Gas sensor array and method

Abstract

The invention relates to a method for analyzing the composition of a gaseous stream comprising at least two gaseous components, one of which is methane; and to a sensor array and a gas sensor comprising such sensor array. The method comprises contacting the gaseous mixture with a sensor, wherein the sensor comprises a sensor array comprising at least two sensor elements, wherein each of said sensor elements comprises a transducer coated with a coating comprising a polymeric material having at least one property that is responsive to one or more of said gaseous components when exposed thereto, wherein said sensor elements differ at least in the composition of the coating, providing an energy input to said transducers that is converted to output signals based on said property, and obtaining said output signals.

Claims

1. A method for analysing the composition of a gaseous mixture, comprising at least two gaseous components, one of which is methane, the method comprising contacting the gaseous mixture with a sensor, wherein the sensor comprises a sensor array comprising at least two sensor elements, wherein each of said sensor elements comprises a transducer coated with a coating comprising a polymeric material having at least one property that is responsive to one or more of said gaseous components when exposed thereto, wherein said sensor elements differ at least in the composition of the coating, providing an energy input to said transducers that is converted to output signals based on said property, and obtaining said output signals, wherein optionally said output signals are data signals, wherein the method is for determining the calorific value of the gaseous mixture.

2. The method of claim 1, wherein the gaseous mixture comprises ethane and/or propane and wherein the coating of at least one sensor element comprises a compound with selectivity for absorption of methane over ethane, and the coating of at least one other sensor element comprises a compound with selectivity for absorption of ethane and/or propane over methane.

3. The method of claim 1, wherein said gaseous mixture is a gaseous stream, wherein the method is for determining the calorific value of said gaseous stream, and comprises passing the gaseous stream over the sensor elements.

4. The method of claim 1, wherein said gaseous stream is a stream of natural gas or biogas or a mixture comprising natural gas and/or biogas.

5. The method of claim 1, further comprising: providing said data signals to a computer processor which is in communication with a computer memory device in which instructions are stored for conversion of said data signals to an estimated composition parameter, and calculating in said processor said estimated composition parameter using said instructions and said data signals from said different sensor elements.

6. The method of claim 5, wherein said estimated composition parameter is the calorific value of said gaseous mixture.

7. The method of claim 5, wherein said estimated composition parameter is the methane concentration.

8. The method of claim 1, wherein said gaseous mixture comprises at least one component selected from the group consisting of ethane, propane, carbon dioxide and water.

9. The method of claim 1, wherein said gaseous mixture is a gaseous stream, the method comprising removing non-gaseous contaminations from the gas stream prior to contacting the gas stream with the sensor.

10. The method of claim 1, wherein said transducers are a capacitive sensor and wherein said responsive property of said polymeric material is the relative permittivity.

11. The method of claim 1, wherein said gaseous mixture is a gaseous stream, wherein the method is a method of in-line analysis of the composition of the gaseous stream and wherein the sensor is an in-line device mountable or mounted to, or integrated in a pipeline segment or flow meter, and wherein the step of contacting the gaseous stream with said sensor comprises flowing at least part of the gaseous stream over the sensor array.

12. The method of claim 1, wherein the sensor array comprises a first sensor element comprising a coating comprising cured epoxy resin, a second element sensor comprising a coating comprising a fluoropolymer, a third sensor element comprising a polymeric organosilicon compound, and wherein said sensors are different from each other, and wherein the coating of at least one sensor element of the sensor array comprises a molecular encapsulation material in a polymeric matrix, wherein said encapsulation material is selected from the group consisting of cryptophane, zeolite and metal-organic framework.

13. A sensor array, comprising at least two sensor elements, wherein each of said sensor elements comprises a transducer coated with a coating comprising a polymeric material having at least one property that is responsive to one or more gaseous components when exposed thereto, wherein said sensor elements differ at least in the composition of the coating, wherein the coating of at least one sensor element comprises a compound with selectivity for absorption of methane over ethane, and the coating of at least one other sensor element comprises a compound with selectivity for absorption of ethane and/or propane over methane.

14. The sensor array of claim 13, wherein said transducer is configured for converting an energy input to a data signal based on said property, preferably wherein said transducers are capacitive sensors.

