METHOD FOR THE DETECTION OF A GAS USING PHOTOACOUSTIC SPECTROSCOPY

Abstract: In a process for the detection of a first gas in a gas mixture comprising a second gas, the absorption spectre of which interferes with the absorption spectre of the first gas, a photoacoustic measurement is carried out in the presence of a third gas which in combination with the first or the second gas exhibits kinetic cooling. During measurement the gas mixture is influenced by pulsating laser light having a constant repetition frequency where the frequency of the laser light is varied gradually. The measurement comprises at least one detection of the phase of the photoacoustic signal as a function of the laser light frequency. The invention further relates to an apparatus for carrying out the invention.

Description: Method for the detection of a gas using photoacoustic spectroscopy. The invention relates to a method for the detection of a gas using photoacoustic spectroscopy. All gases have characteristic absorption spectres disclosing the ability of the material to absorb energy as a function of the wawe length of a direct influx of energy. This absorption spectre is characteristic for each specific gas and may be considered a sort of fingerprint for the gas. Thus, a specific gas in a gas mixture may be detected by measuring the energy absorption of the gas mixture at selected wawe lengths of energy where the gas has a high absorption. A very appropriate measurement method for measuring such absorption is photoacoustic spectroscopy. According to this method the gas mixture is influenced by a pulsated energy source, e.g. a laser. The energy absorption of the gas mixture will cause a pressure increase and a pressure decrease, respectively, proportionally to the ab¬ sorption, the gas being heated during absorption of the energy and being cooled again when releasing the energy absorbed to the surroundings. The absorption may thus be recorded using a pressure transducer wherein the pressure is proportional to the absorption. However, this measurement method gives rise to problems e.g. in case of interference between the absorption spectres of two gases. In that event it is very difficult to distinguish the absorption of each of the specific gases from each other. This problem is known in particular from measurements carried out on atmospheric gas mixtures using a C0 2 laser. Herein the large amount of CO in the gas mixture in combination with the particularly intensive absorption ability of C0 2 at the wawe lengths of radiation which may be generated with a C0 2 laser to a particularly high degree obstructs the detection of less dominant gases. As disclosed in the article "Selectivity in Optoacoustic Trace Gas Monitoring with Waweguide C0 2 lasers": 11th International Conference on Infrared and Milimeter Wawes, Tirreria, Pisa, 24-24 October 1986, the selectivity as regards different gases may be increased by letting the laser scan a wawe length area around a centre frequency instead of measuring the photoacoustic amplitude of some of the fixed wawe lenghts of the laser. The laser is pulsated in a conventional manner where the radiation emitted within each pulsa¬ tion has a constant wawe length, but the wawe length is changed in the course of several pulsations. Combination of this scanning with a reduced pressure in the measurement chamber, thus concentrating the absorption to a narrowed frequency range, has resulted in a significantly increased selectivity. As to measurement in e.g atmospheric environments, however, this method is still far from satisfactory, the C0 2 content of the gas mixture still drowning the photoacoustic signals from less dominant gases. It is the object of the invention to provide a method of the kind disclosed in the preamble of claim 1 wherein, however, the above inconveniences are eliminated. This is achieved with the subject matter of the characterizing part of claim 1. Thus, the phenomenon "kinetic cooling" is used to phase shift a partial absorption contribution in the gas mixture thereby permitting detection of the presence of a gas in substantially lower concentrations than it has been possible so far by regarding the phase course as a function of the frequency. Kinetic cooling is subject to more detailed discussion in the article: "F.G. Gebhardt & D.C. Smith Kinetic Cooling of a Gas by Absorption of CO- Laser Radiation" Appl. Phys. Lett. 20, 129, 1972, but in summary it comprises the following: Generally the laser light will excite a part of the molecules which have an absorption frequ¬ ency close to that of the laser frequency when the light reaches the gas in the measuring cell. These molecules are excited to a higher energy state when they collide with other molecules. This higher energy level is unstable and the energy will be released to the surroundings in the form of heat. This heat generates pressure changes which may be recorded by a pressure transducer due to the pulsation of the laser. For certain combinations of molecules it applies that their high energy levels are close to each other and therefore they are in resonance. Here an excited molecule may lose an energy amount to another molecule which corresponds to twice the amount originally received from the photon. Thereby, the first molecule is in an unstable state, wherein it has an energy deficit as compared to the stable state. The molecule compensates for this through the absorption of energy from the surroundings. Therefore, in this case a subsequent cooling of short duration of the system is recorded when energy is supplied thereto. By detecting the amplitude of the absorption, too, the method may also be used to carry out improved amplitude measurement by photo¬ acoustic spectroscopy since the phase signal may also be used for the detection in the amplitude signal of such information which is overridden by other information having higher amplitude. An appropriate