System and method for gas analysis using photoacoustic spectroscopy
View Patent ↗A method for analyzing a target analyte gas concentration using a photoacoustic spectroscopy cell comprising: i) a modulatable light source which provides optical radiation at an absorption wavelength of a target analyte; ii) a resonant acoustic chamber for containing said analyte; iii) a microphone positioned within said chamber whereby the acoustic reactance of the microphone is substantially equal but opposite in value to the acoustic reactance of the chamber at a selected cell resonance frequency.
1. A photoacoustic spectroscopy cell comprising:
i) a modulatable light source which provides optical radiation at an absorption wavelength of a target analyte;
ii) a resonant acoustic chamber for containing said analyte;
iii) a microphone positioned within said chamber whereby the acoustic reactance of the microphone is substantially equal but opposite in value to the acoustic reactance of the chamber at a selected cell resonance frequency.
2. The photoacoustic cell of claim 1 wherein said light source is a fixed frequency laser using direct or external amplitude modulation.
3. The photoacoustic cell of claim 1 wherein the amplitude of said optical radiation is modulated.
4. The photoacoustic cell of claim 1 wherein said microphone is an electret.
5. The photoacoustic cell of claim 1 wherein said microphone is an acoustic transducer having an acoustic impedance within a factor of 3 of the cavity impedance at the point where the microphone is attached to the cavity and also at the resonant frequency used for detection.
6. The photoacoustic cell of claim 1 wherein said resonant acoustic chamber is a Helmholtz or other lumped resonator.
7. The photoacoustic cell of claim 1 wherein said resonant acoustic chamber is a distributed resonator.
8. The photoacoustic cell of claim 1 wherein said resonant acoustic chamber comprises both lumped and distributed elements.
9. The photoacoustic cell of claim 1 wherein said resonant acoustic chamber is a closed cell.
10. The photoacoustic cell of claim 1 wherein said resonant acoustic chamber is an open cell.
11. The photoacoustic cell of claim 1 wherein said resonant acoustic chamber is substantially cylindrical and comprises an optical window at at least one end.
12. The photoacoustic cell of claim 1 wherein said resonant acoustic chamber is substantially cylindrical and comprises an optical reflector at at least one end.
13. The photoacoustic cell of claim 1 wherein said resonant acoustic chamber has a diameter ranging from about 1 to 4 mm and a length ranging from about 40 to 100 mm.
14. The photoacoustic cell of claim 1 wherein said resonant acoustic chamber has a volume ranging from about 0.1 cc to 1.0 cc.
15. The photoacoustic cell of claim 1 wherein said resonant acoustic chamber is non-cylindrical.
16. The photoacoustic cell of claim 1 wherein the back volume of the microphone provides the dominant system compliance relative to the acoustic chamber.
17. The photoacoustic cell of claim 1 wherein the light source is wavelength modulated so as to sweep back and forth across the absorbance wavelength of the target analyte.
18. The photoacoustic cell of claim 1 wherein said light source is a frequency modulated laser.
19. The photoacoustic cell of claim 18 wherein said laser is a continuous wave laser.
20. The photoacoustic cell of claim 1 wherein said light source is a phase modulated laser.
21. The photoacoustic cell of claim 20 wherein said laser is a continuous wave laser.
22. A method for analyzing a target analyte gas concentration using a photoacoustic spectroscopy cell comprising:
i) a modulatable light source which provides optical radiation at an absorption wavelength of a target analyte;
ii) a resonant acoustic chamber for containing said analyte;
iii) a microphone positioned within said chamber whereby the acoustic reactance of the microphone is substantially equal but opposite in value to the acoustic reactance of the chamber at a selected cell resonance frequency.