15. The sensor array of claim 14, comprising: a first sensor element comprising a coating comprising poly(allylamine), or a coating comprising a polymer and metal organic framework or zeolite, a second sensor element comprising a coating comprising a homopolymer or copolymer of tetrafluoroethylene, a third sensor element comprising a coating comprising cured polyepoxide SU-8 resin giving a polymer comprising bisphenol-A diglicydyl ether residues, a fourth sensor element comprising a coating comprising polydimethylsiloxane and a cryptophane compound, and a fifth sensor element comprising a coating comprising polydimethylsiloxane, optionally a sixth sensor element comprising a coating comprising a polymer and zeolite, wherein each of said sensor elements has a capacitive sensors as transducer.

16. The sensor array of claim 13, wherein at least one sensor element has a coating comprising a cryptophane compound and/or one or more polymers selected from the group consisting of a polymer comprising repeating units comprising an amine group, a fluoropolymer, a polymeric organosilicon compound, a polyisoprene, a polymer of intrinsic microporosity, and cured epoxy resin.

17. The sensor array of claim 13, wherein at least one sensor has a coating comprising one or more polymers selected from the group consisting of: a fluoropolymer selected from the group consisting of polymers and copolymers of tetrafluoroethylene, and polymers and copolymers of vinylidene fluoride, a polymeric organosilicon compound selected from the group consisting of polydimethylsiloxane, polydiethylsiloxane, polydiphenylsiloxane, and copolymers of any of these polymers, and an amine comprising polymer selected from the group consisting of polyallylamine, polyvinylamine, polyethyleneimine, and copolymers of any of these polymers, a cured epoxy resin selected from cured cycloaliphatic epoxides and aromatic epoxides.

18. The sensor array of claim 13, comprising the combination of: a first sensor element comprising a coating comprising a zeolite, a metal organic framework, and/or a polymer comprising repeating units comprising an amine group, a second element sensor comprising a coating comprising a fluoropolymer, a third sensor element comprising a polymeric organosilicon compound, wherein said sensor elements are different from each other, and wherein the coating of at least one sensor element of the sensor array comprises a cryptophane compound in a polymeric matrix.

19. The sensor array of claim 18, wherein the sensor array further comprises a fourth sensor element comprising a coating comprising a cured epoxy resin comprising a cross-linked polymer comprising aromatic rings, and a fifth sensor element comprising said cryptophane compound in a polymeric matrix, and optionally a sixth sensor element comprising a coating comprising a zeolite or a metal organic framework.

20. An inline gas sensor comprising the sensor array of claim 13, wherein in said sensor array the transducer of each of said sensor elements comprises a capacitive sensor coated with said coating, and comprising a casing comprising a chamber in which said sensor elements are exposed and which is provided with at least one opening for a gaseous stream, wherein said casing is mountable or mounted to or integrated in a pipeline segment.

21. A sensor array comprising at least two sensor elements, wherein each of said sensor elements comprises a transducer coated with a coating comprising a polymeric material having at least one property that is responsive to one or more gaseous components when exposed thereto, wherein said sensor elements differ at least in the composition of the coating, wherein at least one sensor element has a coating comprising a cryptophane compound and/or one or more polymers selected from the group consisting of a polymer comprising repeating units comprising an amine group, a fluoropolymer, a polymeric organosilicon compound, a polyisoprene, a polymer of intrinsic microporosity, and cured epoxy resin.

Description

(1) FIG. 1 shows a schematic plan of an example of a sensor array according to the invention. Sensor array 1 on a PCB comprises five sensor elements 2A-2E, each having an exposed surface (in the plan view) and a different coating thereon. The sensor further comprises microprocessor 3 with an integrated memory device 6, and an electric feed 4 and electronic conduits 5 for electronic output signals from each of sensor elements 2A-2E to microprocessor 3, and an outlet 7 for a calculated data signal from microprocessor 3.

(2) FIG. 2 shows a vertical cross section of a sensor element comprising a substrate 8 and electrodes 9 as transducer (capacitive sensor) end coating 10.

