CALIBRATION METHOD, THE USE THEREOF, AND APPARATUS FOR CARRYING OUT THE METHOD

Abstract

The application describes a method for calibrating metal oxide gas sensors using impedance spectroscopy, comprising the steps of: determining the impedance spectrum of the metal oxide gas sensor in a gas mixture in the absence of an analyte in order to determine a base line, and determining the impedance spectrum of the metal oxide gas sensor in the gas mixture in the presence of the analyte in at least a known concentration. The use of this method and an apparatus which can be used to carry out this method are also described.

Claims

1. A method for calibrating metal oxide gas sensors with impedance spectroscopy, comprising the steps of: determining the impedance spectrum of the metal oxide gas sensor in a gas mixture in the absence of an analyte, to specify a baseline, and determining the impedance spectrum of the metal oxide gas sensor in the gas mixture in the presence of the analyte in at least one known concentration.

2. The method as claimed in claim 1, wherein the gas mixture is selected from synthetic air and/or synthetic biogas and/or room air and/or inert gas and/or N.sub.2 and/or at least one noble gas and/or N.sub.2/CO and/or N.sub.2/NO.sub.x and/or N.sub.2/CO.sub.2.

3. The method as claimed in claim 1, wherein the analyte is selected from water and/or carbon monoxide and/or at least one alcohol and/or at least one aldehyde, and/or at least one ketone and/or at least one terpene and/or at least one organic acid and/or at least one aliphatic hydrocarbon and/or at least one thiol and/or at least one sulfide and/or at least one ester and/or at least one compound having an aromatic C6 group and/or at least one lactone and/or at least one halogenated organic compound.

4. The method as claimed in claim 1, wherein the metal oxide gas sensor is selected from oxide ceramics, nonoxide ceramics and clay minerals.

5. The method as claimed in claim 4, wherein the oxide ceramic is selected from at least one SnO.sub.2, AgO, CuO, Al.sub.2O.sub.3, WO.sub.3, GeO.sub.2, SiO.sub.2, TiO.sub.2, ZnO, In.sub.2O.sub.3 or Mn.sub.2O.sub.3 ceramic or mixture of at least two of the stated compounds.

6. The method as claimed in claim 5, wherein the oxide ceramic is doped with at least one metal.

7. The method as claimed in claim 6, wherein the metal is selected from Pd, Pt, Au, Ag, Cd, Ni, Mn, Fe and/or Cu and/or wherein the metal is incorporated in an amount of about 0.2 to about 5 wt %, based on the oxide ceramic.

8. The method as claimed in claim 1, wherein the metal oxide gas sensor is coated with at least one compound selected from at least one polymer, one bioorganic substance, one antibody, one metal-organic cluster, one metal-organic framework compound, one metal organyl, one ionic liquid, one siloxane and/or organic ions.

9. The method as claimed in claim 1, wherein the impedance spectrum is determined in a frequency range from about 1 Hz to about 1 000 000 Hz.

10. The method as claimed in claim 1, wherein the impedance spectrum is determined in a frequency range from about 100 Hz to about 10 000 Hz.

11. The method as claimed in claim 1, wherein the impedance spectrum is determined at an amplitude of about 1 mV to about 12 V.

12. The method as claimed in claim 1, wherein the impedance spectrum is determined at a relative humidity of about 15% to about 60%.

13. The method as claimed in claim 1, wherein the impedance spectrum is determined at an analyte concentration of about 1 to about 100 ppb.

14. The method as claimed in claim 1, wherein the analyte is contacted as gas with the metal oxide gas sensor.

15. The use of the method as claimed in claim 1 for regulating air supply in line with demand, for VOC measurement, in thermal processes, for measuring ammonia and sulfur gas, for controlling technical fermentation processes, in food production, in gas warning systems, in military and security technology, in the realm of private and public transport and for demand warning in vehicles.

16. An apparatus (1) for calibrating metal oxide gas sensors with impedance spectroscopy, comprising: a measuring chamber (2) with a metal oxide gas sensor (3), a facility for determining the impedance spectrum, and a metering facility for metering the gas mixture and optionally the analyte into the measuring chamber (2), the metering facility being connected via a line (5) to the measuring chamber (2).

17. The apparatus as claimed in claim 16, wherein the metering facility comprises a first metering apparatus (6) for the gas mixture and a second metering apparatus (7) for the analyte.

18. The apparatus (1) as claimed in claim 16, wherein the second metering apparatus (7) is adapted to vaporize the analyte so that it is introduced as a gas into the measuring chamber (2).

19. The apparatus (1) as claimed in claim 16, which further comprises a humidifying facility which is connected to the measuring chamber (2) via a line (10) so as to establish a prespecified humidity in the measuring chamber (2).

