Abstract:
The invention relates to a method for detecting gases that can be ionized wherein an atmospheric plasma jet is produced, wherein a gas mixture is brought into interaction with the plasma jet, and wherein an electrical quantity is measured as a measure of the concentration of the gas in the gas mixture. The invention further relates to a device for detecting gases that can be ionized, including a gas inlet, means for ionizing a gas, a voltage source, two electrodes, and means for determining an amperage, wherein the two electrodes are connected to the voltage source, wherein the means for determining an amperage are connected to the electrodes in such a way that the magnitude of the current flowing between the electrodes can be measured, and wherein a plasma nozzle is provided to produce an atmospheric plasma jet.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for detecting ionisable gases, in particular organic molecules, preferably hydrocarbons. The invention also relates to an apparatus for detecting ionisable gases, includins a gas inlet, means for ionising an ionisable gas, including a voltage source, two electrodes and means for determining a current strength, the two electrodes being connected to the voltage source and the means for determining a current strength being connected to the electrodes such that the strength of the current flowing between the electrodes can be measured. 
     2. Description of Related Art 
     In order to detect organic molecules, for example hydrocarbons in a gas mixture, the flame ionisation detection method is frequently used. In such a method, the gas mixture to be examined is introduced into a flame, in particular into an oxyhydrogen flame. In this flame, the ionisable components of the gas mixture, i.e. in particular organic molecules, are ionised by the thermal energy. The free electrons and ions generated in this way, for example CHO + , will then move according to their charge to respectively one of two electrodes provided, between which a voltage is applied. The current flowing between the electrodes in this way is a measure for the proportion of the ionisable gas, i.e. the concentration of the organic molecules, in the overall gas mixture. 
     The flame ionisation detection method has the disadvantage that hydrogen and oxygen have to be provided in order to generate the flame. Further, the environment of the burner is intensely heated by the flame. A further disadvantage is that part of the organic molecules is oxidised by the flame. These will then no longer contribute to the current flowing between the electrodes and will therefore not be detected. Therefore, this method is inaccurate. 
     The concentration of organic molecules in a gas mixture has to be determined, inter alia, in the case of exhaust gases. Thus, for example in the case of exhaust systems, legal requirements in respect of maximum concentrations of certain organic substances have to be complied with. In order to meet such standards, controlled exhaust gas purification systems are often used in the prior art. The measured variable of the concentration of the organic molecules in exhaust gas, which variable is required for controlling, is obtained here, for example, by means of a flame ionisation detector. However, these have the disadvantage that a combustible gas such as for example oxyhydrogen gas is required for the operation of these detectors. Depending on the position of the detector in the exhaust system, however, it is complex, dangerous or even impossible to ensure a supply with this combustible gas. 
     Determining the concentration of organic molecules in a gas mixture is also required in the general area of gas analytics, for example when a process gas is to be analysed. The disadvantage of the use of flame ionisation detectors here is that the process gas is contaminated by any incompletely burnt proportions of the combustible gas used to feed the flame or by combustion products. This may have a negative effect on the process steps following the analysis. 
     SUMMARY OF THE INVENTION 
     The present invention is therefore based on the technical object of providing a method and an apparatus for detecting ionisable gases, in particular organic molecules, preferably hydrocarbons, which at least partially avoids the above-mentioned disadvantages. 
     According to the invention, this object is achieved by means of a method wherein an atmospheric plasma jet is generated, a gas mixture containing the ionisable gas is made to interact with the plasma jet and an electrical variable is measured so as to be used as a measure for the concentration of the ionisable gas in the gas mixture. 
