Background technology
CO, N
2O, SO
2, NO, NO
2, CH
4, CH
3, H
2S, halogenide and organic compound or the like all belong to trace gas, some is natural generation in them, but be the various trace grain kinds that mankind's activity produces to a great extent, these trace grain kinds are subjected to physics, chemistry with the effect of biology and participate in the geochemistry circulation, to the global atmosphere environment and ecological produce significant impact, for example chemical fumes, acid rain, greenhouse effect and ozonosphere go to pot etc. relevant with aforesaid trace gas invariably.
In the trace gas that is not limited to exemplify previously, SO
2Being most important a kind of sulfurous pollutants in the air, is a kind of pungent and for asphyxiant colourless gas, fusing point is-72 ℃, and the solubleness in water is 8.5%(25 ℃), SO
2Mix the back with water vapor metal is had very strong corrosivity, and human health is had great harmfulness.
Along with the fast development of progress of science and technology and industry, human also being accompanied by when the creation material wealth is used the enhancement civilization enters a large amount of toxic and harmfuls in atmosphere.Known reason, the SO in the atmosphere
2Mainly by burning such as fuel such as coal, oil, rock gas and produce in the production run of chemical products, normal concentration is 0.57~5.7ug/m
3Again, SO
2Be one of root that produces photo-chemical smog and acid rain, acid rain has extremely strong corrosivity to buildings and soil. and biological kind and quantity are all subtracted, the existence of the mankind and other biological is had serious harmfulness, for example suck SO for a long time
2Slow poisoning can take place, and breathing problem is increased the weight of, and liver, kidney, heart are all had harm.SO
2The catalysis of iron oxide and very easily be oxidized to SO in solar radiation and some metallic dust such as industrial fumes in atmosphere
3, combine the formation sulfuric acid mist with airborne water vapor, metallic article, buildings, soil and rivers and lakes are produced acidization.
China's environmental quality standards regulation, the SO in the atmosphere of residential district
2Daily mean of concentration should be lower than 0.15mg/m
3, mean annual concentration is lower than 0.06 mg/m
3In the monitoring index of various countries to air quality, all SO
2Be considered as one of emphasis monitoring target.
At SO
2Detection, the Environmental Protection in China standard has mainly defined two kinds of detection methods, a kind of is formaldehyde absorption-Pararosaniline spectrophotometric method, another kind is a tetrachloro mercury salt absorption-Pararosaniline spectrophotometric method.In addition adopt electrical conductivity method and infrared detection method and Ultraluminescence detection method in addition.
Aforesaid tetrachloro mercury salt acid absorption-Pararosaniline spectrophotometric method is for example: SO
2After the absorption of tetrachloro mercury potassium solution, generate stable sulphite complex compound, again with formaldehyde and the effect of hydrochloric acid Pararosaniline, generate the aubergine complex compound, measure absorbance log at the 575nm place.This method is to measure SO in the air both at home and abroad
2The most frequently used method, its advantage be highly sensitive, selectivity good and both applicable to short time sampling, can be suitable for long-time sampling again.Shortcoming is that absorption liquid toxicity is big; Need to rely on the interpolation chemical substance.
Aforesaid formaldehyde absorption-Pararosaniline spectrophotometric method is for example: SO
2After the absorption of buffered formaldehyde solution, generate stable methylol sulfonic acid additive compound, adding NaOH in sample solution decomposes additive compound, sulphuric dioxide that discharges and Pararosaniline, generate the aubergine compound through the formaldehyde effect, measure absorbance at wavelength 577nm place with spectrophotometer.The advantage of this method is to avoid the toxicity of mercury salt, and the absorptivity of this formaldehyde absorption process is compared with tetrachloro mercury potassium absorption process and significantly improved, and stable.Sensitivity and accuracy are suitable with tetrachloro mercury potassium absorption process, and still, operating conditions comparatively harshness and color operation requires comparatively strict and need to rely on to add chemical substance.
Aforesaid electrical conductivity method is by means of the sulfur dioxide concentration measuring instrument, and principle of work is to utilize solution when temperature constant, has and the corresponding conductivity of its concentration.When reacting when this kind solution absorption gas or with gas, its conductivity changes, and obtains gas concentration thereby measure conductivity.The electrode of this kind instrument needs to demarcate with the sulphuric dioxide calibrating gas before dispatching from the factory, but can not a mark ease forever, the conductivity of every conductivity when absorption liquid and timing signal absorption liquid differs than greatly the time, must demarcate again, and sampling pipe must heat use.So the method is not suitable for using in the long-time monitoring of atmospheric environment.
