System and method for high sensitivity optical detection of gases

Abstract

A method and apparatus architecture for detecting gases, particularly hazardous gases which should be detected in miniscule amounts. High sensitivity detection of chemical warfare agents (CWAs) is set forth with very low probability of false positives (PFP) by the use of an innovative laser-photoacoustic spectrometer (L-PAS). Detection of diisopropyl methylphosphonate (DIMP), a decomposition product of Sarin and a relatively harmless surrogate for the nerve gases, is made in the presence of other gases that are expected to be interferences in an urban setting. Detection sensitivity for DIMP in the presence of these interferences of better than 0.45 ppb, which satisfies current homeland and military security requirements is shown as well as the first analysis of optical techniques for the detection of chemical warfare agents (CWAs) and toxic industrial chemicals (TICs) in real world conditions.

1 . A gas detector, comprising: 
 an optical analyzer detecting in a sample of gas optical absorbance of said sample at a plurality of wavelengths, said optical analyzer transmitting an absorbency signal representative of optical absorbency for each respective one of said plurality of wavelengths;    a spectral library of gas species absorbing one or more of said plurality of wavelengths including determined absorptions for expected gases and target gases; and    a processor receiving said absorbency signals and applying pattern recognition with said absorbency signals and said determined absorptions to provide mole fraction quantities for said expected gases and said target gases; whereby    said sample of gas may be analyzed for presence and quantity of expected gases and target gases.    
   
   
       2 . A gas detector as set forth in  claim 1 , further comprising: 
 said optical analyzer including an optical analyzer selected from the group consisting of: tunable laser systems, laser photoacoustic detection systems (L-PAS); long path optical absorption measuring systems, cavity ring-down spectroscopy systems, FTIR systems, and any combinations thereof.    
   
   
       3 . A gas detector as set forth in  claim 1 , further comprising: 
 said optical analyzer including a L-PAS system using a CO 2  laser.    
   
   
       4 . A gas detector as set forth in  claim 1 , further comprising: 
 said optical analyzer including a L-PAS using one or several tunable quantum cascade lasers.    
   
   
       5 . A gas detector as set forth in  claim 1 , further comprising: 
 said optical analyzer including a L-PAS using a tunable parametric oscillator.    
   
   
       6 . A gas detector as set forth in  claim 1 , further comprising: 
 said optical analyzer including a L-PAS using one or several direct bandgap recombination type semiconductor lasers.    
   
   
       7 . A gas detector as set forth in  claim 1 , further comprising: 
 said optical analyzer including a L-PAS using any combination of CO 2  lasers, quantum cascade lasers, parametric oscillators, and direct bandgap recombination type semiconductor lasers.    
   
   
       8 . A gas detector as set forth in  claim 1 , further comprising: 
 said plurality of wavelengths being equal or greater in number than the number of absorbing gas species.    
   
   
       9 . A gas detector as set forth in  claim 1 , further comprising: 
 said spectral library derived from optical analysis of said expected gases and said target gases by said optical analyzer.    
   
   
       10 . A gas detector as set forth in  claim 1 , further comprising: 
 said pattern recognition algorithms including use of a least squares fitting technique.    
   
   
       11 . A gas detector, comprising: 
 an optical analyzer detecting in a sample of gas optical absorbance of said sample at a plurality of wavelengths, said optical analyzer transmitting an absorbency signal representative of optical absorbency for each respective one of said plurality of wavelengths, said optical analyzer selected from the group consisting of: 
 tunable laser systems;  
 laser photoacoustic systems including a L-PAS system using a CO 2  laser;  
 long path optical absorption measuring systems;  
 cavity ring-down spectroscopy systems;  
 FTIR systems;  
 a L-PAS using one or several tunable quantum cascade lasers;  
 a L-PAS using a tunable parametric oscillator;  
 a L-PAS using one or several direct bandgap recombination type semiconductor lasers;  
 a L-PAS using any combination of CO 2  lasers, quantum cascade lasers, parametric oscillators, and direct bandgap recombination type semiconductor lasers; and  
 any combinations thereof;  
   a spectral library of gas species absorbing one or more of said plurality of wavelengths including determined absorptions for expected gases and target gases, said spectral library derived from optical analysis of said expected gases and said target gases by said optical analyzer; said plurality of wavelengths of said optical analyzer being equal or greater in number than the number of absorbing gas species; and    a processor receiving said absorbency signals and applying a least squares fitting technique with said absorbency signals and said determined absorptions to provide mole fraction quantities for said expected gases and said target gases; whereby    said sample of gas may be analyzed for presence and quantity of said expected gases and said target gases.    
   
