying normalized responses.

[0072] Single metal layer coatings: Among all the channels coated and tested with a single metal layers, gold (with chromium underlayer and parylene C second layer, showed the best response (FIGURES 5 and 6), as it is one of the most non-reactive materials in nature and was used here to decrease the chemical cross contamination of the gas molecules to the channel walls which eventually results in faster sensor recovery. The chromium layer was coated to increase the adhesion of the substrate to gold. Similarly, a Si02 first layer with a parylene C
second layer showed a good response (volts) and recovery curve (FIGURE 5).
However, the Cr and Au coated channel without parylene C also showed a reasonable ability to distinguish ethanol, methanol and acetone (FIGURE 5).

[0073] First layer (Bottom layer i.e. closest to the channel surface): In case of channels with multilayer coatings, it is observed that the channels coated with different bottom layer materials (even with the same top layer) provide different responses. For instance, the channel coated with three layers of Cr, Au, and Parylene C (with a gold and chromium layer as the bottom coating layers) and Cu and Parylene C (with the copper layer as the bottom coating layer) show different responses to the same concentration of ethanol. This is due to the permeation of the gas molecules through the top layer and reaction with the bottom coating layer. In choosing a first layer, it is preferred in some embodiments that the first layer physically interacts (i.e. non-specifically and reversibly via van der Wahl's forces) with the VOC, but does not chemically interact with the VOC.

[0074]
Second layer (Top layer i.e. on top of the first layer): The preliminary experiments revealed the importance of the porosity of the top coating layer. In essence, the number of surface adsorption sites available per unit volume of the channel (Ca in equation (3)) is greater in channels with higher porosity. As it is shown in FIGURE 5, the diffuse-in and diffuse-out processes of ethanol was more rapid in the channels with the combination of Cr and Au and Parylene C coatings, whereas, the coating combination of Cr and Au and Cytonix shows the slowest response. This suggests that more physical adsorption occurs in the case of Cr and Au and Cytonix channel coating. Thus, Parylene C is a good candidate for the top layer coating material as it can be coated as a thin polymer film, which is chemically inert. It also has high porosity [25], which increases physical adsorption of the gas molecules to the channel walls that eventually increases selectivity of the sensor. In addition, Parylene C
provides a pinhole free coating and a lower permeability (as compared to other similar polymers) and has been recently used in the development of GC columns [26] as well as a material for moisture barrier in numerous applications [27]. The latter is potentially significant for gas sensing, since the gas sensors are subject to errors as they are vulnerable to ambient fluctuations such as humidity and temperature change. Therefore, the response of a sensor depends on not only the analyte concentration, but also the ambient conditions (particularly humidity).
Therefore, in high precision sensing applications, such as breath analyzers, fluctuation in humidity [28] may result in false signals. Thus, the use of an effective moisture barrier such as Parylene C along the channel may reduce the error caused by humidity. In choosing a second layer, it is preferred in some embodiments that the second layer if used has porosity so that the VOC
has access to the first layer and is also chemically inert. Furthermore, it may also be useful for the second layer to have moisture barrier properties.

[0075] Analytes: Three different analytes including ethanol, methanol, and acetone were tested to compare the selectivity of the fabricated sensors among different gases. These gases were selected to show the capability of the device in differentiating alcohol and ketone vapors.
Four out of the eleven fabricated sensors showed acceptable selectivity among the three selected analytes. The temporal responses obtained from the device are normalized to fit within the magnitude range of [o 1], eliminating the influence of the analyte concentration on the shape of the responses. Normalized responses for each of the sensors to 2000 ppm of each of the three analytes are depicted in FIGURE 6. As it can be seen in FIGURE 6, each of the four sensors give unique responses corresponding to different tested analytes such that the finger-prints of three analytes on each of the four selected sensors were distinct. However, different sensors may distinguish these three analytes differently. In other words, it may be observed that from one sensor to another the level of segregation between analytes may be different, showing different selectivity among the sensors tested. A better quantitative comparison may be evaluated based on calculating indicators of selectivity and the recovery time of the sensor to find the optimum material for the treatment of the channel of the proposed gas detector. For instance, FIGURE 7 shows typical responses of one of the sensors against three different analytes. A selectivity factor is defined as S=S1+S2+S3, in which Si, S2, and S3 are the absolute values of the distances between the amplitude of responses of methanol-acetone, ethanol-methanol, and acetone-ethanol, respectively, at five different time points (t=2os, t=40s, t=6os, t=8os, and t=loos). The square root of the sum of square of the selectivity factors at five points is used as a measure of selectivity of different sensors. Another factor for determining the sensor performance is the recovery time: in essence, the sensor with the lower recovery time is preferable.