Aforesaid infrared detection method is to utilize sulphuric dioxide to carry out measurement of concetration in region of ultra-red (7.3 μ m) light absorption all around, when the infrared light of a branch of constant 6.82~9 μ m when containing the medium of sulfur dioxide gas, absorbed by sulphuric dioxide, luminous flux is attenuated, measure the fast luminous energy of decay, can obtain the concentration of sulphuric dioxide.It is comparatively general that this class methods are used, but must pay attention to influencing the disturbing factor of measuring accuracy when measuring, and wherein, carbon dioxide in the mixed gas and water vapor can have very big influence to accuracy of detection.These class methods must be in conjunction with heat tracing, multistage dedusting and multistage dehumidifying, and the sample gas fed distance can not be oversize, otherwise influences measuring accuracy.The more important thing is that infrared detection sulphuric dioxide method can't detect the sulfur dioxide gas that is low to moderate ppb concentration.
The principle that sulphuric dioxide is measured in aforesaid Ultraluminescence is after the ultraviolet lighting of 220nm is mapped to sulfur dioxide gas, the energy of sulfur dioxide molecule absorption ultraviolet light is stimulated and sends fluorescence when high level returns ground state, and the size of fluorescence intensity reflects the concentration of sulphuric dioxide.The technology key point of this class detection method is how to guarantee that the ultraviolet light intensity of 220nm is constant.In addition, prevent that micronic dust from stopping up and weighing apparatus is released than changing.
In view of above-mentioned prior art, be necessary to be explored even more ideal to SO
2The method that concentration detects, for this reason, the applicant has done a large amount of and useful research, and technical scheme described below produces under this background.
Summary of the invention
Task of the present invention is to provide a kind of and need not to add any chemical substance, need not to carry out secondary or repeatedly calibrate, help avoid sensitivity that water vapor exerts an influence to accuracy of detection and the photocaustic spectroscopy that utilizes of accuracy of detection height and excellent in stability detects the method for sulfur dioxide concentration.
Task of the present invention is finished like this, a kind of method of utilizing photocaustic spectroscopy to detect sulfur dioxide concentration, this method is launched tunable light source by quantum cascade laser and is given chopper, obtain the spectrum of the adjustable system of a beam intensity by chopper, on the sulfur dioxide gas of spectral illumination in the photoacoustic cell, make sulphuric dioxide absorb luminous energy and discharge the energy of absorption by intermolecular collision in the mode of heat, this energy makes the sulfur dioxide gas molecule by radiationless transition described luminous energy is converted into heat energy, make the sulfur dioxide gas temperature variation and cause pressure surge and produce sound wave by heat energy, this sound wave is by being configured in the outer microphone detection of photoacoustic cell and by change-over circuit described Pressure Fluctuation Signal being amplified, the photoacoustic signal that obtains simulating, the photoacoustic signal of simulation converts digital electric signal to by peripheral analog to digital conversion circuit, and by lock-in amplifier digital electric signal is amplified and to send into the data acquisition circuit image data, the data of collection calculate the sulfur dioxide concentration value through formula.
In a specific embodiment of the present invention, it is that the wavelength that quantum cascade laser cooperates chopper to send is the middle infrared spectrum of 8-12 μ m that described quantum cascade laser is launched tunable light source.
In another specific embodiment of the present invention, described photoacoustic cell is the resonant mode photoacoustic cell.
In another specific embodiment of the present invention, described microphone is the electret capacitor type microphone.
In another specific embodiment of the present invention, the microphonic electric capacity of described electret capacitor type is that 10PF, signal to noise ratio (S/N ratio) are that 56dB and sensitivity are-42dB.
Also have in the specific embodiment of the present invention, the luminous energy modulation range of the spectrum that described intensity can be modulated is 15.9x10
-19To 23.9x10
-19Electron volts.
More of the present invention and in specific embodiment, the modulating frequency of described light source is 2.4x10
15To 3.6x10
15Hz.