(3) All references cited herein are hereby completely incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

(4) The use of the terms a and an and the and similar referents in the context of describing the invention (especially in the context of the claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms comprising, having, including and containing are to be construed as open-ended terms (i.e., meaning including, but not limited to) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. The use of any and all examples, or exemplary language (e.g., such as) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

(5) Preferred embodiments of this invention are described herein. Variation of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject-matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context. The claims are to be construed to include alternative embodiments to the extent permitted by the prior art.

(6) For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described.

(7) The invention will now be further illustrated by the following non-limiting examples.

EXAMPLES

Experiment 1

(8) Quartz Crystal Microbalance experiments were carried out for various quartz crystals. The quartz crystals had an operating frequency of 5 MHz, a diameter of 1 inch and were obtained from Inficon. Coating solutions were prepared by mixing a polymeric matrix (resin) material a solvent and optionally an additive and a few drops of the coating solutions were applied using a spincoater. The coated crystals were heated at 80-90 C. for 1-2 hours for curing of the polymeric matrix.

(9) Table 1 gives the coating compositions. Herein, ratios are in weight/weight. NanoZeo100 is NanoLTA-100, Zeolite LTA (4 , Na-form), 100 nm, available from Nanoscape. NanoZeo300 is NanoLTA-300, Zeolite LTA (3 , K-form), 300 nm, available from Nanoscape. The cryptophane compound is cryptophane-A.

(10) For coating 1 the solvent was ethanol, for coating 4 IPA, for coatings 6, 7, and 14-19 THF, for coatings 8, 10-12 CH.sub.2Cl.sub.2, and for coatings 9 and 13 PCF770.

Experiment 2

(11) Gas uptake characteristics of coated crystals 1-19 as shown in table 1 were measured in a Gas Exposure System with a Teflon flow cell, controlled gas flow and measures the temperature and moisture level. Measurements were performed with an Inficon Research Quartz Crystal Microbalance (QCM) in which the gas uptake is measured by change in mass obtained from a change in the resonance frequency of the crystal.

(12) The signal observed for N.sub.2 was the baseline signal. The flow speed with 500 ml/min and the moisture level generally below 1%. Gas stream of a gas component in N.sub.2 were used, percentages of the gas component are in vol. %.

(13) Table 1 shows that HPDMS itself absorbs some methane. Comparing crystal 14 with 15 and crystal 17 with 19, a slight increase in methane selectivity is visible upon addition of a small amount of cryptophane. Without wishing to be bound by way of theory, the small amounts of cryptophane may provide a more finely divided morphology and better distribution in the matrix, which increase its accessibility for methane. Upon increase of the cryptophane content (crystals 16 and 18), the sensitivity is reduced.

(14) Table 2 shows methane uptake for crystal 8 as a function of % methane, the methane uptake for crystal 11, and the uptake of CO.sub.2 onto crystal 9. R.sup.2 is the coefficient of determination of a straight line fit of the data. Compared to crystal 8, the coating is thicker but contains less cryptophane. Nevertheless, crystal 11 has better sensitivity to methane. However, crystal 11 has also a strong mass change upon exposure to e.g. CO.sub.2.

(15) TABLE-US-00001 TABLE 1 Coating composition for QCM and mass changes upon gas exposure of crystals coated with coatings. [a][b] 100% 20% 10% 100% Ratio Layer Crystal Coating CH.sub.4 C.sub.2H.sub.6 C.sub.3H.sub.8 CO.sub.2 CO.sub.2/CH.sub.4 thickness [d] 1 PEI 0.05 n.r n.r. [c] 5.7 4 PAAm 0.1 n.r. n.r. 0.2 2.0 2.9 6 PMDS/NanoZeo100 83/17 0.03 0.17 0.35 0.47 15.7 2.3 7 PMDS/NanoZeo300 82/18 0.03 0.33 0.4 0.79 26.3 1.7 8 PMDS/Cryptophane 0.06 0.15 0.6 0.38 6.3 2.0 50/50 9 Teflon-AF 1600 0.02 0.08 0.22 0.57 28.5 0.63 11 PDMS/Cryptophane 0.45 1.5 2.3 4.3 9.6 4.7 87/13 13 Teflon/Cryptophane 0.02 n.d. n.d. 0.56 28.0 0.93 (0.7) 90/10 14 SU-8 0.03 0.075 n.r. 1.1 36.7 1.1 (1.1) 15 SU-8/Cryptophane 99/1 0.06 0.1 0.05 1.3 21.7 1.2 16 SU-8/Cryptophane 90/10 0.05 0.15 0.1 2.5 50.0 2.2 17 HPDMS 0.11 0.25 0.57 0.82 7.5 1.9 (1.6) 18 HPDMS/Cryptophane 0.26 0.55 10.2 10.8 60.9 3.0 95/5 19 HPDMS/Cryptophane 0.34 0.61 1.4 1.8 5.3 2.5 98/2 [a] Data are given in g/cm.sup.2 relative to the baseline N.sub.2 signal. [b] N.R. = no response. n.d. = not determined. [c] No stable signal could be obtained. [d] Estimation of the layer thickness based on the resonance frequency of the crystal before and after coating. In brackets: direct measurement of the layer thickness on a Dektak apparatus.