     Due to the fact that the gas mixture is made to interact with an atmospheric plasma jet, the ionisable gas contained therein is at least partially ionised by the plasma jet. The free electrons and ions which develop as a result constitute movable charge carriers, through which an electric current is generated that is measured so as to be used as a measure of the concentration of the ionisable gas in the gas mixture. The ionisation of the ionisable gas with the atmospheric plasma has the advantage over thermal ionisation using a flame that no combustible gas such as for example oxyhydrogen or hydrogen and oxygen needs to be supplied in order to generate the flame. Thus, air may be used as the working gas for producing the atmospheric plasma. As a rule, this is available as ambient air. Further, the environment of the plasma jet is heated by the plasma jet to a lesser degree than the environment of a flame, in particular an oxyhydrogen flame, is heated by the flame itself. 
     An electrical variable is understood to be any conceivable electrical variable, in particular a voltage, a current, the electric field or the resonance frequency of a resonance circuit. 
     In a preferred embodiment of the method, the electrical variable is measured between two electrodes, with a voltage being applied between the electrodes. In this way, the measurement of the electrical variable is realised in a very simple manner. Thus, for example the current flowing between the electrodes, the voltage or the capacitance of the electrode arrangement may be measured. The capacitance is influenced by the charged particles present between the electrodes. Further, the electrodes may be integrated as capacitance in a resonance circuit and the resonance frequency of the latter can be measured. 
     A further embodiment of the method is achieved by applying a voltage between 50 V and 350 V, in particular between 50 V and 250 V, preferably between 75 V and 150 V, between the electrodes. It has been shown that the method may be carried out particularly well at voltages within these voltage ranges. If the voltage is too high, there is a risk that the current flowing between the electrodes is too strong, so that the apparatus used for determining the current strength might be damaged. If the voltage is too low, the electric current flowing between the electrodes is very weak, so that the measurement of the current strength may have greater relative errors. Further, if the voltage is too low, the ions and electrons may be influenced to a large degree by the space-charge region occurring between the electrodes during ionisation. 
     In this method, either a DC voltage or an AC voltage can be applied between the electrodes. In the case of a DC voltage, the electrons and ions released during ionisation will always move towards the same electrode. In this way, a continuous measurement of the current flowing between the electrodes becomes possible. The application of an AC voltage has the advantage that the current strength measurement signal can be modulated via the AC voltage. This allows the measurement signal to be isolated from interferences by means of lock-in amplification. Also a square-wave voltage may be applied between the electrodes in order to discretise the measurements in time. 
     In a further embodiment of the method, a less noisy measurement is achieved owing to the fact that the plasma jet is substantially potential-free. As a result, no additional electric field is introduced into the area of ionisation by the plasma jet, by which the free electrons and ions might be influenced. 
     In a further preferred embodiment of the method, the plasma jet is produced by an arc discharge generated using a high-frequency high voltage. A high-frequency high voltage is typically to be understood to mean a voltage in the range of 1 to 50 kV, in particular 1 to 15 kV, at a frequency of 1 to 100 kHz, in particular 10 to 100 kHz, preferably 10 to 50 kHz. A plasma jet thus produced has a particularly low temperature at a high reactivity, so that the environment of the plasma jet is only slightly heated by the plasma jet. 
     In a further preferred embodiment, any influence on the gas mixture by the detection method is reduced by using an inert working gas, preferably nitrogen or an noble gas such as for example argon or helium, for generating the plasma jet. In this way, the gas mixture is not contaminated by any non-inert working gases. This is advantageous in particular if the method is used for analysing gas streams, wherein the gas stream is used further after the analysis, for example in a subsequent process step. This offers in particular also advantages over the flames used in the prior art, since the gas stream is not contaminated by any incompletely combusted gases, for example hydrogen, or by the combustion products generated, for example water or carbon dioxide. 
     In a further embodiment of the method, the gas mixture is derived from a process gas stream or an exhaust gas stream. In the case of a process gas stream, it is important to know the components contained in the gas mixture because they influence the composition, the character and the quality of the products produced by the process. Also in the case of an exhaust gas stream, for example from a combustion process, it is important to control the organic components contained therein, for example in order to comply with the legally stipulated emission limit values. It is therefore advantageous to cause the gas mixture from an exhaust gas stream to interact with the plasma jet, in order to determine in this way the concentration of the ionisable gas in the exhaust gas stream. 