[0076]
Optimization of coating: The selectivity and recovery time of the fabricated sensors are all compared and listed in TABLE 3. In this table, the sensors are listed based on two major categories: coating materials and dimensions. The average pick time of each sensor, which is the mean of three time points for which the sensors have the maximum readout for three different analytes, were also calculated and listed. It is observed that the smaller the pick time value the faster the recovery of the sensor. The average pick time was used to rank (in the order of 1 to 4, from the lowest average pick time to the highest, respectively), and hence compare the speed of the recovery of different detectors. The sensors were also ranked based on their selectivity factor (as explained above). The effect of both coating materials and channel dimensions are separately investigated through the above ranking schemes. The results show that the Cr and Au and Parylene C coated sensor provides the maximum selectivity and the minimum recovery time among all the coating materials tested here. This means that the proposed coating combination decreases the cross contamination and the chemical adsorption and increases the physical adsorption (and hence selectivity). To perform a quantitative comparison of the response of the sensor to different analytes three features (tr, tm, Rf) are extracted from each normalized response. The feature space for the sensor with the coating combination of Cr and Au and Parylene C, which shows the best performance in terms of selectively and recovery time, is shown in FIGURE 8. It will be appreciated by a person of skill that the optimum coating will depend on the VOCs being tested.

[0077] TABLE 3 Comparison of the Separation Factor and Recovery Time Among the Fabricated Sensors Coating Channel Channel Average Peak Selectivity S
Length Depth Peak Time Time Factor (S) Rank (1) (d) (seconds) Rank Cr-Au- 59.37 1 1.49 1 Parylene C
Sensors with Cr-Au 154.07 4 1.07 3 Different 8102- 30 mm 500 jim 72.25 2 1.08 2 Coatings Parylene C
Cu- 98.51 3 1.01 4 Parylene C
20 1111T1 500 AM 51.18 1 1.37 4 Sensors with Cr-Au- 30 mm 500 pm 59.37 2 1.49 3 Different Parylene C 40 mm 500 vun 67.25 3 1.52 2 Dimensions 30 MM. 200 pm 68.56 4 1.74 1

[0078] EXAMPLE 2: Channel Dimensions

[0079]
After choosing the preferred coating combination of the tested coatings listed in TABLES 2 and 3, for the tested VOCs, Cr and Au and Parylene C were preferred.
This coating was then tested to study the effect of the channel dimensions on the response of the sensor, sensors with three different channel lengths and two different channel depths are fabricated and tested (see TABLE 3). The ranking procedure explained above was also used to quantify the effect of the channel dimension on the selectivity and recovery time. In general, there is an opposite trend in rankings based on the selectivity and recovery time for sensors with different dimensions as explained below.

[0080]
Channel depth: Normalized responses for three different analytes (ethanol, methanol and acetone) for four different channel dimensions: (i) 1=20 mm, d=500 pm, (ii) 1=30 mm, d=500 m, (iii) 1=40 mm, d=500 m, and (iv) 1=30 mm, d=200 pm (1 is the length and d is the depth of the channel) are depicted in FIGURE 9. As expected, the sensors with higher channel depths are recovered faster. According to equation (3), increasing the depth of the channel decreases the effect of physical adoption, which will result in changing the diffusion-physisorption equation to only diffusion equation for deep channels. In this case (which is only diffusion-dependent), the only analyte related parameter in the equation is D
(gas diffusivity).
On the other hand, by decreasing the channel depth, the effect of Ca and a in Equation (3) increases and more adsorption and desorption dependency will be observed in the response.
Thus, channels with smaller depths are recommended to differentiate gases with similar diffusion coefficients.