20. A kit comprising the recorded calibration curve of claim 1 and a metal oxide gas sensor calibrated therewith.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The intention below is to elucidate the invention in more detail using figures and exemplary embodiments, without restricting the general concept of the invention. Here

[0034] FIG. 1 shows an apparatus of the invention.

[0035] FIG. 2 shows a metal oxide gas sensor as may be used in the apparatus of FIG. 1.

[0036] FIGS. 3a-3c show the calibration of a Pd/SnO.sub.2 metal oxide gas sensor with a thiol and synthetic air.

[0037] FIGS. 4a-4c show the calibration of a Pd/SnO.sub.2 metal oxide gas sensor with a sulfide and synthetic air.

[0038] FIGS. 5a-5c show the calibration of a CuO metal oxide gas sensor with a thiol and synthetic air.

[0039] FIGS. 6a-6c show the calibration of a CuO metal oxide gas sensor with a sulfide and synthetic air.

[0040] FIGS. 7a and 7b show the calibration of a Pd/SnO.sub.2 metal oxide gas sensor with a thiol and synthetic biogas.

[0041] FIGS. 8a and 8b show the calibration of a sulfide with a Pd/SnO.sub.2 metal oxide gas sensor and synthetic biogas.

[0042] FIGS. 9a-9j show the cross-correlation of a calibrated Pd/SnO.sub.2 metal oxide sensor with various substances in synthetic air.

DETAILED DESCRIPTION

[0043] FIG. 1 serves to elucidate an apparatus of the invention with which the method of the invention can be carried out. The apparatus comprises a measuring chamber 2, in which the metal oxide gas sensor 3 is located. Further, the measuring chamber may contain a temperature/humidity/pressure sensor 9. Using this temperature/humidity/pressure sensor 9, it is possible to control the chamber conditions, which may be adjusted, for example, to about 0.7% to about 100%, or about 15% to about 60%, or about 40-about 60% relative humidity, a temperature of about 55 C. to about +85 C., about 18 C.-about 25 C. and a pressure of about 570 hPa to about 1600 hPa, or about 940-about 1200 hPa, to allow these chamber conditions to be reestablished in the event of deviations. For the sensors in the measuring chamber 2, steel or glass tube T-pieces may be used as sensor passages. The measuring chamber 2 may have an inert surface, achievable for example by powder coating, anodizing or siloxing. Measuring chambers 2 used may further comprise flow tubes and glass vessels, of the kind employed as laboratory reactors. The volume of the measuring chamber may be about 10 cm.sup.3 to about 2000 m.sup.3, as are used in the case of emissions testing chambers, for example. The measuring chamber may be connected to a detector 4, examples being FID (flame ionization detector), PID (photoionization detector), GC-MS (gas chromatography coupled with mass spectrometry), PTR-MS (photon transfer reaction mass spectrometry), FT-IR (Fourier-transform infrared spectrometer) and online NMR (nuclear magnetic resonance spectroscopy), in order to allow further measurements to be performed in order to determine the analyte.

[0044] The apparatus according to FIG. 1 further comprises a metering facility for metering the gas mixture and the optional analyte into the measuring chamber 2, the metering facility being connected via a line 5 to the measuring chamber 2. In FIG. 1, the first metering apparatus 6 of this metering facility comprises a mass flow regulator, allowing regulated supply of the background gasfor example, synthetic air or synthetic biogas. The flow rate may be, for example, about 1 ml/minute to about 100 L/minute, or about 2-about 3 L/min. This mass flow regulator may be connected via a line to a metering unit as second metering apparatus 7 for the analyte, said unit being connected via a line 5 to the measuring chamber 2. The second metering unit 7 may allow the metering of liquid by microdrop, or a piezoelectric crystal may be used as vaporizer.

[0045] The apparatus of FIG. 1 additionally has a humidifying apparatus, which in the present case comprises a mass flow regulator 10 whose mass flow is passed through a wash bottle 11 which is connected via a line 8 to the measuring chamber. The mass flow for the mass flow regulator 6 of the humidifying apparatus may be 0.4-0.5 L/min.

[0046] FIG. 2 shows a metal oxide gas sensor 3. In this case there is a ceramic layer 31, examples being the ceramics set out above, located on a heated carrier 32. A metal oxide gas sensor of this kind allows the impedance spectrum to be determined, for example, in a range from about 100 Hz to about 1 000 000 Hz. In FIG. 2, the reaction equation is shown schematically, for better illustration of the reaction occurring in the metal oxide gas sensor. Furthermore, FIG. 2 also shows the circuit 12 for the heating of the carrier 32 and the circuit 13 for the impedance measurement.

[0047] In the examples below, the impedance spectrum of various organic compounds was determined.