(16) TABLE-US-00002 TABLE 2 Gas uptake of selected crystals Crystal 8 Crystal 11 PDMS/Cryptophane PDMS/Cryptophane, Crystal 9 50/50, CH.sub.4 87/13, CH.sub.4 Teflon, CO.sub.2 % gas m % gas m % gas m 20 0.032 20 0.125 10 0.052 40 0.037 60 0.269 20 0.111 60 0.044 80 0.35 30 0.169 80 0.042 100 0.447 50 0.32 100 0.061 100 0.573 R.sup.2 0.8221 R.sup.2 0.9951 R.sup.2 0.9942

Experiment 3

(17) Bare comb electrodes obtained from NXP were coated with two coatings. The chip was placed in the Teflon holder of the Gas Exposure System and connected to an LCR analyzer (Iviumstat, Ivium). The change in capacitance was measured upon exposure to N.sub.2, CH.sub.4 and CO.sub.2.

(18) Table 3 shows the results for a coating of HDPMS with 10% w/w cryptophane. The chip 1 was subsequently exposed to air, then to N.sub.2, then to CH.sub.4 and finally to CO.sub.2, each stream 100 vol. %. The difference in capacitance upon each change of gas is shown. Table 3 also shows the change in capacitance for chip 2 which was subsequently exposed to N.sub.2, then to CH.sub.4, then to CO.sub.2, and finally to N.sub.2. The difference in capacitance (in pF) upon each change of gas is shown.

(19) TABLE-US-00003 TABLE 3 Change in capacitance (pF) 0.1 kHz 1 kHz 10 kHz 100 kHz chip 1 Air 0 0 0 0 N.sub.2 0.85 0.57 0.40 0.26 CH.sub.4 0.72 0.43 0.30 0.76 CO.sub.2 0.65 0.32 0.22 0.78 chip 2 N.sub.2 0.010 0.033 0.1 0.037 CH.sub.4 0.074 0.085 0.22 0.049 CO.sub.2 0.30 0.29 0.22 0.057 N.sub.2 0.010 0.03383 0.1 0.037

(20) A further experiment was carried out for a capacitance sensor coated with SU-8/10 wt. % cryptophane. The loss of water when exposed to dry N.sub.2 had a significant influence on the capacitance; in addition equilibration took 30 minutes. Moreover, the decrease in capacitance upon exposure to CH.sub.4 is small and the difference with the CO.sub.2 signal was small.

(21) The results can be summarized as follows. Polymeric coatings readily (ab)sorb propane and CO.sub.2, but sorb methane and ethane to a much smaller extent. Teflon is particularly sensitive and selective to CO.sub.2. PAAm is very sensitive to water and is a promising sensor for detection of the water content of fuel gas streams. Teflon, SU-8 and HPDMS and their mixtures with cryptophane gave better results than PDMS in terms of reproducibility and reliability of the QCM measurements, but have lower sensitivities for methane. In the case of SU-8 and HPDMS, the presence of a small amount of cryptophane (1-2 wt. %) provided in a increase in the methane sensitivity compared to the coatings without cryptophane. Higher amounts of 5-10 wt. % cryptophane surprisingly resulted in a loss of the additional methane sensitivity. Without wishing to be bound by way of theory, this may be due to a change in the morphology of the cryptophane.