[0081]
Channel length: When examining two gases (with different diffusion coefficients), increasing the length of the channel increases the diffusion time, which results in a larger difference in the temporal responses of the sensor (see FIGURE 9). In other words, increasing the length of the channel slows down the diffusion process and increases the selectivity of the sensor. However, longer channels result in longer recovery time for the sensor. Therefore, considering the trade-off between the selectivity and the recovery time of the sensor, the preferred dimension of the channel for the VOCs tested and with the tested coatings was 1=3o mm, d=200 gm (see TABLE 3).

[0082] EXAMPLE 3: Analyte Concentration

[0083] After adjusting the sensor coating and dimensions, the coating of Cr and Au and Parylene C and the dimensions of 1 = 30 mm and d = 200 gm are used for verifying the selectivity of the sensor. A wide range of concentration (250-4000 ppm) of 6 different target gases were selected among alcohols (including 2-pentanol, ethanol and methanol) and ketone vapours (including acetone, 2-butanone and 2-pentanone). As recorded transient responses for 8 different concentrations for 6 different targets is shown in FIGURE lo; the sensor differentiated among different concentration of gases. As presented in FIGURE
ii, the feature space shows the analytes are successfully separated in the 3D space. The feature vectors of the responses related to each analyte at different concentrations form a clear-cut cluster in the feature space (see FIGURE 11). No mathematical tool was needed for mapping the responses into the feature space, and only one simple feature extraction method [20] was adequate for the determination of the positions of the target analytes in the feature space.
The feature space of a particular device was universal and requires hardly any modification when applied to different analytes.

[0084] The gas detector operation is humidity and temperature dependent.
Ambient temperature and humidity dependence of the responses provided for a specific analyte may be considered as sources of error, which causes displacement of the feature vector related to each analyte in the feature space. This arises from the fact that the analyte diffusion/physisorption along the channel/to the channel walls are both strongly temperature-dependent processes.

These errors caused by ambient fluctuations introduce drift-like terms into the responses of the sensor, which causes false measurements. Therefore, the ambient temperature and humidity are controlled during all the experiments. The apparatus may be further optimized to minimize the effect of humidity and temperature fluctuation on the response of the sensor.

[0085] EXAMPLE 4: Detection of Tetrahydrocannabinol (THC)

[0086] An embodiment of the apparatus was also tested for detection of cannabis in human exhaled breath. The tested embodiment was capable of differentiating small concentrations of Tetrahydrocannabinol (THC) in presence of other volatile organic compounds (VOCs). The main advantage of the proposed device over previous microfluidic-based gas sensors [30-31] is the integration of heaters along the micro-channels to enhance the diffusion rate of the THC
molecules in the channel and decreasing the sensor response and recovery time from 15 minutes to below 200 s. Detection of THC in breath has been used as an indicator of cannabis use [32].
However, as there are traces of other VOCs in the breath, it is important to differentiate among different gases, and pinpoint the distinct "smell print" of THC. General purpose Metal Oxide Semiconductor (MOS) gas sensors are sensitive and not selective of different gases [33]. As described above, micro-channels may be integrated with these sensors to enhance their selectively (FIGURE 2A and 2C) [30]. However, these microfluidic gas sensors are not suitable for detection of large molecule gases (such as THC) as the diffusion process is slow and takes more than few minutes [31]. In this example, the sensor response time was decreased by modulating the temperature of the diffusion channel. The sensor assembly was fabricated using a similar method as explained in [30]. To control the temperature of the diffusion channel, a platinum heater wire is integrated along the channel. The response time and selectivity of the sensor for THC-methanol binary mixture (1 mg/mL solution in methanol) and pure methanol were studied at different temperatures (25 C, 40 C and 8o C). A method described in [31]
was used to characterize the sensor response (FIGURE 13). The sensor recovery time for THC-methanol mixture at 25 C was approximately 1.5 minutes, and as the temperature is increased to 80 C, the recovery time is reduced to under 3 minutes. The slow recovery, which is attributed to high molecular weight of THC, was not observed for pure methanol. Therefore, the overall sensor response time was decreased drastically for THC detection by addition of the heater. Increasing the micro-channel temperature has another important effect:
enhancing the selectivity. As can be seen in FIGURE 14, the selectivity of the device is increased at higher temperatures as bigger molecules of THC in the binary mixture are more actively involved in the diffusion process and react with the sensor. It must be noted that the observed response for the binary mixture of THC-methanol is distinct for each THC concentration, and we have successfully detected THC concentrations as low as 50 ppm. In contrast to previous microfluidic gas sensor designs, the selectivity of the sensor was not compromised when achieving faster response times such that the heater embedded channel design would be suitable for detection of larger molecules including THC. This embodiment may provide a low-cost breath analyzer device, which may provide a powerful tool for roadside testing or also for personal monitoring purposes.

