System and method for isotope ratio analysis and gas detection by photoacoustics

• US 5,900,533 A
• Filed: 08/03/1995
• Issued: 05/04/1999
• Est. Priority Date: 08/03/1995
• Status: Expired due to Term

• Current Assignee: Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Original Assignee: TRW Limited (Zeppelin - Stiftung der Stadt Friedrichshafen)
• Inventors: Chou, Mau-Song
• Primary Examiner(s): LARKIN, DANIEL SEAN
• Application Number:
  US08/592,748
• Time in Patent Office: 1,370 Days
• Field of Search: 73/24.01, 73/24.06, 73/24.02, 73/31.01, 73/31.02, 73/31.03, 73/61.41, 73/61.45, 73/61.49, 73/61.75, 73/61.79, 73/64.53, 250/339.13, 250/340, 250/343, 250/351, 356/51, 356/432, 356/437
• US Class Current: 73/24.01
• CPC Class Codes: G01N 21/1702 with opto-acoustic detection, e.g. for gases or analysing solids

Abstract

A system and method are set forth for photoacoustical analysis of isotope and other compounds having telltale absorption wavelengths between 1700-2500 nm. The system and method includes a Co:Mg F₂, optical parametric oscillator (OPO) or diode laser tunable between 1700-2500 nm which is directed into a sample at energies sufficient to generate detectable acoustical emissions. A microphone detects the emissions for processing and analysis. The system and method is adapted to detect stable isotope compounds such as ¹³ CO₂ as well as other chemical compounds. For non-gaseous compounds, a CO₂ or diode laser is used to photoablate the gaseous sample contacting the suspected compound.

Claims

1. A photoacoustic system for detecting a suspected constituent in a vapor sample, the constituent having at least one absorption wavelength in the range of approximately 1700-2500 nm, the system comprising:

a source of electromagnetic radiation tunable to wavelengths between 1700 to 2500 nm to correspond to at least one absorption wavelength of the suspected constituent and having a pulsed energy output level in the range of 1 to 160 mJ to generate acoustical waves in the sample in response to said absorption, said source directed into the sample;

a microphone to detect acoustic waves generated in response to absorption and produce an electronic signal proportional to the intensity of the acoustic wave detected;

a processor for processing the electronic signals to extract acoustic emissions from background acoustical emissions generated in response to absorption by the constituent from any extraneous detected acoustic emissions and to average constituent acoustic emissions over a plurality of pulses to improve the signal to noise ratio; and

a gas chromatograph to isolate various components of the sample and a combustor to convert the effluents of the gas chromatograph to the sample form of CO₂.

2. A photoacoustic system for detecting a selected constituent in a mixture of background constituents in a sample, said selected constituent having at least one absorption wave length in the range of approximately 1700 to 2500 nm comprising:

a gas chromatograph to isolate various components of the sample;

a combustor to convert the effluents from the gas chromatograph to a CO₂ form of the sample;

a source of electromagnetic radiation for producing a selected wavelength between 1700 to 2500 nm to correspond to at least one absorption wavelength of the selected constituent and having a pulsed energy output level in the range of 1 to 160 mJ sufficient to generate acoustical wave indicating the presence of this selected constituent said radiation being directed into the sample;

a sound detector for detecting the acoustic wave and producing an electric signal proportional to the acoustical emissions; and

a processor for processing the signal to extract the selected constituent acoustic emissions from background acoustical emissions and to average the constituent acoustic emissions over a plurality of pulses to improve the signal-to-noise ratio.

3. The system of claim 2 wherein the source of radiation is an optical parametric oscillator (OPO) laser.

4. The system of claim 2 wherein the source of radiation is a diode laser.

5. The system of claim 2 including a fiber optic conduit to transmit the output of the source of the electromagnetic radiation and a lens to focus the output into the sample.