   
       12 . A high sensitivity gas detector for detection of hazardous gases with reduced probability of false positive and false negative signals, comprising: 
 a light source transmitting modulated tunable radiation;    a photoacoustic test cell illuminated by said radiation, said photoacoustic test cell having a microphone system transmitting a photoacoustic signal;    a signal receiver receiving said microphone signal and transmitting a normalized signal; and    a signal processor receiving said normalized signal, analyzing said normalized signal in conjunction with at least one entry in a library for signal signatures for gases detectable by said radiation, said signal processor transmitting a resulting signal indicating a quantity of the hazardous gases in said test cell; whereby    detection of known hazardous gases can be made by illuminating sample air or other gas in said photoacoustic test cell.    
   
   
       13 . A high sensitivity gas detector for detection of hazardous gases as set forth in  claim 12 , wherein said light source transmitting modulated tunable radiation further comprises: 
 a tunable CO 2  laser.    
   
   
       14 . A high sensitivity gas detector for detection of hazardous gases as set forth in  claim 13 , wherein said tunable CO 2  laser further comprises: 
 a tunable CO 2  laser tunable to wavelengths inclusively between 9.0 μm and 11.5 μm.    
   
   
       15 . A high sensitivity gas detector for detection of hazardous gases as set forth in  claim 12 , wherein said light source transmitting modulated tunable radiation further comprises: 
 one or more tunable quantum cascade lasers.    
   
   
       16 . A high sensitivity gas detector for detection of hazardous gases as set forth in  claim 15 , wherein at least one of said one or more tunable tunable quantum cascade lasers further comprises: 
 a tunable quantum cascade laser tunable to wavelengths inclusively between 3.0 μm and 15 μm.    
   
   
       17 . A high sensitivity gas detector for detection of hazardous gases as set forth in  claim 12 , wherein said light source transmitting modulated tunable radiation further comprises: 
 a tunable parametric oscillator.    
   
   
       18 . A high sensitivity gas detector for detection of hazardous gases as set forth in  claim 17 , wherein said tunable parametric oscillator further comprises: 
 a tunable parametric oscillator tunable to wavelengths inclusively between 2.0 μm and 15 μm.    
   
   
       19 . A high sensitivity gas detector for detection of hazardous gases as set forth in  claim 12 , wherein said light source transmitting modulated tunable radiation further comprises: 
 one or more tunable direct bandgap recombination type semiconductor lasers.    
   
   
       20 . A high sensitivity gas detector for detection of hazardous gases as set forth in  claim 19 , wherein at least one of said one or more tunable direct bandgap recombination type semiconductor lasers further comprises: 
 a tunable direct bandgap recombination type semiconductor laser tunable between wavelengths inclusively between 1.0 μm and 15 μm.    
   
   
       21 . A high sensitivity gas detector for detection of hazardous gases as set forth in  claim 12 , wherein said light source transmitting modulated tunable radiation further comprises: 
 any combination of CO 2  lasers, quantum cascade lasers, parametric oscillators, and direct bandgap recombination type semiconductor lasers.    
   