     In a further preferred embodiment of the method, the gas mixture is generated by applying the plasma jet onto a contaminated surface. In this way, the plasma jet fulfils two functions at the same time. The plasma jet releases any contaminants from the surface and these contaminants are then ionised as a gas in the plasma jet. The measured electric current will then be a measure of the contamination removed from the surface. The application of the plasma jet onto the surface can then be carried out for example until the concentration of the ionisable gas has fallen below a certain level. In this way it is ensured that a certain degree of purity is achieved when cleaning the surface. 
     In the case of a conductive surface, for example a metallic one, this surface can replace one of the electrodes. In this case, the voltage is applied between the remaining electrode and the surface. 
     Alternatively, the remaining electrode may also be dispensed with by applying the voltage between the surface and the plasma nozzle. 
     In a further embodiment of the method, an analysis that deviates from the analysis of the gas mixtures as so far described, which is based on an electrical variable, is carried out by analysing the light generated in the interaction region of the plasma jet with the gas mixture and by determining the intensity of the light in at least one spectral range as a measure of the concentration of at least one substance in the gas mixture. 
     In the area of the plasma, substances of the gas mixture, in particular individual molecules and atoms, are excited by the energy contained in the plasma and emit light. Since these substances have a characteristic wavelength dependent emission behaviour, the concentration of the substance in the plasma can be readily determined by way of a wavelength selective analysis of the light emitted from the plasma. The concentration of an individual substance can be determined here via the intensity of the emitted light in a spectral range that includes a characteristic emission wavelength of the substance. 
     Preferably, optical emission spectroscopy (OES), which is a widely used technology, is used for determining the spectra. In this spectroscopy, a light beam is spectrally decomposed using a diffraction grating and is subsequently recorded by means of a line camera or a CCD camera. The spectra determined in this way show an intensity distribution as a function of the wavelength, so that a wavelength selective analysis of the light obtained from the plasma will become possible. Of course, also other spectroscopes can be used. 
     In a preferred manner it is further possible to determine the concentration of a substance in two different ways at the same time, which means both by means of an electrical variable and via the spectral light yield. In this way, accuracy may be enhanced. Alternatively, also a measurement signal for calibrating the other signal may be used. Further, it is possible to determine the concentration of substances that are not or not completely ionised in the plasma jet by analysing the light. 
     The analysis of the light is particularly advantageous when applying the plasma jet onto a contaminated surface. 
     The object that forms the basis of the invention is further achieved by means of an apparatus having the features further described herein. 
     The advantage of providing a plasma nozzle for generating an atmospheric plasma jet as a means for ionising ionisable gases lies in the fact that no combustible gas, in particular hydrogen and/or oxygen, needs to be provided in order to operate the plasma nozzle. Air that can be taken for example from the environment of the apparatus may be used as the working gas for generating the atmospheric plasma jet. Thus, the apparatus may be used in places where the supply of a combustible gas or the supply of a working gas other than air is not possible, is complex or dangerous. The gas inlet may be disposed in such a way that the gas mixture is introduced into the plasma jet before the plasma nozzle. It is also possible to provide a tubular extension of the outlet area of the plasma nozzle, into which the gas mixture is introduced. 
     A gas inlet is therefore understood to mean, for example, a feed line for the gas mixture. Alternatively, however, the apparatus may also be arranged in an environment containing a gas mixture, for example a gas stream, in such a way that the gas mixture to be analysed reaches the apparatus as a result of pressure, diffusion, lift or in a similar way. The gas inlet is then formed by at least one open side of the apparatus, into which a flow may be directed. Therefore, the term gas inlet is to be understood in a very broad sense. 