Technical scheme provided by the invention has with respect to prior art need not be used any chemicals and use and embody safety, the feature of environmental protection; After the adjustment of demarcating, need not to set up once more and use and embody the convenience of using; Because water does not have absorption peak on light source, thereby is not subjected to steam-laden the influence; Have the accuracy of detection height, sensitivity is good and the strong point of excellent in stability.
Embodiment
Embodiment:
Ask for an interview Fig. 2 and Fig. 4, monochromatic light with the adjustable system of a beam intensity shines on the sample that is sealed in the photoacoustic cell, absorption of sample luminous energy, and to discharge the mode de excitation of heat energy, the heat energy that discharges makes sample molecule produce periodically heating by the modulating frequency of light, thereby causes medium to produce the periodic pressure fluctuation, and this pressure surge can detect with sensitive microphone, and obtain photoacoustic signal, optoacoustic effect that Here it is by amplification.Utilize optoacoustic spectroscopy sensor sulfur dioxide concentration just to be based on the optoacoustic effect of sulphuric dioxide, this optoacoustic spectroscopy sensor such as Fig. 2 and shown in Figure 4 comprise: quantum cascade laser 1, chopper 2, photoacoustic cell 3, microphone 4 and peripheral circuit.Use amount qc laser 1 transmitting illuminant, and the monochromatic light that cooperates chopper 2 to obtain the adjustable system of a beam intensity shines on the sulfur dioxide gas that is sealed in the photoacoustic cell 3, sulphuric dioxide absorbs luminous energy, and release heat energy, the heat energy that discharges makes sulfur dioxide molecule produce periodically heating by the modulating frequency of light, thereby produce the periodic pressure fluctuation, this pressure surge can be surveyed with sensitive microphone 4, and obtain photoacoustic signal by the change-over circuit amplifying signal, photoacoustic signal converts electric signal to by peripheral circuit, again by sending into data acquisition circuit behind the phase-locked amplifying signal in the peripheral circuit, calculate by formula at last and finish the sulfur dioxide concentration quantitative test, with this detection of finishing sulfur dioxide concentration, Here it is, and optoacoustic spectroscopy is surveyed the sulphuric dioxide principle.
For light source, the applicant uses the quantum cascade laser 1 of 8-12 micron, light intensity is very high, at room temperature just can reach very high absorption coefficient, cooperate chopper 2 to obtain 8.7 microns spectrum, sulfur dioxide gas absorbs the absorption line that does not have water vapor on the spectral coverage at this to be disturbed, and sulfur dioxide gas to be measured needn't purify through aqueous vapor.Quantum cascade laser and traditional pn pn junction p n diode laser are completely different, the emission wavelength of traditional semiconductor diode laser is by the energy gap decision of semiconductor material, quantum cascade laser is an one pole type laser instrument, it has only electronics to participate in, transitting to ground state by the sub-energy level electron resonance of quantum well conduction band excited state releases energy, ballistic phonon also is tunneling to next stage, the one-level one-level hands on, up to launching laser, its emission wavelength is by the decision of the energy difference between two excited state of quantum limitation effect decision.It is big that it possesses output power, but working and room temperature, wavelength tuning range is wide, and the tuning precision advantages of higher is so selected amount qc laser 1 cooperates 8.7 micron wavebands as light source.
For photoacoustic cell 3, we adopt the photoacoustic cell 3 of resonant mode.The principle of resonant mode photoacoustic cell is that sound wave transmits in cavity, and sound wave resonance in chamber forms standing wave, need not sealed chamber, and plays the effect that resonance is amplified.The eigenfrequency that itself and sound wave is propagated by the modulated light source irradiation frequency in chamber overlaps formation resonance, the amplification of photoacoustic signal can being resonated like this.Resonant mode optical cavity chamber has characteristics such as practicality, easily making, sensitivity height.The expand border of propagation of the size that produces hot sound wave in the gas and the luminous energy of gas absorption and gas is relevant.In the optoacoustic gas detection system, gas is in the photoacoustic cell of certain design, by the structure sensitivity that can improve gas of design photoacoustic cell.For certain gas, own specific absorption spectrum is arranged, by selecting the wavelength of modulated light source, thereby make and have only certain specific gas to produce bigger absorption, just have only this gas absorption luminous energy to produce hot sound wave, thereby can judge the concentration of this kind gas by the size that detects hot sound wave, also realize the high selectivity of detection of gas simultaneously.The energy of gas absorption is relevant with gas concentration in the absorption coefficient and the intensity of light source at this wavelength place with gas, the size that produces hot sound wave is directly proportional with the heat of gas absorption, can realize high selectivity, the high sensitivity of detection system by correct selection photoacoustic cell.