[0087] The embodiment shown in FIGURE 12, shows one way to control the temperature of the diffusion channel, a heater is shown integrated along the channel. A
benefit of the embodiment shown in FIGURE 12 is the integration of a heater along the micro-channels to enhance the diffusion rate of the THC molecules in the channel and decreasing the sensor response and recovery time to below 200S. In contrast, some microfluidic gas sensor designs do not have the selectivity of the sensor in combination with a faster response when used to detect larger molecules, including THC.

[0088] The sensor selectivity may be further be enhanced by creating a flow (advection) of gas inside the micro-channels. Also, a water trap is shown in FIGURE 12 to minimize large droplets of moisture entering device. The sample enters an antechamber; the force of exhalation drives the sample through a humidity filter and into the sampling chamber. A
one way valve can be used ensure gas does not escape through the inlet. A small vacuum pump draws in fresh air from inlet and out the exhaust port to recover the sensor.

[0089] A 3D-printed microfluidic platform is fabricated by integrating a chemo-resistor with a channel. Using a novel coating combination, a surface treatment on the inner walls of the microfluidic channel is carried out, which enhances the selectivity power of the device. Different coating materials are tested and compared to choose the best material in terms of giving the maximum selectivity and the minimum sensor recovery time. The geometry of the channel is then optimized after comparison of the results of sensors fabricated with different channel dimensions. Embodiments may be developed as low-cost (¨$10), portable and highly selective gas detectors, which provide a powerful tool for numerous applications including personal monitoring of exhaled breath for patients suffering from different diseases, biological analysis, safety and environmental monitoring, and analytical chemistry.

[0090] A different method of feature extraction is also used for characterization of the concentration of the analyte. Three different features are extracted from each transient response (see FIGURE i6A). The signal maximum response level (F1), the response level for the final readout (F2), and the surface area underneath the response (F3) are the three extracted features from each transient response. The feature vector (0) extracted from the transient response is shown in a 3D space in FIGURE 16B. The transient responses of the sensor with Cr and Au and Parylene C channel coating and dimensions of 1 = 40 mm, w= 3 mm, d = 500 m are shown in FIGURE 17A. The transient responses are shown in FIGURE 17A, representing the repeatability of the device for each concentration. Some parts of the transient responses are magnified to show the reproducibility of the response for each concentration.
The feature vectors related to each concentration are segregated (see FIGURE 17B) in the feature space.
The results show three separated spheres, representing the separation capability of the device between different concentrations of the same analyte. A regression model is used to show the linear relation between the concentration and the area underneath the curve (see FIGURE 18).

[0091] EXAMPLE 5: Natural Gas Leakage Detection

[0092] An embodiment of the apparatus was also tested as an automated and reliable means for monitoring of natural gas leakage in pipelines and around pump stations.
In particular, a microfluidic-based sensor as described herein may be deployed using an unmanned aerial vehicle (UAV) for timely and precise detection of natural gas leakage at storage sites and along pipelines. Such a device may be operated easily by pipeline maintenance technicians with basic training to remotely inspect natural gas infrastructure including pumps, tanks and pipes wherein the natural gas infrastructure may have limited everyday access. The sensor can be used for detection of methane, ethane and pentane.

