CN105241814A - Apparatus and method for measurement of trace gas with photoacoustic spectroscopy technology - Google Patents

Abstract

The invention discloses an apparatus and method for measurement of trace gas with photoacoustic spectroscopy technology. On the basis of the first-order longitudinal resonance photoacoustic spectroscopy technology, a differential photoacoustic signal method is utilized to significantly weaken the influence of correlated noise caused by photoacoustic effect in solid and improve the detection signal-to-noise ratio of a photoacoustic signal. Compared with existing photoacoustic spectroscopy detection methods, the method provided by the invention further improves the detection limit of trace gas.

Description

A kind of apparatus and method utilizing photoacoustic spectroscopy measurement trace gas

Technical field

The present invention relates to gas concentration measurement system regions, specifically a kind of apparatus and method utilizing photoacoustic spectroscopy measurement trace gas.

Background technology

Spectrographic detection technology is divided into Emission Spectroscopy and absorption spectroscopy techniques, optoacoustic spectroscopy (photo-acousticspectroscopy) is the same with difference absorption spectrum technology (DOAS), belonging to absorption spectroscopy techniques, is the one of indirect absorption spectroscopy techniques.The feature of optoacoustic spectroscopy has high sensitivity and selectivity characteristic, is a kind of important limit detection method.

The ultimate principle of optoacoustic spectroscopy is based on optoacoustic effect, and namely when the light that gas absorption frequency a branch of and to be measured is close irradiates the gas in a closed container intermittently, have sound wave in container and produce, this phenomenon is the optoacoustic effect of gas.When light gas molecule to be measured or Atomic absorption are in the light of resonant frequency, gas molecule is in excited state.Excited molecule and surrounding ground state molecule interact or self relaxation, and return ground state by excited state transition, excess energy becomes heat with radiationless transition.Heat radiation out after, become gas internal energy (being proportional to temperature).Gas temperature change in a confined space causes pressure change.If periodically modulated (wavelength-modulated or light intensity periodic modulation) incident light, then can cause air pressure cyclical variation, if modulating frequency is near audio frequency, then produce acoustic pressure, sound pressure is proportional to gas concentration.Detect sound wave with detector and indirectly can measure gas concentration.

Tradition optoacoustic spectroscopy detection system comprises light source, modulating device, photoacoustic cell, microphone, demodulating equipment etc., as shown in Figure 1.Chopper is modulated light source intensity under certain frequency, and the light after modulation is in photoacoustic cell, and the wavelength of light modulated is in by the wave band of gas strong absorption to be measured, causes the absorption light of gases cycle to be measured and produces relaxation, produces photoacoustic signal.When certain one class resonant frequency of the photoacoustic cell after optimal design is equal with modulation of source frequency, namely resonate, photoacoustic signal is by gain.Utilize a sensitive microphone to detect photoacoustic signal, then after enlarge leadingly, utilize lock-in amplifier to detect.The photoacoustic signal measured is proportional to gas concentration to be measured, so just indirectly can measure gas concentration.Light source, photoacoustic cell and microphone are the core devices of whole system.

As shown in Figure 2, photo-acoustic spectrometer light source used divides coherent source and incoherent light source, and detection single gas, multi-purpose laser, monochromaticity is good, and intensity is high.During detection multiple gases, realize wavelength chooses with incoherent light source+optical filter+monochromator.Photoacoustic cell divides resonance photoacoustic cell and off-resonance photoacoustic cell, and off-resonance photoacoustic cell structure is simple, and volume is little, and cost is low, but measuring limit is not as resonance mode.When the resonant frequency of the modulating frequency of incident light and certain single order normal mode of photoacoustic cell is equal, can carry out gain amplification to photoacoustic signal, now photoacoustic cell works in resonance mode.The gain of photoacoustic signal is represented by the Q value of PA cell.Because photoacoustic signal is general all very weak, is easily submerged in noise signal, generally adopts single order longitudinal resonance photoacoustic cell to improve signal to noise ratio (S/N ratio) to the detection of trace gas now.