Experiment 4

(22) An analysis of table 1 was carried out, based on five sensor elements and four components (methane, ethane, propane and CO.sub.2). This provided the Example Sensor Array 1 consisting of five sensor elements as in table 4, with the Am (mass change) as obtained with the QCM experiments as indicated in the table.

(23) TABLE-US-00004 TABLE 4 Example Sensor Array 1 m for m for Layer 100% 20% m for m for thickness Coating CH.sub.4 C.sub.2H.sub.6 10% C.sub.3H.sub.8 100% CO.sub.2 (m) PAAm (a) 0.1 n.r. n.r. 0.2 2.9 Teflon 0.02 0.08 0.22 0.57 0.63 AF 1600 SU-8 0.03 0.075 n.r. 1.1 1.1 (1.1) HPDMS 0.11 0.25 0.57 0.82 1.9 (1.6) HPDMS/ 0.34 0.61 1.4 1.8 2.5 Cryptophane 98/2 w/w (a) PAAm gave a response of 3 g/cm.sup.2 for 4000 ppm water.

(24) Assuming an error of 0.01 fF for a capacitance sensor in capacity reading for each electrode and of 5 Hz for QCM, multiple regression analysis gives an accuracy for the calorific value (CV) for Example Sensor Array 1 as in table 5. For each gas stream, 50 ppm water was included.

(25) TABLE-US-00005 TABLE 5 Accuracy of Example Sensor Array 1 Error in CV for 5 Hz response deviation HPDMS/ Gas CV Cryptophane stream [MJ/m.sup.3] 98/2 SU-8 HPDMS Teflon PAAm Total GG 34.74 0.37% 0.17% 0.67% 0.54% 0.00% 1.77% BioGas 25.07 0.52% 0.24% 0.93% 0.75% 0.00% 2.45% HG 40.01 0.32% 0.15% 0.59% 0.47% 0.00% 1.53% FHG 44.16 0.29% 0.14% 0.53% 0.43% 0.00% 1.39% Error in CV for 0.01 fF response deviation HPDMS/ Gas Cryptophane stream CV 98/2 SU-8 HPDMS Teflon PAAm Total GG 34.74 0.20% 0.09% 0.34% 0.27% 0.01% 0.91% BioGas 25.07 0.26% 0.12% 0.48% 0.39% 0.00% 1.25% HG 40.01 0.17% 0.09% 0.29% 0.23% 0.01% 0.79% FHG 44.16 0.15% 0.07% 0.27% 0.22% 0.00% 0.71%

(26) Example Sensor Array 2 has sensor elements as in table 6, with the m (mass change) as obtained with the QCM experiments as indicated in the table.

(27) TABLE-US-00006 TABLE 6 Example Sensor Array 2 Layer m for m for m for m for thickness Coating 100% CH.sub.4 20% C.sub.2H.sub.6 10% C.sub.3H.sub.8 100% CO.sub.2 (m) PAAm (a) 0.1 n.r. n.r. 0.2 2.9 Teflon-AF 0.02 0.08 0.22 0.57 0.63 1600 SU-8 0.03 0.075 n.r. 1.1 1.1 (1.1) PDMS/ 0.45 1.5 2.3 4.3 4.7 Cryptophane 87/13 w/w HPDMS/ 0.34 0.61 1.4 1.8 2.5 Cryptophane 98/2 w/w (a) PAAm gave a response of 3 g/cm.sup.2 for 4000 ppm water.

(28) Example Sensor Array 2 has accuracies as indicated in table 7. As can be seen, the error levels on the QCM are now ranging between 0.9% and 1.6% and on the capacitive sensor even below 1%, which meets the desired accuracy.

(29) TABLE-US-00007 TABLE 7 Accuracy of Example Sensor Array 2 HPDMS/ PDMS/ Gas CV Cryptophane Cryptophane stream [MJ/m.sup.3] 98/2 SU-8 87/13 Teflon PAAm Total Error in CV for 5 Hz response deviation GG 34.74 0.16% 0.20% 0.02% 0.77% 0.00% 1.15% BioGas 25.07 0.22% 0.28% 0.02% 1.06% 0.00% 1.59% HG 40.01 0.14% 0.17% 0.02% 0.67% 0.00% 1.00% FHG 44.16 0.13% 0.16% 0.01% 0.60% 0.00% 0.90% Error in CV for 0.01 fF response deviation GG 34.74 0.09% 0.11% 0.00% 0.39% 0.01% 0.59% BioGas 25.07 0.12% 0.14% 0.01% 0.54% 0.00% 0.82% HG 40.01 0.08% 0.10% 0.00% 0.33% 0.01% 0.52% FHG 44.16 0.07% 0.08% 0.01% 0.31% 0.00% 0.46%

(30) In the calculations, the used gas compositions GG (Groningen Gas), BioGas, HG (High calorific value gas) and FHG (Future HG) are as in table 8.