     In a preferred embodiment of the apparatus, a voltage between 50 V and 350 V, in particular between 50 V and 250 V, preferably between 75 V and 150 V can be generated using the voltage source. It has been shown that a voltage within these voltage ranges, which is applied between the electrodes, is particularly well suited for the operation of the apparatus. 
     A DC voltage source or an AC voltage source may be provided in the apparatus as the voltage source. A DC voltage source allows a continuous detection of the concentration of the ionisable gas, an AC voltage source can be used to suppress any disturbances. 
     A particularly compact and thus space-saving apparatus is achieved by arranging the plasma nozzle between the two electrodes. By means of this arrangement, a recombination of the electrons and ions released during ionisation of the gas is prevented since the differently charged particles flow immediately in opposite directions to the electrode that is correspondingly oppositely charged. The gas inlet is preferably located here in such a way that the gas mixture is directed into the plasma jet from a direction that is not parallel, preferably substantially vertical to the plane fixed by the direction of the plasma jet and the direction of a connection line between the two electrodes. 
     In a further preferred embodiment of the apparatus, the plasma nozzle is arranged in the area of the cathode. A cathode is here understood to mean the positively charged electrode. In this way, due to their lower the more mobile electrons are immediately sucked off, so that the risk of recombination with the ions is further reduced. 
     A further preferred embodiment of the apparatus is achieved by arranging the plasma nozzle between the gas inlet and the cathode. The anode is located on the side of the cathode that faces away from the plasma nozzle. The cathode preferably has an opening through which the ions can get into the area between the cathode and the anode. In this embodiment, the plasma jet is thus located outside of the space between the cathode and the anode. Thus, the electric field between the cathode and the anode is not influenced by the plasma jet. 
     A particularly low temperature of the plasma jet and consequently a low degree of heating of the environment of the plasma jet is achieved in a further embodiment of the apparatus in that the plasma nozzle has at least two electrodes and a voltage source for generating a high-frequency high voltage, said voltage source being connected to the at least two electrodes. 
     The method and the apparatus for detecting ionisable gases may be used in an exhaust gas purification system. If such a system includes means for removing ionisable molecules, in particular organic molecules, preferably hydrocarbons, and an apparatus for detecting ionisable gases, with said means for removing ionisable molecules and the gas inlet of the apparatus being arranged in the area of an exhaust gas line, then these means for removing ionisable molecules may be controlled via the value of the concentration of ionisable gases as determined by the apparatus by means of additional control means. In an analogous way, the method and the apparatus may also be used in a process with at least one process gas stream for controlling and/or adjusting the process gas composition. 
     By means of the apparatus, the concentration of ionisable gases in the exhaust gas mixture is detected. The means for removing these gases may be controlled via the detected concentration in such a way that the concentration falls below a certain limit value. Thus, such an exhaust gas purification system can be used to control the concentration of the ionisable gases and, for example, to comply with stipulated emission limit values. This is necessary because as a rule, a complete removal with the means for removing ionisable molecules is not possible. 
     In a preferred embodiment, the apparatus according to the invention is installed upstream or downstream of the means for removing ionisable molecules. In the case of an upstream arrangement, the concentration of the ionisable gases is determined prior to their removal, so that the means can be adjusted to the concentrations without delay. This is particularly advantageous in the case of rapidly fluctuating concentrations. In a downstream arrangement, the concentration of the ionisable gases is measured after the removal thereof, i.e. the end concentration is measured. As a result, it is simpler to control the means so as to achieve certain limit values. As a rule, a removal of the ionisable molecules is not to be understood to mean a complete removal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the present invention will be explained in more detail in the description of five exemplary embodiments with reference to the attached drawings, wherein: 
         FIG. 1  shows a first exemplary embodiment of the apparatus according to the invention as well as a first exemplary embodiment of the method according to the invention, 
         FIG. 2  shows a second exemplary embodiment of the apparatus according to the invention as well as a second exemplary embodiment of the method according to the invention, 
         FIG. 3  shows a third exemplary embodiment of the apparatus according to the invention as well as a third exemplary embodiment of the method according to the invention, 
         FIG. 4  shows a fourth exemplary embodiment of the apparatus according to the invention as well as a fourth exemplary embodiment of the method according to the invention, 
         FIG. 5  shows a fifth exemplary embodiment of the apparatus according to the invention as well as a fifth exemplary embodiment of the method according to the invention, 
         FIG. 6  shows an exemplary embodiment of an apparatus for a spectroscopic gas analysis, and 
         FIG. 7  shows a diagram of an exemplary embodiment of an exhaust gas purification system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a first exemplary embodiment of the apparatus according to the invention as well as a first exemplary embodiment of the method according to the invention. The apparatus  2  comprises a plasma nozzle  4 , a gas inlet  6 , a first electrode  8  and a second electrode  10 . The first electrode  8  is disposed between the gas inlet  6  and the second electrode  10 . The plasma nozzle  4  is disposed between the first electrode  8  and the second electrode  10 . 