As shown in Figure 3, soft boundary light acoustic resonance chamber is diameter 10-20mm, and length is the cylindrical cavity of about 100mm.This chamber can form single order longitudinal resonance pattern, and vibration frequency is v/2L, v is the velocity of sound, and L is effective cavity length.During single order longitudinal resonance mode oscillation, the wave amplitude of standing wave is in the center of chamber length, and chamber end positions is node.

Due to gas to be measured trace gas often, photoacoustic signal is often very faint, is even submerged in ground unrest.Noise in gas photo acoustic spectrometry system can be divided into correlation noise and the large class of uncorrelated noise two, utilize phase lock amplifying technology effectively can suppress uncorrelated noise, but correlation noise due to photoacoustic signal same frequency, remain the principal element of influential system limit detection sensitivity at present.Correlation noise mainly comprises chopper noise, and window and pool wall absorb the noise produced.General correlated noise is more much bigger than uncorrelated noise, and when correlation noise is suppressed preferably, uncorrelated noise is just manifested.Researchist have employed a lot of measure and has carried out restraint speckle, improves signal to noise ratio (S/N ratio), such as: the good metal material of cavity temperature conductivity is made (as brass), reduces the impact of solid optoacoustic effect.The two ends of resonant cavity respectively have a gas buffer air chamber to absorb luminous energy in order to eliminate window.Buffer air chamber length is the half of resonant cavity, and radius is more than three times (through checking such noise isolation effect best) of resonant cavity radius.The window of two turnover light adopts Brewster angle to place, and is positioned over the node place of standing-wave sound field, makes the window absorption of noise of generation minimum like this.The node that air inlet/outlet is positioned at standing-wave sound field sentences reduction eddy current crack.

But, although have employed noise reduction schemes as above, due to the broadband response characteristic of microphone, be still difficult to the impact of the stress release treatment of essence.In correlation noise, especially window and pool wall absorb the noise produced is overriding noise.Its mechanism comes from solid optoacoustic effect.Window and pool wall are heated by alternation by the exciting light of intensity modulated, and partial heat energy is still the principal element of influential system limit detection sensitivity to correlation noise by heat exchange pattern.

Summary of the invention

The object of this invention is to provide a kind of apparatus and method utilizing photoacoustic spectroscopy measurement trace gas, there is based on the gas concentration measuring apparatus of photoacoustic cell the problem that window and pool wall absorb the noise produced to solve prior art.

In order to achieve the above object, the technical solution adopted in the present invention is:

A kind of device utilizing photoacoustic spectroscopy measurement trace gas, include light source, photoacoustic cell, chopper, prime amplifier, lock-in amplifier, digital signal processing unit, light power meter, it is characterized in that: the inner chamber of described photoacoustic cell is by before being positioned at photoacoustic cell, the buffer air chamber of rear end, and with photoacoustic cell coaxially and the longitudinal resonator cavity of single order being communicated with two buffer air chambers form, the air intake opening be communicated with front end buffer air chamber is provided with on front side of the corresponding resonator cavity of photoacoustic cell pool wall, the gas outlet be communicated with rear end buffer air chamber is provided with on rear side of the corresponding resonator cavity of photoacoustic cell pool wall, photoacoustic cell pool wall be also provided with a pair respectively in the same way vertical connection to the acoustic aperture of resonator cavity, one of them acoustic aperture is communicated to resonator cavity centre position, another acoustic aperture is communicated to resonator cavity front end or rear end, microphone is respectively arranged with in each acoustic aperture, described light source emergent light is incident to photoacoustic cell through chopper, in photoacoustic cell after front end buffer air chamber, resonator cavity, rear end surge chamber, from photoacoustic cell outgoing to light power meter, gas to be measured enters photoacoustic cell Inner Front End surge chamber from photoacoustic cell air intake opening, rear end surge chamber is entered again through resonator cavity, finally flow out photoacoustic cell from gas outlet, microphone signal output terminal in photoacoustic cell acoustic aperture is connected with prime amplifier input end, prime amplifier output terminal is connected with lock-in amplifier input end, and lock-in amplifier output terminal is connected with digital signal processing unit input end.