(31) TABLE-US-00008 TABLE 8 Gas streams (volume concentration [%]) GG HG FHG Biogas CH.sub.4 81.30 91.4 80.3 60 C.sub.2H.sub.6 2.85 3.0 11.7 0 C.sub.3H.sub.8 0.37 1.5 3.9 0 C.sub.4H.sub.10 0.14 0.5 0 0 C.sub.5H.sub.12 0.04 0.1 0 0 C.sub.6H.sub.14 0.05 0 0 0 N.sub.2 14.35 2.0 4.1 0 O.sub.2 0.01 0 0 0 CO.sub.2 0.89 1.5 0 35 Other (NH.sub.3, H.sub.2O, HS) 0 0 0 5 Density (1 bar, 273 K) 0.831 0.795 0.865 1.172 [kg/m.sup.3] Calorific Value [MJ/m.sup.3] 34.95 40.70 44.16 23.88 Calorific Value [MJ/kg] 42.08 51.20 51.08 20.37

Experiment 5

(32) A set of nine coatings was tested for sensitivity to gas exposure, by measuring the capacitance changes of coated electrodes. Nine bare comb electrodes obtained from NXP were coated with different coatings according to table 9. Each of these coated chips were subsequently placed in a Teflon gas flowcell of the Gas Exposure System and connected to an LCR analyzer (Iviumstat, Ivium). The gas flowcell contains a holder for NXP test chips, a temperature sensor and an in- and outlet for a gas stream. The measurements were done at room temperature; the temperature was not controlled. In order to monitor the relative humidity of the gas stream, a second flow cell, containing a moisture sensor, was connected to the exit stream of the first cell. The moisture level was kept to a minimum by flushing the entire system with N.sub.2 gas for up to 16 hours, prior to each measurement. The change in capacitance was measured upon exposure to continuous flows of 500 ml/min 100% CH.sub.4, 250 ml/min 20% C.sub.2H.sub.6 (in N.sub.2), 500 ml/min 10% C.sub.3H.sub.8 (in N.sub.2), 500 ml/min 100% CO.sub.2, and 500 ml/min 5% relative humidity (in N.sub.2). The capacitance was measured at 16 different frequencies, ranging from 100 Hz to 100 000 Hz. The measurement at 720 Hz was used for determining the capacitance changes as the noise level was lowest at this frequency. For each coating, the capacitance at 1 bara of N.sub.2 was regarded as the baseline signal. In all cases, the observed capacitance change upon exposure to a gas was determined with respect to the baseline signal. In table 9, the nominal capacitance values of the uncoated chips, as well as the capacitance values of the chips after coating and at exposure to 1 bara of N.sub.2 (i.e. the baseline value) are listed. In table 10, the capacitance changes upon gas exposure are given as absolute values and in table 11 as percentages of the baseline signal.

(33) For all data presented in tables 10 and 11, the observed capacitance change upon exposure to a gas was determined with respect to exposure to pure N.sub.2 gas. To enable analysis of natural gas in which N.sub.2 is present, the response of the chips to N.sub.2 gas with respect to vacuum was determined by extrapolation from the response to N.sub.2 at various pressures. A high-pressure gas exposure chamber was used which was integrated into the Gas Exposure System. The capacitance was measured at 2, 3 and 6 bara and the capacitance changes were determined relative to the baseline signal at 1 bara. The responses to 1 bara N.sub.2 with respect to vacuum were then determined by extrapolation and are given in table 12.

