Dual Wavelength Isotope Ratio FS-CRDS Thinh Q. Bui California Institute of Technology ISMS 2014

Motivation Stable-isotope analysis can provide valuable constraints on the global budgets of many important species: e.g. H 2 O, CO 2, CH 4, N 2 O, and CO. Valuable tool for studying kinetic isotope effects from relative rates (need <1‰ precision) For species other than CO 2, IRMS (isotope-ratio mass spectrometry) requires elaborate pre-treatment procedures that hinder in-situ applications. IR spectroscopy can alleviate problems of mass resolution and pre-treatment, as long as precision requirements can be met

Fundamental Challenges Sensitivity & Dynamic Range: measuring small variations (~ 0.1 %) in a rare isotopologue  Need extraordinary sensitivity—at least 10 5 times better than mixing-ratio measurements, and large dynamic range. Calibration: stable-isotope ratios are defined in per-mil (‰) units against ratios found in conventionally defined standards, which were not designed for convenience of laser techniques. Introduce a dual-wavelength* FS-CRDS* isotope ratio spectrometer for high precision measurements of CH 4 isotopes ( 13 C & D) *K. Uehara et al. (2001) Sensor Actuat. B 74, *L. Gianfrani et al. (2003 Opt. Express 11, 1566 *D. A. Long et al. (2011) Appl. Phys. B 105, * Y. Chen et al (2013) Anal. Chem. 85,

DFB 1 OI Input 0th 1st Servo Frequency Stabilized HeNe PD AOM DFB 2 or ECDL OI FP DDG s p s p PD1 PD2 WP FPC -meter AB TTL control computer RDC Instrumentation R = % L eff = 47km FSR = 200 MHz CW cavity ringdown specifications:  s RF switch

FS-CRDS allows for long-term averaging useful for high precision isotope analysis Spectrometer noise performance

Spectroscopy Laser 1 Automated, simultaneous acquisition of two distant spectral regions!

Advantages: 1)Arbitrary isotope spectral lines can be chosen and measured simultaneously 2)Use of a single gas sampling cell 3)Minimizes errors due to temperature dependent intensities 4)Highly precise FS-CRDS Peak Wavelengths 13 C/ 12 C Acquisition Disadvantage: 1)Short term drifts (~15 min) were observed with single wavelength measurements, caused by laser frequency drifts  transition to integrated area measurements

CH 4 Lineshapes -300 ringdown averages/wavelength -100 mTorr total pressure -99% 13 CH 4 + N 2 ZOOM IN At low pressures << 50 torr: 1)Small contribution of Dicke narrowing 2)Galatry profile is more ideal than the Voigt profile

IsotopologuePosition (cm -1 ) HITRAN Intensity (cm/molec.)E" (cm -1 ) Molec. Density (molec/cm 3 ) Sensitivity (ppb)* 12 CH x x CH 3 D x x , CH x x CH x x D/H 13 C/ 12 C Integrated Area 13 C/ 12 C and D/H Acquisition   D /  T = 4 ‰/K   13C /  T = 0 ‰/K Enriched Samples *Sensitivity in natural abundance

 Long term stability – greater than 7 hours of averaging!   D /  T = 4 ‰/K   13C /  T = 0 ‰/K D/H 13 C/ 12 C T~ 65 mK drift (close to measured cell temperature drift for one D/H measurement) Integrated Area 13 C/ 12 C and D/H Allan Deviation 14 continuous acquisitions of D/H and 13 C/ 12 C

Current Total Precision (  13 C and  D) 1) Temperature2) Isotope Ratio (integrated areas) u  = 0.119‰ (cell temperature) u  spectroscopy ~0.26‰ u D,total = 0.286‰ 1) Temperature2) Isotope Ratio (integrated areas) u  = 0.119‰ (cell temperature) u  spectroscopy ~0.11‰ u 13C,total = 0.162‰

Conclusion Methane  13 C and  D precisions of 0.162‰ and ‰, respectively were achieved with enriched samples (current design has limited dynamic range) Using integrated areas from modeling with Galatry profile was necessary for achieving highest precision Simultaneously sampling (not sequential) was necessary for averaging down Temperature drifts limits  D but not  13 C precisions, which could average > 7 hours.

Linhan Shen, Daniel Hogan, Mitchio Okumura Caltech Pin Chen Jet Propulsion Laboratory $$$ NASA & NESSF Fellowship Acknowledgements