For microphone 4, the applicant selects the electret capacitor type microphone, and the electret capacitor microphone has two sheet metals, and wherein a surface scribbles electret film, and another piece sheet metal is connected to the grid of field effect transistor, is connected to a diode between grid and the source electrode.The characteristics of electret diaphragm are: when diaphragm is subjected to vibration, friction, surface charge can occur on the diaphragm.If surface charge is Q, cartridge electric capacity is C, the voltage U=Q/C on the cartridge then, and electric capacity is constant, and the quantity of electric charge on the electret diaphragm is owing to the sound variations in flow changes.Acoustic pressure is big more, and electric weight is big more, and it is big more to produce voltage.The electric quantity change speed has reflected the variation speed of voltage, has also reflected the frequency of sound.The electret capacitor type microphone sensitivity that we use is-42dB, 10pF electric capacity, and signal to noise ratio (S/N ratio) is greater than 56dB, and its sensitivity is high.
Ask for an interview Fig. 1 and Fig. 3, Fig. 1 is inherent 4 dew points of test experience, and the detection curve of sulphuric dioxide and water under the situation of 3% fluctuation, this spectrogram have proved that the optoacoustic spectroscopy detection sulphuric dioxide with 8.7 microns light sources can not had influence on testing result by the aqueous vapor in the environment.
Sulphuric dioxide all has bigger absorption peak near ultraviolet l90 nm~230 nm, 200 nm~320 nm, 350 nm~390 nm and infrared 3.98 microns, 7.35 microns, 8.70 microns.And sulphuric dioxide does not have the absorption line of water vapor to disturb on 8.7 microns infrared absorption spectral coverage. sulfur dioxide gas to be measured needn't be through purifying, so the applicant selects this wave band as light source.Photoacoustic cell 3 is closed containers, in be full of the sulphuric dioxide sample, and place microphone 4.Because photoacoustic spectroscopy is the size of absorption of sample luminous energy, thereby reflected light, scattered light etc. are very little to measuring interference, absorb light intensity and very little weak absorption sample and the low concentration sample of incident intensity ratio so optoacoustic spectroscopy is suitable for measuring, be suitable for very much detecting the sulfur dioxide gas of trace.
Can provide the mathematical model that hot sound wave produces in the gas by the fluid mechanics and the law of thermodynamics.Acoustic disturbance in the gas can be used acoustic pressure p, and (r t) describes, and acoustic pressure is the poor of general pressure P and average value P 0: p=P-P0.Because of absorbing light produce hot H (r, t) and the excitation acoustical signal, this process can be described as:
Wherein: r is the gas coefficient of heat insulation, and Co is the velocity of propagation of sound wave in the gas.The boundary condition of equation is by light path decision in optoacoustic chamber and the optoacoustic chamber.The applicant adopts the vertical acoustic resonance of one dimension chamber: if the sectional dimension of resonant cavity is much smaller than wave length of sound, the acoustic signals that then is excited only changes along the resonant cavity length direction, this cavity can be considered to the one dimension resonant cavity can be by shown in Figure 3, two ends all the cavity of opening or sealing to require length be the integral multiple of half-wavelength, resonant frequency is:
The cavity that one end opening, one end is remained silent, requiring length is the odd-multiple of 1/4 wavelength, resonant frequency is:
Wherein: △ l is the open end correction, generally approximate △ l=6r, and r is a section radius, is zero to blind end △ l.The cavity of both ends open or sealing, the standing wave mode difference: the cavity hatch place of both ends open is a node, the cavity end of closed at both ends is an antinode.
It is as follows that the vertically harmless optoacoustic chamber of one dimension acoustic equation can be write an accepted way of doing sth, and wherein p is a gas density, and S is a sectional area in the optoacoustic chamber, and f is the velocity of sound:
Its in form with four-pole network in the equation (2) that satisfies of the relation of voltage and current close, set up mathematical model
Thereby by LC oscillatory circuit simulation sonic transmissions.