(34) TABLE-US-00009 TABLE 9 Nominal capacitance values for the chips, before and after coating C (coated) C (uncoated) (at 1 bara N2) Chip # Coating (pF) (pF) 1 SU-8 31.0 54.9 2 Teflon 30.0 35.9 3 PDMS 30.2 45.8 4 PDMS/Cryptophane (50/50) 29.5 48.3 5 PDMS/MOF Z1200 (50/50) 30.0 42.7 6 PDMS/Zeolite NH4 CZP200 30.8 46.3 (MFI) (50/50) 7 PDMS/Cryptophane (83/17) 41.8 69.7 8 HPDMS/Cryptophane (98/2) 31.1 42.6 9 HPDMS 30.9 43.9

(35) TABLE-US-00010 TABLE 10 Absolute capacitance changes upon gas exposure. C (pF) (absolute value) Coating 100% 20% 10% 100% Chip # CH.sub.4 C.sub.2H.sub.6 C.sub.3H.sub.8 CO.sub.2 5% RH 1 SU-8 n.r. 0.014 0.030 0.17 0.88 2 Teflon n.r. 0.016 0.033 0.037 n.r. 3 PDMS n.r. 0.013 n.r. 0.027 0.018 (6% RH) 4 PDMS/ 0.11 0.014 n.r. 0.16 0.24 Cryptophane (50/50) 5 PDMS/ 0.013 0.025 0.094 0.037 0.028 MOFZ1200 (50/50) 6 PDMS/ 0.022 0.36 0.44 0.22 2.87 Zeolite NH4CZP200 (MFI) (50/50) 7 PDMS/ 0.11 n.r. 0.0096 0.19 0.30 Cryptophane (83/17) 8 HPDMS/ 0.033 0.021 n.r. 0.16 n.m. Cryptophane (98/2) 9 HPDMS 0.0067 0.0095 0.027 0.016 n.r. n.r. = no response. n.m. = not measured.

(36) TABLE-US-00011 TABLE 11 Capacitance changes upon gas exposure, as percentages of the N.sub.2 baseline signal. C (pF) (percentage of baseline value) 100% 20% 10% 100% Chip # Coating CH.sub.4 C.sub.2H.sub.6 C.sub.3H.sub.8 CO.sub.2 5% RH 1 SU-8 n.r. 0.025 0.060 0.31 1.6 2 Teflon n.r. 0.044 0.093 0.10 n.r. 3 PDMS n.r. 0.028 n.r. 0.058 0.038 (6% RH) 4 PDMS/ 0.23 0.030 n.r. 0.33 0.49 Cryptophane (50/50) 5 PDMS/ 0.029 0.059 0.22 0.086 0.067 MOFZ1200 (50/50) 6 PDMS/ 0.048 0.76 0.96 0.48 6.20 Zeolite NH4CZP200 (MFI) (50/50) 7 PDMS/ 0.16 n.r. 0.014 0.28 0.43 Cryptophane (83/17) 8 HPDMS/ 0.077 0.048 n.r. 0.35 n.m. Cryptophane (98/2) 9 HPDMS 0.015 0.022 0.061 0.036 n.r. n.r. = no response. n.m. = not measured.

(37) TABLE-US-00012 TABLE 12 Capacitance change for exposure to 1 bara of N.sub.2, with respect to vacuum, as determined by extrapolation. Response to 1 bara N.sub.2 Chip # Coating C (pF) 1 SU-8 0.098 2 Teflon 0.005 3 PDMS 0.001 4 PDMS/Cryptophane (50/50) 0.018 5 PDMS/MOFZ1200 (50/50) 0.005 6 PDMS/Zeolite NH4CZP200 (MFI) (50/50) 0.251 7 PDMS/Cryptophane (83/17) 0.033 8 HPDMS/Cryptophane (98/2) 0.019 9 HPDMS 0.007
Coating Selection for Sensor Array

(38) Out of the nine tested coatings (tables 9-12), six were selected for a gas sensor array. Coating selection was based on the following considerations: (1) The number of coatings should be minimal, but sufficient for estimating partial pressures of the 5 object gases; (2) The standard deviation of the Caloric Value (CV) should be as small as possible. Based on these criteria, the following 6 coatings were selected:

(39) 1) SU-8; 2) Teflon; 3) PDMS; 4) PDMS/Cryptophane (50/50); 5) PDMS/MOF Z1200 (50/50); 6) PDMS/Zeolite NH4CZP200 (MFI) (50/50)

