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Infrared-Laser Induced Fluorescence and Instrumental Design Michael W. Morton Geoffrey A. Blake Division of Geology and Planetary Science California Institute of Technology Pasadena, CA 91125
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Stable Isotope Studies Applications include Atmospheric chemistry Geochemistry Bio-geochemistry Planetary Exploration Specific examples Atmospheric photochemistry Climate studies Biological influences
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Instrumentation Mass Spectrometers Utilize mass differences in isotopes, Great for laboratory studies, but not in situ Limitations in planetary exploration: Size, weight Remote operation Sample preparation Sample destruction Isotopomers/ – N 15 NO 15 NNO NN 17 O Isotopologues in situ Stable Isotope Ratio Spectrometer ? Utilize spectroscopic differences in isotopes
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A Brief Look at Photoacoustic Detection Figures courtesy of – Miklos, A. & P, Hess (2001). Am. Inst. Phys., 72, 1938, 1951.
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Photoacoustic Considerations Indirect absorption technique signal strength depends on Beer’s law and cell geometry Cell volume and sample size Non-selective detection of mixed samples HDO - D 2 O Short path length Good sensitivity
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Sensitivity of PA Detection Show high laser power spectrum with small peak detection cm -1
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IR-LIF Technique Small instrument size Remote operation Little to no sample preparation non-destructive Isotopomer resolution Small sample size Figure courtesy of – Geoff Blake
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Instrumental Setup chopper PA cell dewar detector filter wheel lock-in amplifier scope PC wavemeter PC
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HDO Transitions 1 – OD stretch2726.73 cm -1 – bend1402.2 3 – OH stretch3707.47 Pump v1+v3 near 1550 nm (can use telecommunications diode lasers). cm -1 6434 2727 3707 0 0 1 1 0 1 1 0 0 0 0 0 0 2 1 6512 0 2 0
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HDO Vapor Spectrum S/N ~ 90 ~1 pMole HDO in volume imaged onto detector.
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Signal and Noise Analysis 1 Why is the signal ~independent of pressure in a pure gas?
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Signal and Noise Analysis S/N a √ t out to ~10 sec Microphonic peak (vibrations)
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Signal and Noise Analysis Figure from – Bernath, P.F. (1995). Oxford University Press, Inc., 8. Limited by warm window?
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Future Work Characterize noise H 2 O quenching of HDO emissions FM modulation instead of chopper wheel New integrated sample cell on the order of < cm 3 Measure isotopic fractionation of water Low pressure studies < 100 mtorr New detector, lasers 12347561234756
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Acknowledgements Professor Geoff Blake Vadym Kapinus Dr. Pin Chen Dr. Keith Matthews The Blake Group
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