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Dual Wavelength Isotope Ratio FS-CRDS
Thinh Q. BuiCalifornia Institute of Technology
ISMS 2014
Motivation
• Stable-isotope analysis can provide valuable constraints on the global budgets of many important species: e.g. H2O, CO2, CH4, N2O, and CO.
• Valuable tool for studying kinetic isotope effects from relative rates (need <1‰ precision)
• For species other than CO2, 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 105 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 CH4 isotopes (13C & D)
*K. Uehara et al. (2001) Sensor Actuat. B 74, 173-178
*L. Gianfrani et al. (2003 Opt. Express 11, 1566
*D. A. Long et al. (2011) Appl. Phys. B 105, 471-477
* Y. Chen et al (2013) Anal. Chem. 85, 11250-11257
DFB 1 OI
Input0th
1st
Servo
Frequency Stabilized HeNe
PD
AOMDFB 2 or ECDL
OI
FP
FP
DDG
s
p
s
p
PD1
PD2
WP
FPC
l-meter A BTTL control
computer
RDC
Instrumentation
R = 99.9995%Leff = 47kmFSR = 200 MHz
CW cavity ringdown specifications:
ms RF switch
FS-CRDS allows for long-term averaging useful for high precision isotope analysis
Spectrometer noise performance
Spectroscopy Laser 1
Laser 2
Automated, simultaneous acquisition of two distant spectral regions!
Advantages:1) Arbitrary isotope spectral lines can be chosen and measured simultaneously2) Use of a single gas sampling cell3) Minimizes errors due to temperature dependent intensities4) Highly precise FS-CRDS
Peak Wavelengths 13C/12C Acquisition
Disadvantage:1) Short term drifts (~15 min) were observed with single wavelength measurements,
caused by laser frequency drifts transition to integrated area measurements
p.]rare isotoisotop.]/[ [primary where R
, R
RRδX(‰)
i
S
SX
1000
CH4 Lineshapes
-300 ringdown averages/wavelength-100 mTorr total pressure-99% 13CH4 + N2
ZOOM IN
At low pressures << 50 torr:
1)Small contribution of Dicke narrowing
2)Galatry profile is more ideal than the Voigt profile
Isotopologue Position (cm-1)HITRAN Intensity (cm/molec.) E" (cm-1 )
Molec. Density (molec/cm3) Sensitivity (ppb)*
12CH4 6006.06590 6.060x10-24 293.1542 9.05x1014 12 12CH3D 6457.03268 8.149x10-27 46.554 1.89x1014 16,373 12CH4 6004.86265 3.245x10-22 10.4817 4.78x1013 0.6 13CH4 6008.46523 5.793x10-24 10.4821 2.04x1013 25
D/H 13C/12C
Integrated Area 13C/12C and D/H Acquisition
sdD / sT = 4 ‰/K sd13C / sT = 0 ‰/KEnriched Samples
*Sensitivity in natural abundance
Long term stability – greater than 7 hours of averaging!
sdD / sT = 4 ‰/K sd13C / sT = 0 ‰/K
D/H 13C/12C
T~ 65 mK drift (close to measured cell temperature drift for one D/H measurement)
Integrated Area 13C/12C and D/H Allan Deviation
14 continuous acquisitions of D/H and 13C/12C
Current Total Precision (d13C and dD)
1) Temperature 2) Isotope Ratio (integrated areas)
u ,d T = 0.119‰ (cell temperature)
u ,d spectroscopy~0.26‰
uD,total = 0.286‰
1) Temperature 2) Isotope Ratio (integrated areas)
u ,d T = 0.119‰ (cell temperature)
u ,d spectroscopy ~0.11‰
u13C,total = 0.162‰
Conclusion
• Methane d13C and dD precisions of 0.162‰ and 0.286 ‰, 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 dD but not d13C precisions, which could average > 7 hours.
Linhan Shen, Daniel Hogan, Mitchio Okumura
Caltech
Pin Chen
Jet Propulsion Laboratory
$$$ NASA & NESSF Fellowship
Acknowledgements