Example 1

[0048] The substances were investigated in the apparatuses described in FIGS. 1 and 2 above. The conditions in the measuring chamber were 21 C./50% relative humidity/940 hPa. The background, i.e., the gas mixture used for determining the baseline (without analyte), and into which the analytes were then metered in concentrations of 1 ppb, 10 ppb and 100 ppb, were either synthetic air (20% O.sub.2 and 80% N.sub.2) or synthetic biogas with 60% methane, 38% CO.sub.2 and 2% O.sub.2. The overall volume flow was 2.51/min.

[0049] In preliminary investigations with direct-current measurement, the two sensor types SnO.sub.2 with 3% Pd and pure CuO were found to be the most sensitive for sulfur-organic compounds. They were operated with a heating voltage of 2.7 V and over a frequency range of 100 to 1 000 000 Hz (amplitude 100 mV).

[0050] FIGS. 3a-3c show the calibration of the metal oxide gas sensor composed of SnO.sub.2 with 3% Pd (referred to hereinafter as Pd/SnO.sub.2 sensor) with butanethiol, the background used for determining the baseline being synthetic air. In FIG. 3a, the resistance R in ohms is plotted over the frequency range investigated. Here, in a first measurement, the baseline was determined, i.e., the impedance spectrum of the background, i.e., of the synthetic air, without the addition of butanethiol as analyte. This is described as Background in the figures. Then the impedance spectra at 1 ppb, 10 ppb and 100 ppb of butanethiol as analyte were determined. The impedance spectra, as frequency [Hz] versus R [ohms], are represented in FIG. 3a. For better illustration for the spectra found at low concentrations, the relevant part from FIG. 3a has been shown in enlarged form in FIG. 3b. In FIG. 3c, the concentration is plotted over the distance to the baseline (Background) in ohms. From FIG. 3c, it is possible to recognize a virtually linear curve profile which can be used as a calibration curve. If the sensor calibrated in this way is used for determining an unknown concentration of butanethiol by impedance spectroscopy, then the concentration can be taken directly, on the basis of the impedance determined, from FIG. 3c.

Example 2

[0051] Example 2 was carried out in a similar way to example 1, with the difference that the analyte used was dimethyl sulfide.

[0052] The results are shown in FIGS. 4a-4c. The results obtained were analogous to those in example 1, and accordingly reference is made to the comprehensive discussion of FIGS. 3a-3c above.

Example 3

[0053] Example 3 was carried out in a similar way to example 1, with the difference that the metal oxide gas sensor used was CuO.

[0054] The results are shown in FIGS. 5a-5c. These correspond, in their implementation and in the result, to the above-discussed FIGS. 3a-3c, and hence for the interpretation of FIGS. 5a-5c, reference is made to the observations above.

Example 4

[0055] Example 4 was carried out in a similar way to example 2, with the difference that the metal oxide sensor used was CuO.

[0056] The results are shown in FIGS. 6a-6c. These correspond, in their implementation and in the result, to the above-discussed FIGS. 3a-3c, and hence for the interpretation of FIGS. 6a-6c, reference is made to the corresponding observations above.

Example 5

[0057] Example 5 was carried out in a similar way to example 1, with the difference that synthetic biogas with 60% methane, 38% CO.sub.2 and 2% O.sub.2 was used instead of synthetic air.

[0058] The results are shown in FIGS. 7a and 7b. Here, FIG. 7a shows the resistances found relative to the frequency range investigated, in the concentrations indicated in the figures, and FIG. 7b shows in graph form the concentration relative to the resistance, at the specified frequencies.

Example 6

[0059] Example 6 was carried out in a similar way to example 2, with the synthetic biogas indicated in example 5 being used instead of the synthetic air.

[0060] The results are shown in FIGS. 8a and 8b. Here, FIG. 8a shows the resistance found relative to the frequency applied, at the concentrations indicated in FIG. 8a. In FIG. 8b, these concentrations are plotted against the resistance found, at the specified frequencies.

Example 7

[0061] In example 7, the substances ethanol, decanol, acetone, hexanal, -pinene, limonene, acetic acid and octanoic acid, octane and isoprene as analyte were investigated by impedance spectroscopy using SnO.sub.2 with 3% Pd as metal oxide gas sensor against the background of synthetic air. The above analytes were metered in a concentration of 100 ppb. The results are shown in FIGS. 9a-9j.

[0062] The results show that with a specified sensor, it is possible to test other analytes, in order to determine the sensitivity of the sensor for the other analytes.

[0063] The invention is of course not confined to the examples and embodiments represented in the figures. The above description should therefore be regarded not as restricting, but instead as illustrative. The claims which follow should be understood to mean that a stated feature is present in at least one embodiment of the invention. This does not exclude the presence of further features. Where the claims and the above description define first and second features, this designation serves for distinguishing two features of the same kind, without specifying any hierarchy.