6. The system of claim 5 wherein the fiber optic conduit is adapted to have low absorption in the 1700-2500 nm range.

7. A photoacoustic system for detecting a suspected constituent in a vapor sample that is a mixture of background constituents, the suspected constituent having at least one absorption wavelength in the range of approximately 1700 to 2500 nm, the system comprising:

a gas chromatograph to isolate various components of the vapor sample;

a combustor to convert the components to a form of CO₂ ;

a source of electromagnetic radiation tunable to wavelengths between 1700 to 2500 nm to correspond to at least one absorption wavelength of the suspected constituent and having an output pulse level of energy in the range of 1 to 160 mJ sufficient to generate acoustical waves in the vapor sample in response to said absorption, said source being directed into the sample;

a microphone to detect acoustic waves generated in response to absorption and produce an electronic signal proportional to the intensity of the acoustic waves detected; and

a processor for processing the electronic signals to extract acoustic emissions generated in response to absorption by the constituent from any extraneous detected acoustic emissions.

8. The system of claim 7 wherein the source of electromagnetic radiation is a tunable Co:

Mg F₂ laser.

9. The system of claim 8 wherein the output of the Co:

MgF₂ laser source is pulsed and said processor is adapted to average constituent acoustic emissions over a plurality of pulses.

10. The system of claim 8 further including a lens to focus the output of the laser into the sample.

11. The system of claim 7 wherein the source of radiation is an optical parametric oscillator (OPO) laser.

12. The system of claim 7 wherein the source of radiation is a diode laser.

13. The system of claim 7 wherein the electromagnetic radiation is directed into the sample comprising an optical fiber conduit.

14. The system of claim 13 wherein the optical fiber is a silicon-dioxide based fiber.

15. A photoacoustic system for detecting the presence of a selected constituent and a sample having at least one absorption line in the range of approximately 1700 to 2500 nm in a non-gaseous-sample retained in a test cell comprising:

a laser source selected from the group consisting of a CO₂ laser or a diode laser for photoablating volatiles from a sample as a vapor containing the selected constituent;

a source of pulsed collimated electromagnetic radiation being tunable through wavelengths between 1700 to 2500 nm and having an output energy level in the range of 1 to 160 mJ;

means for directing the radiation into a vapor to be absorbed by the selected constituent, said electromagnetic radiation source having energy sufficient to generate an acoustic signal in the vapor indicating the presence of the selected constituent;

a microphone for detecting the acoustic signals; and

a processor for processing the detected signal for each pulse of radiation to extract constituent gas acoustic emissions from background acoustical emissions and averaging the same for a plurality of pulses to improve the signal to noise ratio.

16. The system of claim 15 wherein the source of pulsed collimated electromagnetic radiation is a laser tunable between 1700-2500 nm and has an output energy in the range of 1-160 mJ and is selected from the group consisting of a Co:

MgF₂ laser, an optical parametric oscillator laser, or a diode laser.

17. The system of claim 15 further including a test cell to retain the sample.

18. A method for detecting the presence of a selected constituent in a non-gaseous composition, said constituent having at least one absorption line in the range of approximately 1700 to 2500 nm comprising:

photoablating volatiles from the composition, said volatiles containing said constituent;

directing electromagnetic radiation into said volatiles, said radiation tunable through wavelengths between 1700 to 2500 nm and having sufficient energy to generate an acoustic signal in the volatiles in response to said constituent absorbing said electromagnetic energy;

detecting said acoustic signal; and

processing said detected acoustic signals to extract in a readable form acoustic emission signals generated in response to absorption.

19. The method of claim 18 including providing a Co:

Mg F₂ laser as a source of said electromagnetic radiation.

20. The method of claim 18 including providing an optical parametric oscillator laser as a source of said electromagnetic radiation.

21. The method of claim 18 including providing a diode laser as a source of said electromagnetic radiation.

22. The method of claim 18 further including pulsing said radiation.

23. The method of claim 18 including modulating a continuous wave source of electromagnetic radiation.