     The plasma nozzle  4  includes a nozzle tube  12  made from metal which conically tapers towards a nozzle tube outlet  14 . At the end opposite to the nozzle tube outlet  14 , the nozzle tube  12  includes a twist device  16  having an inlet  18  for a working gas, for example for nitrogen or air. An intermediate wall  20  of the twist device  16  includes a ring of bores  22  arranged at an angle in the circumferential direction, by which bores the working gas is twisted. Therefore, the working gas flows through the conically tapered downstream part of the nozzle tube in the shape of a vortex  24 , the core of which extends along the longitudinal axis of the nozzle tube. 
     An electrode  26  is centrally arranged on the underside of the intermediate wall  20  and coaxially protrudes into the nozzle tube in the direction of the tapered section. The electrode  26  is electrically connected to the intermediate wall  20  and the remaining parts of the twist device  16 . The twist device  16  is electrically isolated from the nozzle tube  12  by a ceramic tube  28 . A high-frequency high voltage is applied to the electrode  26  via the twist device  16 , which voltage is generated by a transformer  30 . The inlet  18  is connected to a pressurised working gas source having a variable throughput via a hose (not shown). The nozzle tube  12  is grounded. 
     The applied voltage generates a high-frequency discharge in the form of an arc  32  between the electrode  26  and the nozzle tube  12 . The nozzle tube  12  thus constitutes the second electrode. An arc discharge is understood here to be such an arc. The term “arc” or the term “arc discharge” as synonymously used in this document is used here as a phenomenological description of the discharge, since the discharge occurs in the form of an arc. 
     Due to the twisted flow of the working gas this arc, however, is channelled in the vortex core along the axis of the nozzle tube  12 , so that it does not branch out to the wall of the nozzle tube  12  until it reaches the area of the nozzle tube outlet  14 . The working gas that rotates with a high flow speed in the area of the vortex core and thus in the immediate vicinity of the arc  32 , comes into intimate contact with the arc and is therefore partially transferred into the plasma state, so that an atmospheric plasma jet  34  exits through the nozzle tube outlet  14 , through the outlet area  35  and through the outlet opening  36  from the plasma nozzle  4 . Therefore, the outlet area  35  may optionally include an isolating ceramic tube  37 . 
     Alternatively, also a controlled DC voltage source may be provided instead of the transformer  30 . This preferably includes a current controller or a current limiter. On account of the applied DC voltage, a brief discharge in the form of a spark or a permanent discharge in the form of an arc is then generated between the electrode  26  and the nozzle tube  12 . 
     The first electrode  8  has an opening  38 . A gas mixture  40  entering through the gas inlet  6  flows through this opening  38  into the area between the first electrode  8  and the second electrode  10 . The gas mixture  40  interacts there with the plasma jet  34 . As a result, the molecules of the ionisable gas are split into ions and electrons. In the case of hydrocarbons, for example CHO +  ions are generated. 