Utilize the method for photoacoustic spectroscopy measurement trace gas, it is characterized in that: the exciting light of light source incides photoacoustic cell after chopper modulation, buffer air chamber successively through front end in photoacoustic cell, resonator cavity, after the buffer air chamber of rear end, outgoing is to light power meter, after gas to be measured enters the buffer air chamber of photoacoustic cell Inner Front End from photoacoustic cell air intake opening simultaneously, the buffer air chamber of rear end is entered again through resonator cavity, last gas to be measured flows out photoacoustic cell from gas outlet, chopper is modulated exciting light light intensity under certain frequency, the wavelength of light modulated is in by the wave band of gas strong absorption to be measured, cause the absorption light of gases cycle to be measured and produce relaxation, thus generation photoacoustic signal,

When beam modulation frequency reaches consistent with the resonance frequency of the first order resonant normal mode of resonator cavity, resonator cavity resonance, now utilize microphone to gather the photoacoustic signal standing-wave sound field at antinode place in resonator cavity in resonator cavity centre position, and gather the photoacoustic signal standing-wave sound field at node place in resonator cavity at resonator cavity back-end location;

The signal of collection is sent into digital signal processing unit process through prime amplifier, lock-in amplifier after amplifying process by microphone successively, after digital signal processing unit carries out difference processing to the signal of two microphones, and then can obtain gas concentration to be measured.

Photoacoustic spectroscopy scheme proposed by the invention, utilize the method for difference photoacoustic signal, significantly can weaken the impact of the correlation noise that solid optoacoustic effect causes, improve the detection signal to noise ratio (S/N ratio) of photoacoustic signal, thus improve the detection limit of trace gas further.

Accompanying drawing explanation

Fig. 1 is that optoacoustic spectroscopy detects trace gas apparatus structure schematic diagram.

Fig. 2 is longitudinal resonance photoacoustic cell sectional view.

Fig. 3 is the formation basic theory figure of standing-wave sound field in single order longitudinal resonance photoacoustic cell.

Fig. 4 is photoacoustic cell structural representation of the present invention.

Embodiment

As Fig. 1, shown in Fig. 4, a kind of device utilizing photoacoustic spectroscopy measurement trace gas, include light source, photoacoustic cell, chopper, prime amplifier, lock-in amplifier, digital signal processing unit, light power meter, the inner chamber of photoacoustic cell is by before being positioned at photoacoustic cell, the buffer air chamber 1 of rear end, 2, and with photoacoustic cell coaxially and the longitudinal resonator cavity 3 of single order being communicated with two buffer air chambers form, the air intake opening 4 be communicated with front end buffer air chamber 1 is provided with on front side of the corresponding resonator cavity 3 of photoacoustic cell pool wall, the gas outlet 5 be communicated with rear end buffer air chamber 2 is provided with on rear side of the corresponding resonator cavity 3 of photoacoustic cell pool wall, photoacoustic cell pool wall be also provided with a pair respectively in the same way vertical connection to the acoustic aperture 6 of resonator cavity 3, 7, one of them acoustic aperture 6 is communicated to resonator cavity 3 centre position, another acoustic aperture 7 is communicated to resonator cavity 3 front end or rear end, each acoustic aperture 6, microphone 8 is respectively arranged with in 7, light source emergent light is incident to photoacoustic cell through chopper, front end buffer air chamber 1 in photoacoustic cell, resonator cavity 3, after rear end surge chamber 2, from photoacoustic cell outgoing to light power meter, gas to be measured enters photoacoustic cell Inner Front End surge chamber 1 from photoacoustic cell air intake opening 4, rear end surge chamber 2 is entered again through resonator cavity 3, finally flow out photoacoustic cell from gas outlet 5, microphone signal output terminal in photoacoustic cell acoustic aperture is connected with prime amplifier input end, prime amplifier output terminal is connected with lock-in amplifier input end, lock-in amplifier output terminal is connected with digital signal processing unit input end.