special embodiment of the method may be used for the measurement of atmospheric gas mixtures resulting from e.g. co bus- tion processes or from particularly exposed environments where it is desirable to detect polluting gas. In particular it is the absorp¬ tion into C0 2 which interferes with the measurement and it is therefore desirable to eliminate part of this contribution. C0 2 exhibits distinct kinetic cooling in connection with N 2 , and N 2 being present already in atmospheric gas mixtures in high concentrations, the C0 2 contribution may be eliminated to a large extent simply by detecting the phase course of the photoacoustic signal as a function of the frequency. As stated in the characterizing part of claim 7 the effect may also be used without the laser emitting pulsations of varying frequen¬ cies, the only condition being that both amplitude and phase masurements of the photoacoustic signal are carried out prior to and following the addition of the gas which triggers the kinetic cool- ing. In the following the invention will be described with reference to the accompanying drawings, wherein: Figure 1 is a schematic view of the apparatus for carrying out the method according to the invention, Figure 2 shows the summation of the signals in vector form, Figures 3 and 4 show absorption spectres for gas mixtures having trace elements as measured according to the method of the invention, Figures 5, 6 and 7 show absorption spectres for a trace element which is present in an increasing concentration of interfering gases. Figure 1 illustrates the optoacoustic gas detection system compri¬ sing a laser 10 which, according to a preferred embodiment, is a C0 2 laser due to the high output of such a laser. The laser 10 may be set to several frequencies which will be referred to as center frequencies in the following. Furthermore, the laser 10 may be tuned within a range of 250 MHz to either side of the center frequency. Passage through the frequency spectre in this way is called a window for the spectre. The C0 2 laser operates pulsatedly in the infrared area of the spectre. The frequency is changed gradually from one series of pulsations to the next series of pulsations. The laser 10 lights up a cell 20 containing a sample of the gas to be scanned for trace elements. According to the preferred embodiment the cell 20 is constructed as an acoustic resonator in which a microphone 30 is mounted. The light enters through a window 21 in the measuring cell 20, where the window 21 absorbs a part of the light. The gas within the cell will absorb another part of the light, which is converted into heat and resulting pressure changes. The laser 10 being pulsated at a repetition frequency of about 700 Hz causes an emission of sound which is recorded by the microphone. The microphone 30 and a light detector 40 transforms the sound and the light, respectively, to electric signals. The microphone 30 receives a signal which is a measure for the amount of absorbed light whereas the detector 40 receives a signal which is consider¬ ably stronger, being the original laser pulsation minus the absorb¬ ing contribution. in a lock-in amplifier 60 one part of the signal recorded with the microphone 30 is multiplied with a periodical signal having the same period as the microphone signal. This may e.g. be a sinosoidal signal or a square-wawe signal. Simultaneously the other part is multiplied with a periodical signal which differs from the first periodical signal only in the phase being displaced by 90". Then the two product signals are integrated over a period and thus two figures are obtained which are said to be the real part and the imaginary part of a complex number. This nummber expresses the size and the phase of the absorption signal. A trigger 70 partly controls the repetition frequency of the laser 10, partly syncronizes the periodical signals in the lock-in ampli¬ fier 60. Scanning may advantageously be carried out over several frequencies using the laser 10 thus permitting the generation of an absorption spectre containing both amplitude and phase. The data may then be printed out by a plotter or be displayed on a monitor (not shown) or the like. Implementation of a catalogue of the absorption spectres of the various gases permits quantitative determination of the gases constituting a given sample. In case of e.g. a sample comprising atmospheric air and some polluting trace elements the fact that, contrary to most other gases, e.g. C0-, and N 2 jointly exhibit kinetic cooling may be exploited. Thereby it is possible to exploit the fact that the electric signal recorded by the microphone is ideally 180° out of phase for gases exhibiting kinetic cooling as compared to gases which do not. If e.g. atmospheric air constitutes a substantial part of the sample a background signal apart from that of CO,, and N 2 may be recorded which is coherent from the absorption of the windows 21, 22, and moreover a signal from the H 2 0 contained in the sample may be detected which signal has long tails, i.e. the signal interferes far into the spectre and acts as noise far away from the H-,0 absorption frequency. Consequently, when detection of trace elements is carried out on the background of C0 2 and N 2 the background signal does not have a phase shift of 180° but typically only one of about 120°. Therefore, a phase shift is detectable prior to the occurrence of a change of amplitude which is particularly advantageous when detection of trace elements is carried out. Other parameters, e.g. the pressure, may influence the actual phase shifts, but the crucial aspect of the invention is that the absorp¬ tion contributions are phase shifted to a substantial extent thus causing the resulting phase course to indicate clearly the presence of the gas to be detected. Figure 2 shows a summation of a strong signal A (which may be the result of the presence of C0 2 + 2 ) and a weak