   
       22 . A high sensitivity gas detector for detection of hazardous gases as set forth in  claim 21 , wherein said any combination of CO 2  lasers, quantum cascade lasers, parametric oscillators, and direct bandgap recombination type semiconductor lasers further comprises: 
 any tunable combination of CO 2  lasers, quantum cascade lasers, parametric oscillators, and direct bandgap recombination type semiconductor lasers, such tunable combination tunable between wavelengths inclusively between 1.0 μm and 15 μm.    
   
   
       23 . A high sensitivity gas detector for detection of hazardous gases as set forth in  claim 12 , further comprising: 
 a detector detecting said radiation and a characteristic thereof, said detector transmitting a reference signal; and    a beam splitter splitting said radiation to illuminate both said test cell and said detector.    
   
   
       24 . A high sensitivity gas detector for detection of hazardous gases as set forth in  claim 23 , further comprising: 
 said signal receiver receiving said reference signal and said photoacoustic signal; and    said signal receiver communicating bi-directionally with said light source.    
   
   
       25 . A high sensitivity gas detector for detection of hazardous gases as set forth in  claim 12 , further comprising: 
 said library having entries for analysis with said normalized signal, said entries including signal signatures for chemical warfare agents (CWAs): Lewisite, Nitrogen Mustard (H—N3), Sulfur mustard (HD), 4-dithiane, diisopropyl methylphosphonate (DIMP), dimethyl methylphosphonate (DMMP), isoamyl alcohol, methylphosphonic difluoride (DIFLUOR), Cyclosarin (GF), Sarin (GB), Soman (GD), Tabun (GA), VX, triethyl phosphate, and 2-diisopropy laminoethanol (DIP AE); and    said library having entries for comparison with said normalized signal, said entries including signal signatures for toxic industrial chemicals (TICs): ammonia, arsine, boron trichloride, ethylene oxide, and nitric acid.    
   
   
       26 . A high sensitivity gas detector for detection of hazardous gases as set forth in  claim 12 , further comprising: 
 said library having entries for comparison with said normalized signal, said entries including signal signatures for chemical warfare agents (CWAs): Mustard (H—N3), Sulfur mustard (HD), 4-Dithiane;    said library having entries for comparison with said normalized signal, said entries including signal signatures for toxic industrial chemicals (TICs): boron trifluoride, carbon disulfide, diborane, formaldehyde, hydrogen cyamide, hydrogen sulfide, nitric acid, phosgene, sulfur dioxide, tungsten hexafluoride; and    said library having entries for comparison with said normalized signal, said entries including signal signatures for explosives: TNT and PETN.    
   
   
       27 . A high sensitivity gas detector for detection of hazardous gases as set forth in  claim 12 , further comprising: 
 said library having entries for comparison with said normalized signal, said entries including signal signatures for toxic industrial chemicals (TICs): HBr, HCl, and HF.    
   