[0093]
Features of this embodiment may include: a sensor recovery process which is capable of automatically regenerating the saturated sensors using a compressed air recovery chamber and electrically actuated solenoid valves in order to continuously monitor the infrastructure for leakage detection; the slope of the "exposure to pentane", which is representative of a gas concentration, may be chosen as the main feature of the response, whereby this feature extraction process allows the device to determine the concentration of the desired analyte; the capability to switch between multiple channels for an uninterrupted detection operation wherein there may be a manifold controlled by micro-valves are used; the sensor may be installed in a mobile platform such as a UAV to enable mobile detection of different gases and to achieve this goal a novel sampling procedure was developed to enable sampling consistent amount of gas as the platform is moving; and an onboard microprocessor may be used to relate the UAV flight path to sensor readings of the methane concentration (see FIGURES 19 A and B).

[0094] EXAMPLE 6: Nuisance Sewer Gas Detection

[0095] An embodiment of the apparatus is also envisaged, wherein the sensor technology may be used to monitor sewer gases and identify "hotspots" of gas production for targeted treatment. Particularly, the gas sensor may be used for detection of nuisance gases, some of which are odorous or even hazardous. For example, hydrogen sulfide, ammonia, carbon dioxide, methane and nitrous oxide, among other greenhouse gases. The embodiment may be relatively independent and low-maintenance, and may have a streamlined data communications to collect, transmit, analyze and store data to inform users' mitigation strategies in real-time.

[0096] Features of this embodiment may include: an aerofoil design is used to minimize the risk of obstruction in the turbid environment, wherein the configuration may be developed to allow the device to be positioned along the side of the pipe to avoid large sediments at the bottom of the pipeline; a shared inlet/outlet channel positioned on the downstream end of the apparatus to avoid blockage due to fast-flowing suspended organics and other waste, which may be combined with a high pressure air source which may be used to purge the previous sample and dislodge any debris build-up and wherein negative pressure may be used to draw the next sample through the inlet; a membrane-less microfiltration mechanism may be used to ensure that the sensing unit is not in contact with microorganisms or debris that can interact with the sample and bias the sensor reading or create nuisance compounds, whereby the microfiltration mechanism is based on the use of inertial microfluidic particle sorters; and since the sensor may use oxygen (02) to recover between samples, onboard compressed gas may be used to flush the micro-chamber and channel, whereby the sensor may recover to the baseline, and a neutral gas (N2) may be used to purge 02 and any remaining sample from the sensing unit and into the surrounding environment through an exhaust outlet (see FIGURE 20).

[0097] Although embodiments described herein have been described in some detail by way of illustration and example for the purposes of clarity of understanding, it will be readily apparent to those of skill in the art in light of the teachings described herein that changes and modifications may be made thereto without departing from the spirit or scope of the appended claims. Such modifications include the substitution of known equivalents for any aspect of the invention in order to achieve the same result in substantially the same way.
Numeric ranges are inclusive of the numbers defining the range. The word "comprising" is used herein as an open ended term, substantially equivalent to the phrase "including, but not limited to", and the word comprises" has a corresponding meaning. As used herein, the singular forms "a", "an" and "the" include plural referents unless the context clearly dictates otherwise.
Thus, for example, reference to "a thing" includes more than one such thing. Citation of references herein is not an admission that such references are prior art to an embodiment of the present invention. The invention includes all embodiments and variations substantially as herein described and with reference to the figures.

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Claims (35)

What is claimed is:

1. A gas detection apparatus, the apparatus comprising:
(a) a channel having an inner surface and having at least one opening, such that the channel is optionally in fluid communication with a sample gas when the opening is in an open position and optionally having a closed position, the inner surface having a coating comprising:
(i) a first layer comprising a non-reactive metal or non-reactive metalloid compound;
(ii) a second layer comprising a moisture barrier;
and (b) a gas sensor disposed within the channel.

2. The apparatus of claim 1, wherein the second layer comprising a moisture barrier has a gas permeability sufficient to absorb the gas particles being sampled.