The exciting light of light source incides photoacoustic cell after chopper modulation, buffer air chamber successively through front end in photoacoustic cell, resonator cavity, after the buffer air chamber of rear end, outgoing is to light power meter, after gas to be measured enters the buffer air chamber of photoacoustic cell Inner Front End from photoacoustic cell air intake opening simultaneously, the buffer air chamber of rear end is entered again through resonator cavity, last gas to be measured flows out photoacoustic cell from gas outlet, chopper is modulated exciting light light intensity under certain frequency, the wavelength of light modulated is in by the wave band of gas strong absorption to be measured, cause the absorption light of gases cycle to be measured and produce relaxation, thus generation photoacoustic signal,

When beam modulation frequency reaches consistent with the resonance frequency of the first order resonant normal mode of resonator cavity, resonator cavity resonance, now utilize microphone to gather the photoacoustic signal standing-wave sound field at antinode place in resonator cavity in resonator cavity centre position, and gather the photoacoustic signal standing-wave sound field at node place in resonator cavity at resonator cavity back-end location;

The signal of collection is sent into digital signal processing unit process through prime amplifier, lock-in amplifier after amplifying process by microphone successively, after digital signal processing unit carries out difference processing to the signal of two microphones, and then can obtain gas concentration to be measured.

When unique distinction of the present invention is single order longitudinal direction resonance occurs, detects the photoacoustic signal standing-wave sound field at node and antinode place simultaneously, difference is done to these two signals.Because in two signals, standing-wave sound field has very big-difference, and the correlated noise signal difference of two microphone measurements is little.Therefore the difference of two photoacoustic signals can improve signal to noise ratio (S/N ratio) greatly.Chopper is modulated light source intensity under certain frequency, and the light after modulation is in photoacoustic cell, and the wavelength of light modulated is in by the wave band of gas strong absorption to be measured, causes the absorption light of gases cycle to be measured and produces relaxation, produces photoacoustic signal.When certain one class resonant frequency of the photoacoustic cell after optimal design is equal with modulation of source frequency, namely resonate, photoacoustic signal is by gain.Arrange two highly sensitive microphones in photoacoustic cell centre position and port to detect photoacoustic signal simultaneously, then by two access prime amplifiers, after amplifying differential signal, utilize lock-in amplifier to detect differential signal.

In the present invention, photoacoustic cell still adopts single order longitudinal resonance resonator cavity two ends to add the structure of two Buffer Pools.Light cross section is Gaussian distribution; Z-axis along its center is incident.When the resonant frequency of the longitudinal normal mode of single order of the modulating frequency of incident light and photoacoustic cell is equal, the single order vertical pattern of photoacoustic cell is exaggerated gain, forms the longitudinal compressional wave sound field of single order.According to the equation of state of ideal gas, the equation of motion and continuity equation, ideal gas can write containing source undulating manner in a closed chamber:

${\▿}^{2}P(r,t)-\frac{1}{V^2}\frac{{\∂}^{2}P(r,t)}{{\∂}^{2}t}=-\frac{(k-1)1}{V^2}\frac{\∂H(r,t)}{\∂t}$

Wherein P is acoustic pressure pattern, and r is displacement vector, and V is the optoacoustic pool gas velocity of sound, and k=Cp/Cv is gas ratio of specific heat (molar heat capacity under constant pressure is than molar heat capacity at constant volume); H is thermal power densities, is determined by the coupling of luminous power and cavity.Suppose the rigid walls condition of photoacoustic cell, boundary condition can be write:

$\▿P(r,t)*n{|}_{surface}=0$

Use this condition, and PA cell is separate containing source wave equation in the right cylinder situation of both ends open, can obtain normal mode solution P can be expressed as:

$P_{qmn}\left(r\right)=cos\left(m\mathrm{\θ}\right)J_m\left(\frac{{\mathrm{\π\α}}_{mn}}{R}r\right)sin\left(\frac{\mathrm{\π}q}{L_{eff}}z\right)$

Wherein Jm is m rank first kind Bezier (Bessel) functions; R is resonator cavity radius, L _effthat resonator is long.Resonant frequency f _jcan be expressed as:

