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