3. The apparatus of claim 1, 2 or 3, wherein:
(i) the non-reactive metal is selected from one or more of the following:
copper; chromium; ruthenium; rhodium; palladium; gold; silver; osmium;
iridium;
platinum; titanium; niobium; tantalum; bismuth; tungsten; tin; nickel; cobalt;

manganese; and zinc; or (ii) is metalloid compound is SiO,

4. The apparatus of claim 1, 2 or 3, wherein the moisture barrier with high porosity is Parylene or Polydimethylsiloxane (PDMS).

5. The apparatus of claim 4, wherein the Parylene is selected from Parylene C, Parylene N
or Parylene D.

6. The apparatus of claim 5, wherein the Parylene is Parylene C.

7. The apparatus of any one of claims 1-6, wherein the non-reactive metal is selected from one or more of the following: copper; chromium; ruthenium; rhodium; palladium;
gold; silver;
iridium; platinum; titanium; niobium; and tantalum.

8. The apparatus of any one of claims 1-7, wherein the coating is chromium, gold and Parylene C.

9. The apparatus of any one of claims 1-6, wherein the channel further comprises a heater.

10. The apparatus of claim 9, wherein the heater is operable to increase the channel temperature to at least 8o°C.

11. The apparatus of any one of claims 1-10, wherein the gas sensor is a Metal Oxide Semiconductor (MOS).

12. The apparatus of any one of claims 1-11, wherein the gas sensor is a tin oxide-based chemoresistive gas sensor.

13. The apparatus of any one of claims 1-12, wherein there is more than one gas sensor in the channel.

14. The apparatus of any one of claims 1-13, wherein the channel length to channel depth ration is 150:1.

15. The apparatus of any one of claims 1-14, wherein the channel width to channel depth ration is 3:1.

16. The apparatus of any one of claims 1-15, wherein the channel length is 3mm wide, 30 mm long and 200 µm deep.

17. The apparatus of any one of claims 1-16, wherein the first layer comprises chromium and gold.

18. The apparatus of claim 17, wherein the chromium is applied to the channel prior to the gold.

19. The apparatus of claim 18, wherein the second layer comprises Parylene C.

20. The apparatus of any one of claims 1-16, wherein the first layer comprises SiO2.

21. The apparatus of claim 20, wherein the second layer comprises Parylene C.

22. The apparatus of any one of claims 1-21, wherein the opening further comprises a closed position.

23. The apparatus of any one of claims 1-22, wherein the apparatus further comprises a second opening.

24. The apparatus of claim 23, wherein the second opening has both an open and closed position.

25. The apparatus of any one of claims 1-24, wherein the apparatus further comprises a liquid trap positioned in fluid communication with the at least one opening.

26. The apparatus of any one of claims 1-25, wherein the apparatus further comprises a humidity filter positioned in fluid communication with the at least one opening.

27. The apparatus of any one of claims 1-26, wherein the apparatus further comprises a pump which is optionally in fluid communication with the at least one opening.

28. The apparatus of any one of claims 23-26, wherein the apparatus further comprises a pump which is optionally in fluid communication with the second opening.

29. The apparatus of any one of claims 1-28, wherein the apparatus further comprises a compressed air source which is optionally in fluid communication with the channel.

30. The apparatus of any one of claims 1-28, wherein the apparatus further comprises a compressed gas source which is optionally in fluid communication with the channel.

31. The apparatus of any one of claims 1-28, wherein the apparatus further comprises a pentane plume which is optionally in fluid communication with the channel.

32. The apparatus of any one of claims 1-28, wherein the apparatus further comprises a compressed O2 source or N2 source or separate O2 and N2 sources which are optionally in fluid communication with the channel.

33. The apparatus of any one of claims 1-32, wherein the apparatus further comprises a cleaning solution which is optionally in fluid communication with the channel.

34. The apparatus of claim 30, wherein the compressed gas source is selected from one or more of the following: air; pentane; CO2; O2; or N2.

35. The apparatus of claim 30, wherein there is more than one compressed gas source, selected from the following: air; pentane; CO2; O2; or N2.