     A voltage source  42  applies a voltage of for example 100 V between the first electrode  8  and the second electrode  10 . This voltage source  42  may be a DC voltage source, with the first electrode  8  being connected to the positive pole. The first electrode  8  thus constitutes the cathode. The second electrode  10  connected to the negative pole correspondingly constitutes the anode. Alternatively, the voltage source  42  may also be an AC voltage source. As a result of the electric field between the first electrode  8  and the second electrode  10 , the free electrons  44  are then attracted by the first electrode  8  and the positively charged ions  46  by the second electrode  10 . The electric current flowing through is measured by a current measurement device  48  connected in series to the voltage source  42 . The measured current value is a measure of the concentration of the ionisable gas in the gas mixture  40 . 
       FIG. 2  shows a second exemplary embodiment of the apparatus according to the invention and a second exemplary embodiment of the method according to the invention. This exemplary embodiment differs from the previous one in that a module  49  is connected between the first electrode  8  and the second electrode  10 . The module  49  forms here, together with the electrodes  8 ,  10 , an electric resonance circuit. For example, the module  49  includes a resistor and an inductance. The electrode arrangement of the electrodes  8 ,  10  essentially constitutes a capacitance. Also an additional capacitance may be provided in the module  49 . The module  49  may further include a voltage source by which the resonance circuit may be excited. Further, means are provided in the module in order to determine the resonance frequency of the resonance circuit. As a result of the ions  46  or the electrons  44 , the capacitance of the electrode arrangement and thus the resonance frequency of the resonance circuit change. This is therefore a measure of the ions  46  or the electrons  44 . 
       FIG. 3  shows a third exemplary embodiment of the apparatus according to the invention and a third exemplary embodiment of the method according to the invention. The apparatus  50  comprises a plasma nozzle  52 , a gas inlet  54 , a first electrode  56  and a second electrode  58 . The plasma nozzle  52  differs from the plasma nozzle  4  shown in  FIG. 1  in that the plasma nozzle  52  has an extended outlet region  60 . The gas inlet  54  is located in the outlet region  60 , so that the gas mixture flowing in through the gas inlet  54  will reach the plasma jet  34  whilst still in the outlet region  60 . 
     The ionisable gas contained in the gas mixture is ionised in the plasma jet  34 . As soon as the ionised gas reaches the area between the first electrode  56  and the second electrode  58 , the free electrons  44  generated during ionisation move towards the first electrode  56  and the positively charged ions  46  move towards the second electrode  58 . By introducing the gas mixture into the outlet region  60  any non-ionised part of the ionisable gas is prevented from reaching the area between the two electrodes and from influencing there, for example, the free electrons  44  or the ions  46 . 
     Of course, also a module as in the second exemplary embodiment may be arranged between the electrodes  56 ,  58  as an alternative to the voltage source  42  shown and the current measurement device  38 . 
       FIG. 4  shows a fourth exemplary embodiment of the apparatus according to the invention and a fourth exemplary embodiment of the method according to the invention. The apparatus  70  includes a plasma nozzle  4  as shown in  FIG. 1 , a first electrode  56  and a second electrode  58 . In the apparatus  70 , the gas inlet is formed as a bottom opening  72  between the two electrodes. The apparatus  70  is positioned above a surface  74  that has contaminations  76 . These contaminations may for example be organic contaminations, in particular hydrocarbons. 
     The plasma jet  34  exiting from the plasma nozzle  4  extends through the area between the two electrodes and impinges onto the surface  74 . As a result of the energy provided in the plasma jet  34 , the contamination  76  is gradually released from the surface  74 . The ionisable molecules released from the surface  74  will then mix with the ambient gas to form a gas mixture which reaches the apparatus  70  through the bottom opening  72 . The molecules are ionised in the plasma jet  34 . The free electrons  44  generated thereby move towards the first electrode  56  and the positively charged ions  46  move towards the second electrode  58 . 