In actual applications, as shown in Figure 4, sulphuric dioxide optoacoustic spectroscopy pick-up unit is mainly by photoacoustic cell 3, microphone 4, chopper 2 is with quantum cascade laser 1, prime amplifier 5, lock-in amplifier 6, motor 7, digital to analog converter 8 and DSP data transmitter 9 with power supply 11 and USB circuit 12 and detect controller 10 compositions.The draft tube in optoacoustic chamber links to each other with the valve of gas circuit controller respectively with escape pipe, and the gas circuit controller is controlled by control circuit board.Quantum cascade laser 1 and chopper 2(Fig. 2 show) the composition light path, motor 7 control choppers 2 move to the light inlet in optoacoustic chamber, motor 7 links to each other with detection controller 10, photoacoustic cell 3 outer walls link to each other with microphone 4, microphone 4 lower ends link to each other with prime amplifier 5, prime amplifier 5 cooperates lock-in amplifier 6 to be connected with digital to analog converter 8 and with quantum cascade laser 1, and digital to analog converter 8 links to each other with DSP data transmitter 9 electricity.
Cooperating chopper 2 to launch adjustable 8-12(present embodiment by quantum cascade laser 1 is 8.7 μ m) a micron quanta cascade LASER Light Source is middle infrared spectrum, shine on the chopper 2, make chopper 2 obtain the spectrum of the adjustable system of a beam intensity, chopper 2 is by motor 7 mechanical modulation, making accurately, 8.7 microns LASER Light Source incides in the resonant mode photoacoustic cell 3, owing to be flooded with sulfur dioxide gas to be measured in the photoacoustic cell 3, therefore make sulphuric dioxide absorb luminous energy and discharge the energy of absorption by intermolecular collision in the mode of heat, make the sulfur dioxide gas molecule luminous energy is converted into heat energy by the energy that discharges again, make the sulfur dioxide gas local temperature change and cause pressure surge by heat energy by radiationless transition., when luminous energy was modulated, the heat energy of generation was periodic, therefore produces modulated pressure surge, and sound wave occurred.This pressure surge is that to be 56dB and sensitivity detect for the microphone 4 of-42dB for 10PF, signal to noise ratio (S/N ratio) by being configured in outer electret capacitor type of photoacoustic cell 3 and electric capacity, and Pressure Fluctuation Signal is amplified by change-over circuit, obtain photoacoustic signal, photoacoustic signal is a simulating signal, convert digital electric signal to by peripheral analog to digital conversion circuit, and digital electric signal is amplified and send into the data acquisition circuit image data by lock-in amplifier 6.Wherein, the concentration of photoacoustic signal and sulfur dioxide gas is proportional.The data of being gathered are after formula calculates the sulfur dioxide concentration value.In this flow process, detect controller 10 and be used to control chopper 2 drivings, phase-locked amplifying circuit, data acquisition circuit, microphone 4 and DSP circuit 9.In the present embodiment, the setting range of the light source luminous energy that aforesaid intensity can be modulated is 15.9x10
-19To 23.9x10
-19Electron volts (eV), the modulating frequency of aforesaid tunable light source are 2.4x10
15To 3.6x10
15Hz.
In sum, technical scheme provided by the invention has the little strong point of high sensitivity, high precision, high stability and device volume.The principle of optoacoustic spectroscopy gas sensor survey sulphuric dioxide is based on the optoacoustic effect of gas, by selecting the high-sensitivity detection of 8.7 microns light sources and acoustic sounding equipment microphone realization to sulphuric dioxide.This inventive method need not to add any chemical substance, has avoided the shortcoming of tetrachloro mercury potassium solution absorption process and formaldehyde absorption process.And after this inventive method was once dispatched from the factory and demarcated calibration, optical texture was stable, need not to do secondary or repeatedly calibration, had also avoided the shortcoming of electrical conductivity method.Compare with the non-dispersion infrared analytic approach, he uses 8.7 microns light source, has avoided the interference of aqueous vapor to detection signal, because water does not have absorption peak on this light source.And ultraviolet fluorescence method is very high to the ultraviolet light intensity requirement of 220nm, and photocaustic spectroscopy can use wide spectrum to mix filter plate and obtain light source, and dynamic range is wide, the precision height, and selectivity is strong.