1. Linhan Shen1, Thinh Bui1, John Eiler2, Mitchio Okumura1
MEASUREMENTS OF DOUBLY-SUBSTITUTED METHANE ISOTOPOLOGUE ABUNDANCE USING FREQUENCY STABILIZED MID-IR CAVITY-RINGDOWN SPECTROSCOPY
Linhan Shen1, Thinh Bui1, John Eiler2, Mitchio Okumura1
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA

2. Methane Isotopologue Abundances
Cardinal Mass
Isotopologue
Relative Abundance
Exact Mass 16
12CH4
98.8%
16.031 17
13CH4
1.11%
17.035
12CH3D
616 ppm
17.038 18
13CH3D
6.92 ppm
18.041
12CH2D2
144 ppb
18.044
Other
13CH2D2, 12CHD3, 13CHD3, 12CD4, 13CD4
1.64 ppb --
Carry Important Molecular Information about the Formation Conditions of Methane (e.g. Temperature)

3. Current State of Clumped Isotope Methane Measurements
Isotope Ratio Mass Spectrometry (IRMS)
The Panorama
Mat 253-Ultra
Eiler Group (Caltech)
Young Group (UCLA)
Limitations of IRMS Measurements
Difficult to differentiate 13CH3D and 12CH2D2
Long averaging time in order to reach < 1‰ precision
High cost and large size
Isotopologue
Precision (‰)
Time Average
13CH3D
0.43
4 hours
13CH3D + 12CH2D2
0.3
Isotopologue
Precision (‰)
Time Average
13CH3D
0.2
26 hours
12CH2D2
0.7
Courtesy of the John Eiler Group

4. Current State of Clumped Isotope Methane Measurements
Tunable Infrared Laser Direct Absorption Spectroscopy (TILDAS)
Measure abundance of 13CH3D
Astigmatic Herriott Cell
0.2‰ precision with 0.42 mmol of pure methane.
Limitations of TILDAS Measurements
Can only measure 13CH3D
Requires large amount of sample
S. Ono, et al. Anal. Chem., 2014, 86 (13),

5. Goals and Difficulties
Measure abundance of 12CH2D2 to less than 1‰ precision.
Difficulties:
Small abundance
Large interferences from higher abundant isotopologues
Hard to standardize

6. Mid-IR FS-CRDS Spectrometer Tuning Range (2200 cm-1 – 2300 cm-1)
L ~ 150cm
4.5 μm QCL
Tuning Range (2200 cm-1 – 2300 cm-1)
InSb Detector
PZT
AOM
Avalanche Detector
FS-HeNe
λ/4
125 MHz
Si Detector
λ/2
Polarizer
EOM
Servo

7. Mid-IR FS-CRDS Spectrometer
For 12CH2D2 detection at ~ 2268 cm-1:
δτ/τ ~ 0.2 %
αmin ~ 4.7 × cm-1
Data acquisition rate ~ 50 Hz
NEA ~ × 10-9 cm-1Hz-1/2

8. Mid-IR Methane Lineshapes
12CH3D
Pressure: 20 Torr
Pure Methane
Natural Abundance
Line Center: cm-1
E” = cm-1

9. Mid-IR Methane Lineshapes
12CH3D
Pure Methane
Natural Abundance
Center: cm-1
E” = cm-1

10. 12CH2D2 Abundance Measurements
Goal: Measure δ12CH2D2
[12CH2D2]/[12CH4]
Very low Abundance
Unassigned line strength
Difficult to standardize
[12CH3D]/[12CH4] Can be easily measured
[12CH2D2]
[12CH2D2]/[12CH3D]
Higher Abundance
Assigned line strength
Can be calibrated with IRMS Measurements
[12CH3D]

11. Spectra of Methane Isotopologues
Simulation Conditions:
1 Torr Pure Methane
HITRAN 2012
Kitt Peak Data: Courtesy of JPL High Resolution Spectroscopy Group (Keeyoon Sung, Linda Brown)

12. Line Selections for [12CH2D2]/[12CH3D] Measurements
HITRAN Simulation Conditions: 1 Torr Pure Methane
Kitt Peak Line Intensities Unassigned.
HITRAN 2012
Kitt Peak Data: Courtesy of JPL High Resolution Spectroscopy Group (Keeyoon Sung, Linda Brown)

13. Pure Methane 12CH3D and 12CH2D2 Measurements
Pressure: 5 Torr
Pure Methane
Natural Abundance
12CH3D
Line Center: cm-1
E” = 58.8 cm-1
With known line strength, [12CH3D] can be derived.
Galatry Profile Fit
SNR = 300:1

14. Pure Methane 12CH3D and 12CH2D2 Measurements
Pressure: 5 Torr
Pure Methane
Natural Abundance
Unassigned 12CH2D2 Line
Identified from Kitt Peak FTIR Data
Line Center: cm-1
Galatry Profile Fit
SNR = 85:1

15. Natural Abundant 12CH2D2 Measurement
Total Average Time: 1.5 Hours
Comparable to 26 hours of averaging on IRMS.

16. Future Work What we need to reach [12CH2D2]/[12CH4] measurement?
Calibrate 12CH3D abundance measurement with IRMS
Calibrate 12CH2D2 abundance measurement using an equilibrated pure methane sample with known isotope abundances.
What we need to reach δ12CH2D2 measurements?
Improve sample input system to avoid fractionation.
Calibrate δ12CH3D and δ13CH3D measurements with IRMS.

17. Acknowledgement Funding:
Student Fellowships: Linhan Shen and Thinh Q. Bui are both supported by NASA NESSF fellowship.
Support:
David Long for providing us ringdown mirrors
JPL high resolution spectroscopy group for providing us FTIR spectra of 13CH3D and 12CH2D2.