signal B (e.g. S0 2 ). No change in the amplitude of the total signal C is detectable whereas a clear change of several degrees in the phase may be detected. Figure 3 shows a window of the absorption spectre for a gas mixture (C0 2 , 2 and S0 2 ) shown by the apparatus used in the present inven¬ tion. It appears tha the amplitude curves are similar to a Gaussian curve and there are no signs of the presence of other gases having absorption lines in that part of the spectre. Observation of the phase clearly indicates the presence of a trace element (S0 2 ) in the gas mixture. Moreover, it appears that the peak occurs at a frequency which is about 110 MHz lower than the centre frequency of the window. Figure 4 shows that it is also possible to employ the method where the trace element (C0 2 ) exhibits kinetic cooling in a gas mixture (NH 3 and N 2 ), wherein the amplitude spectre has a peak (NH,) for a frequency which is 200 MHz lower than the center frequency of the window and otherwise decreases with increasing frequencies. The phase indicates a clear peak (C0 2 ) at the center frequency. The method permits the detection of the frequencies of the absorp¬ tion lines for determination of the gases. Moreover, the phase shift is shown as a measure of the mutual ratio between the phases. A complex catalogue of the absorption spectre of the gases present permits determination of the quantitative gas content of a gas mixture from a series of windows measured in the absorption spectre. Figures 5, 6, and 7 illustrate how the absorption of S0 2 as a trace element by C0 2 is drowned by C0 2 absorption with the increase of the C0 2 concentration in the gas mixture. In Figure 5 there is about 1% S0 2 in 2 with 500 ppm S0 2 (pp = molecules per million). The amplitude clearly indicates the presence of two lines. The phase shift is also strong. In Figure 6 there is 2% C0 2 in N 2 with 500 ppm S0 2 . So far there are no problems associated with distinguishing the presence of two elements and the position of the adsorption lines. In Figure 7 there is 5% C0 2 in N 2 with 500 ppm S0 2 - Here it is impossible to distinguish the presence of S0 2 from the amplitude. The phase information still indicates the presence of a gas apart from C0 2 and also the position of the absorption line of the second gas.

Claims: C 1 a i s 1. A method for the detection of a first gas in a gas mixture using photoacoustic spectroscopy, wherein the gas mixture further 5 comprises a second gas the absorption spectre of which interferes with the absorption spectre of the first gas, wherein the gas mixture is influenced by pulsating laser light having constant pulsation frequency during the measurement and wherein the frequency of the laser light is varied gradually and wherein the measurement ° comprises at least one detection of the phase of the photoacoustic signal as a function of the frequency, c h a r a c t e r i z e d in that a photoacoustic measurement is carried out in the presence of a third gas in the gas mixture, said third gas exhibiting kinetic cooling in combination with the first or the second gas. 5 2. A method according to claim 1, c h a r a c t e r i z e d in that, during the measurement, the gas mixture is provided in a measurement chamber where the pressure is reduced as compared to the atmospheric pressure. 0 3. A method according to claims 1 or 2, c h a r a c t e r i z e d in that, during measurement, the gas mixture is provided in a measurement chamber which is constructed as an acoustic resonator and in that the resonance frequency of the measurement chamber is 5 substantially identical with the pulsation frequency of the laser light. 4. A method according to claims 1-3, c h a r a c t e r i z e d in that the third gas is added to the gas mixture. 0 5. A method according to claims 1-3, c h a r a c t e r i z e d in that the third gas is present in the gas mixture. 6. A method according to claims 1-5, c h a r a c t e r i z e d in 5 that the measurement also comprises the detection of the amplitude of the photoacoustic signal as a function of the frequency. 7. A method for the detection of a first gas in a gas mixture using photoacoustic spectroscopy, wherein the gas mixture further comprises a second gas the absorption spectre of which interferes with the absorption spectre of the first gas, c h a r a c t e r ¬ i z e d in that the absorption of the gas mixture is measured both as to amplitude and phase at a minimum of one wawe length at which the first gas exhibits absorption, that the third gas is added to the gas mixture which third gas coacts with either the first or the second gas in such a way that they exhibit kinetic cooling, that the absorption of the gas mixture is measured again at the same wawe length(s), and that the minimum of two amplitude and phase measure- ments are combined to determine the absorption amplitude of the first gas. 8. An apparatus for carrying out the method according to claim 1 comprising in combination a laser, means for pulsation of the laser light, a measuring cell exhibiting acoustic resonance at a frequency which is substantially identical with the repetition frequency of the laser light, a detector for the detection of the radiation passing through the measuring cell, a detector for the detection of an acoustic signal from the measuring cell, and an electronic measurement circuit for processing the signal from the detectors, c h a r a c t e r i z e d in that the frequency of the laser is settable within certain frequency intervals, and in that the elect¬ ronic circuit comprises phase detection means constructed to record the phase course of the acoustic signal as a function of the frequ- ency within a frequency interval.

Inventors: HAMMERICH MADS, HENNINGSEN JES O, OLAFSSON ARI, HAMMERICH, MADS, HENNINGSEN, JES, O., OLAFSSON, ARI
Applicants: FLS AIRLOQ AS
Application year: 1989
Main classification: G01N21
IPC codes: G01N21, G01N29