   
       28 . A high sensitivity gas detector for detection of hazardous gases with reduced probability of false positive and false negative signals, comprising: 
 a light source transmitting modulated tunable radiation, said light source selected from the group consisting of: 
 a tunable CO 2  laser;  
 a tunable CO 2  laser tunable to wavelengths inclusively between 9.0 μm and 11.5 μm;  
 one or more tunable quantum cascade lasers;  
 a tunable quantum cascade laser tunable to wavelengths inclusively between 3.0 μm and 15 μm;  
 a tunable parametric oscillator;  
 a tunable parametric oscillator tunable to wavelengths inclusively between 2.0 μm and 15 μm;  
 one or more tunable direct bandgap recombination type semiconductor lasers;  
 a tunable direct bandgap recombination type semiconductor laser tunable between wavelengths inclusively between 1.0 μm and 15 μm;  
 any combination of CO 2  lasers, quantum cascade lasers, parametric oscillators, and direct bandgap recombination type semiconductor lasers;  
 any tunable combination of CO 2  lasers, quantum cascade lasers, parametric oscillators, and direct bandgap recombination type semiconductor lasers, such tunable combination tunable between wavelengths inclusively between 1.0 μm and 15 μm; and  
 any combinations thereof;  
   a photoacoustic test cell illuminated by said radiation, said photoacoustic test cell having a microphone system transmitting a photoacoustic signal;    a detector detecting said radiation and a characteristic thereof, said detector transmitting a reference signal;    a beam splitter splitting said radiation to illuminate both said test cell and said detector;    a signal receiver receiving said microphone signal and transmitting a normalized signal;    said signal receiver receiving said reference signal and said photoacoustic signal;    said signal receiver communicating bi-directionally with said light source;    a signal processor receiving said normalized signal, analyzing said normalized signal in conjunction with at least one entry in a library for signal signatures for gases detectable by said radiation, said signal processor transmitting a resulting signal indicating a quantity of the hazardous gases in said test cell    said library having entries for analysis with said normalized signal, said entries including signal signatures for chemical warfare agents (CWAs): Lewisite, Nitrogen Mustard (H—N3), Sulfur mustard (HD), 4-dithiane, diisopropyl methylphosphonate (DIMP), dimethyl methylphosphonate (DMMP), isoamyl alcohol, methylphosphonic difluoride (DIFLUOR), Cyclosarin (GF), Sarin (GB), Soman (GD), Tabun (GA), VX, triethyl phosphate, and 2-diisopropylaminoethanol (DIPAE);    said library having entries for analysis with said normalized signal, said entries including signal signatures for toxic industrial chemicals (TICs):ammonia, arsine, boron trichloride, ethylene oxide, nitric acid, borontrifluoride, carbon disulfide, diborane, formaldehyde, hydrogen cyamide, hydrogen sulfide, nitric acid, phosgene, sulfur dioxide, tungsten hexafluoride, HBr, HCl, and HF; and    said library having entries for analysis with said normalized signal, said entries including signal signatures for explosives: TNT and PETN; whereby    detection of known hazardous gases can be made by illuminating sample air or other gas in said photoacoustic test cell.    
   
   
       29 . A high sensitivity gas detector for detection of hazardous gases as set forth in  claim 28 , wherein said light source transmitting modulated tunable radiation further comprises: 
 a light source transmitting modulated tunable radiation tunable to wavelengths including those between 1 μm and 15 μm, inclusively;    
   
   
       30 . A method for detecting gases, comprising: 
 providing a gas sample to be tested;    providing an optical analyzer which transmits a signal proportional to optical absorbance;    analyzing said gas sample with said optical analyzer to obtain an absorbency signal    analyzing said absorbency signal in conjunction with a spectral library of determined absorptions for expected gases and target gases; and    determining mole fractions of said expected gases and said target gases; whereby    said gas sample may be analyzed for presence and quantity of expected gases and target gases.    
   
   
       31 . A method for detecting gases as set forth in  claim 30 , further comprising: 
 wherein said optical analyzer includes an optical analyzer detecting in a sample of gas optical absorbance of said sample at a plurality of wavelengths, said optical analyzer transmitting said absorbency signal representative of optical absorbency for each respective one of said plurality of wavelengths.    
   
   
       32 . A method for detecting gases as set. forth in  claim 31 , further comprising: 
 said spectral library includes a spectral library of gas species absorbing one or more of said plurality of wavelengths including determined absorptions for expected gases and target gases.    
   
   
       33 . A method for detecting gases as set forth in  claim 31 , further comprising: 
 said step of analyzing said absorbency signal and said step of determining mole fractions achieved by providing a processor receiving said absorbency signals and applying pattern recognition with said absorbency signals and said determined absorptions to provide mole fraction quantities for said expected gases and said target gases    
   
   
       34 . A method for detecting gases as set forth in  claim 30 , further comprising: 
 said gas sample being a sample of ambient or other air.    
   
   
       35 . A method for detecting gases as set forth in  claim 30 , further comprising: 
 determining the probability that the gas detector will transmit a false positive (PFP) signal for a first gas;    determining the probability that the gas detector will transmit a false negative (PFN) signal for said first gas; and    selecting a sensitivity for said optical analyzer according to said PFP and PFN.