$f_j=\frac{V}{2}\sqrt{{\left(\frac{{\mathrm{\α}}_{mn}}{R}\right)}^2+{\left(\frac{q}{L_{eff}}\right)}^2}$

Because the resonance mode of resonant cavity exists end effect, the effective cavity length of resonator cavity need be modified to:

$L_{eff}=L+\frac{16}{3\mathrm{\π}}R$

When forming single order longitudinal resonance (q=1, m=0, n=0), pattern solution and resonant frequency just can be write:

$P_{100}=sin\left(\frac{\mathrm{\π}z}{L_{eff}}\right);f_j=\frac{V}{2L_{eff}}$

As can be seen from the pattern solution of first order resonant, to reach intensity maximum in long centre position in chamber for the standing-wave sound field of photoacoustic signal, is antinode; Minimum in the position intensity at two ends, chamber, be node.There is obvious space distribution in standing-wave sound field intensity.

Claims (2)

1. one kind utilizes the device of photoacoustic spectroscopy measurement trace gas, include light source, photoacoustic cell, chopper, prime amplifier, lock-in amplifier, digital signal processing unit, light power meter, it is characterized in that: the inner chamber of described photoacoustic cell is by before being positioned at photoacoustic cell, the buffer air chamber of rear end, and with photoacoustic cell coaxially and the longitudinal resonator cavity of single order being communicated with two buffer air chambers form, the air intake opening be communicated with front end buffer air chamber is provided with on front side of the corresponding resonator cavity of photoacoustic cell pool wall, the gas outlet be communicated with rear end buffer air chamber is provided with on rear side of the corresponding resonator cavity of photoacoustic cell pool wall, photoacoustic cell pool wall be also provided with a pair respectively in the same way vertical connection to the acoustic aperture of resonator cavity, one of them acoustic aperture is communicated to resonator cavity centre position, another acoustic aperture is communicated to resonator cavity front end or rear end, microphone is respectively arranged with in each acoustic aperture, described light source emergent light is incident to photoacoustic cell through chopper, in photoacoustic cell after front end buffer air chamber, resonator cavity, rear end surge chamber, from photoacoustic cell outgoing to light power meter, gas to be measured enters photoacoustic cell Inner Front End surge chamber from photoacoustic cell air intake opening, rear end surge chamber is entered again through resonator cavity, finally flow out photoacoustic cell from gas outlet, microphone signal output terminal in photoacoustic cell acoustic aperture is connected with prime amplifier input end, prime amplifier output terminal is connected with lock-in amplifier input end, and lock-in amplifier output terminal is connected with digital signal processing unit input end.

2. based on the method utilizing photoacoustic spectroscopy measurement trace gas of device described in claim 1, it is characterized in that: the exciting light of light source incides photoacoustic cell after chopper modulation, buffer air chamber successively through front end in photoacoustic cell, resonator cavity, after the buffer air chamber of rear end, outgoing is to light power meter, after gas to be measured enters the buffer air chamber of photoacoustic cell Inner Front End from photoacoustic cell air intake opening simultaneously, the buffer air chamber of rear end is entered again through resonator cavity, last gas to be measured flows out photoacoustic cell from gas outlet, chopper is modulated exciting light light intensity under certain frequency, the wavelength of light modulated is in by the wave band of gas strong absorption to be measured, cause the absorption light of gases cycle to be measured and produce relaxation, thus generation photoacoustic signal,

When beam modulation frequency reaches consistent with the resonance frequency of the first order resonant normal mode of resonator cavity, resonator cavity resonance, now utilize microphone to gather the photoacoustic signal standing-wave sound field at antinode place in resonator cavity in resonator cavity centre position, and gather the photoacoustic signal standing-wave sound field at node place in resonator cavity at resonator cavity back-end location;

The signal of collection is sent into digital signal processing unit process through prime amplifier, lock-in amplifier after amplifying process by microphone successively, after digital signal processing unit carries out difference processing to the signal of two microphones, and then can obtain gas concentration to be measured.