System and method for gas analysis using photoacoustic spectroscopy
View Patent ↗A system and 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 a substantial factor in determining the acoustic resonant frequency of the acoustic chamber and where the magnitude of the acoustic reactance of the microphone is at least two times the acoustic resistance of the microphone.
1. A photoacoustic spectroscopy instrument 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 a substantial factor in determining the acoustic resonant frequency of the acoustic chamber and wherein the magnitude of the acoustic reactance of the microphone is at least two times the acoustic resistance of the microphone.
2. The instrument of claim 1 wherein said light source is a frequency modulated laser.
3. The instrument of claim 2 wherein said laser is a continuous wave laser.
4. The instrument of claim 1 wherein said light source is a phase modulated laser.
5. The instrument of claim 4 wherein said laser is a continuous wave laser.
6. The instrument of claim 1 wherein said light source is a fixed frequency laser using direct or external amplitude modulation.
7. The instrument of claim 1 wherein the amplitude of said optical radiation is modulated.
8. The instrument of claim 1 wherein said microphone is an electret.
9. The instrument 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 attaches to the cavity and also at the resonant frequency used for detection.
10. The instrument of claim 1 wherein said resonant acoustic chamber is a Helmholtz or other lumped resonator.
11. The instrument of claim 1 wherein said resonant acoustic chamber is a distributed resonator.
12. The instrument of claim 1 wherein said resonant acoustic chamber comprises both lumped and distributed elements.
13. The instrument of claim 1 wherein said resonant acoustic chamber is a closed cell.
14. The instrument of claim 1 wherein said resonant acoustic chamber is an open cell.
15. The instrument of claim 1 wherein said resonant acoustic chamber is substantially cylindrical and comprises an optical window at least one end.
16. The instrument of claim 1 wherein said resonant acoustic chamber is substantially cylindrical and comprises an optical reflector at least one end.
17. The instrument 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 about 100 mm.
18. The instrument of claim 1 wherein said resonant acoustic chamber has a volume ranging from about 0.1 cc to 1.0 cc.
19. The instrument of claim 1 wherein said resonant acoustic chamber is non-cylindrical.
20. The instrument of claim 1 wherein the back volume of the microphone provides the dominant system compliance relative to the acoustic chamber.
21. The instrument of claim 1 wherein the light source is wavelength modulated so as to sweep back and forth across an absorbance wavelength of a target analyte present within the chamber.