     The plasma jet  34  may be applied to the surface  74  for example until the electric current measured by the current measurement device  48  falls below a specified value. In this way it is achieved that the application of the plasma jet onto the surface  74  will not cease until a certain part of the contamination  76  has been removed. 
       FIG. 5  shows a fifth exemplary embodiment of the apparatus according to the invention and a fifth exemplary embodiment of the method according to the invention. This exemplary embodiment differs from the fourth exemplary embodiment in that an electrode  78  with an opening  80  is provided, through which the plasma jet  34  flows. Further, a module  82  is electrically connected to the electrode  78  and to the surface  74 . The surface  74  thus constitutes the second electrode. The surface is therefore preferably electrically conductive. The module  82  can include for example a voltage source and a voltage measurement device. Alternatively, the module  82  may form a resonance circuit with the electrode arrangement of the electrode  78  and the surface  74 . If a voltage that is negative relative to the surface  74  is applied to the electrode  78 , the ions  84  generated during ionisation by the plasma jet  34  are moved towards the electrode  74 . The electrons (not shown) flow off over the surface  74 . In the case of the opposite polarity, the opposite correspondingly applies. 
       FIG. 6  shows an exemplary embodiment of an apparatus for spectroscopic gas analysis. The plasma nozzle  85  differs from the plasma nozzle  2  shown in  FIG. 1  in that, in addition, optical detection means  87  fixed to a bracket  86  are provided, which detection means are orientated towards the area in which the gas mixture  40  is made to interact with the plasma jet  34 . The light emitted by the excited substances from this area is at least partially detected by the detection means  87  and is directed to a spectrometer  89  via a light conductor  88 . In the spectrometer  89 , the intensity is then measured in at least one spectral range, and by means of this measured value, the concentration of at least one substance in the gas mixture  40  is determined. In order to improve the detection of light, the detection means  87  may optionally include collection optics. The spectrometer  89  is preferably formed as a spectrometer for optical emission spectroscopy. Also an energy-dispersive detector may be used as the detection means  87 , so that a separate spectrometer may be dispensed with. The detection means  87  may further be arranged to be separate from the plasma nozzle  85 . 
     Of course it is possible to combine the features of the previous exemplary embodiments with each other. Thus, the gas mixture  40  ionised or excited by the plasma jet  34  may for example advantageously be analysed at the same time via provided electrodes as shown in  FIGS. 1 to 5  and spectroscopically as shown in  FIG. 6 . 
       FIG. 7  shows a diagram of an exemplary embodiment of an exhaust gas purification system. The exhaust gas purification system  90  has an exhaust gas duct  92  that carries an exhaust gas  94 . The exhaust gas  94  may for example be an exhaust gas mix from a combustion process, which contains an ionisable gas. The exhaust gas  94  first reaches the means  96  for removing ionisable molecules. The means  96  may for example be an adjustable catalyst. From there, the exhaust gas  94  flows to an apparatus  98  according to the invention for detecting ionisable gases, where the concentration of the ionisable gas in the exhaust gas  94  is detected. 
     The detected concentration of the ionisable gas is fed into a control unit  100  which generates a control signal for controlling the means  96 . The control unit is designed here in such a way that the removal efficiency of the means  96  is enhanced in case the apparatus  98  detects an excessively high concentration of the ionisable gas. This ensures that after flowing through the exhaust gas purification device  90 , the concentration of the ionisable gas in the exhaust gas  94  will be below a specified limit value. 
     Alternatively, it is also possible to arrange the apparatus  98  upstream of the means  96 . 
     Instead of an exhaust gas purification device, the diagram shown in  FIG. 6  may also be related to a process gas stream. Here, a process gas duct is provided instead of the exhaust gas duct  92  and a process gas instead of the exhaust gas  94 . The concentration of the ionisable molecules in the process gas can then be controlled in an analogous manner using the means  96 , the apparatus  98  and the controller  100 . In this way, the controlled process gas stream can be fed to the next process step.