(40) CV and its standard deviation (.sub.CV) were estimated as follows. The CV is estimated from the partial pressures p.sub.j of the object gases j=1-5 and the p.sub.j values are estimated from the capacity changes C.sub.i of coating i=1-6 via the experimentally obtained response matrix C.sub.i/p.sub.j. From this response matrix and the standard deviation of the capacity measurements (.sub.i=5 fF for the present experiments), we can obtain the covariance matrix R.sub.jj of the estimated partial pressures, from which .sub.CV follows. Table 13 contains the CV for all 5 object gases and table 8 the CV for a number of typical gas mixtures. Table 14 contains .sub.CV for 4 different cases. Rows 1 and 2 are computed for the response matrix with N.sub.2 background. Row 1 contains .sub.CV for all 9 coatings, giving a best CV accuracy of 7.18 MJ/m.sup.3 for any selection of 6 coatings. For the above selection of 6 coatings, we obtain 8.27 MJ/m.sup.3, the value of row 2. In the possible presence of other gases besides the object gases, one should rather work with vacuum background. For that purpose, we subtracted the N.sub.2 background from the response matrix using our measurements of pure N.sub.2 responses for all coatings. Rows 3 and 4 contain .sub.CV for vacuum background, giving a lower bound of the CV accuracy of 7.47 MJ/m.sup.3 for 9 coatings and 8.41 MJ/m.sup.3 for the selected 6 coatings. Note that these values are based on the estimated capacity accuracy of 5 fF. It is expected that in actual operational conditions, capacity measurements will be considerably more accurate, giving more accurate CV estimations. .sub.CV of 8.41 MJ/m.sup.3 amounts to a relative accuracy of about 19-35% of the CV of the different gas mixtures of table 8.

(41) TABLE-US-00013 TABLE 13 Caloric Values of object gases CH.sub.4 C.sub.2H.sub.6 C.sub.3H.sub.8 CO.sub.2 H.sub.2O CV [MJ/m.sup.3] 39.79 70.58 101.20 0 0

(42) TABLE-US-00014 TABLE 14 Standard deviation CV for a coating standard deviation of 5 fF. Case .sub.CV [MJ/m.sup.3] 1 All 9 coatings, N.sub.2 background 7.18 2 Selected 6 coatings, N.sub.2 background 8.27 3 All 9 coatings, vacuum background 7.47 4 Selected 6 coatings, vacuum background 8.41

Experiment 6

(43) A gas sensor array was constructed, consisting of the six coated chips mentioned in the previous experiment. A gas measuring chamber containing all the six chips was used. The array of chips was analyzed using a LCR analyzer and a multiplexer. With this setup, all six chips were simultaneously exposed to gas mixtures and all analyzed at the same time. In order to establish the relationship between gas concentration and response for each chip and each gas, a series of measurements was carried out using the sensor array at different concentrations of each gas. Furthermore, several gas mixtures mimicking the natural gas types mentioned in Table 8 were measured as well. With the measured responses of the six individual chips, the gas composition and its CV can be calculated according to the method described in experiment 5. Table 15 shows a selected number of measured responses for the gas sensor array.

(44) TABLE-US-00015 TABLE 15 Capacitance measurements (C, pF, 720 Hz) with the gas sensor array containing six chips. Chip # Gas 1 2 3 4 5 6 CH.sub.4 n.r. 0.006 0.003 0.101 0.009 0.182 CH.sub.4:N.sub.2 = 75:25 n.r. 0.004 0.005 0.099 0.006 0.134 CH.sub.4:N.sub.2 = 50:50 n.r. 0.007 n.r. 0.068 0.004 0.074 CH.sub.4:C.sub.2H.sub.6:N.sub.2 = n.r. 0.012 0.011 0.110 0.017 0.216 80:10:10 CH.sub.4:C.sub.3H.sub.8:N.sub.2 = 0.030 0.033 0.005 0.077 0.043 0.075 80:5:15 CH.sub.4:C.sub.2H.sub.6:C.sub.3H.sub.8:N.sub.2 = 0.019 0.035 0.015 0.084 0.051 0.103 75:10:5:10 n.r. = no response.