1. National Institute of Standards and Technology
High precision 2.0 µm Photoacoustic Spectrometer for Determination of the 13CO2/12CO2 Isotope Ratio
Z.D. Reed and J.T. Hodges
National Institute of Standards and Technology

2. Photoacoustic Spectroscopy
Absorption
Excitation
Relaxation
PL =peak-to-peak laser power (W)
Bm = microphone response (V Pa-1 )
n=PA resonator response at frequency f
R = efficiency of acoustic relaxation
Detection
Sound Generation
Local Heating

3. Advantages of Photoacoustic Spectroscopy
Optically simple
High sensitivity (αmin =1 x cm-1 W)
Excellent linearity over wide range of concentration (several orders of magnitude)
Measures absorption coefficient but not scattering or extinction
Sensitivity proportional to laser power
Benefits from high powered laser sources

5. Applications of Photoacoustic Spectroscopy
Previously demonstrated for determination of atmospheric CO2 mixing ratio at 1.57 µm [1]
Oxygen A-band sensing [2]
Widely employed for concentration determination in various species (CO2, CO, HCN, NH3, etc) [3-4]
QEPAS (Quartz enhanced Photoacoustic Spectroscopy) previously employed to probe CO2 near 2.0 µm [5]
[1] Z. Reed et al App Phys B 117 (2), 2014,
[2] K.A Gillis, D.K. Havey, J.T. Hodges Rev. Sci. Instruments 81, 2010,
[3] A.A. Kosterev et al App Phys B 100, 2010,
[4] P. Patimisco, G. Scamarcio, F. Tittel, V. Spagnolo Sensors 14, 2014, 6165
[5] G. Wysoscki, A.A. Kosterev, F.K. Tittel Appl. Phys. B. 85, 301, 2006

6. 13CO2/12CO2 Isotope Ratio
Source apportionment (anthropogenic/biogenic sources), biomedical tracing and diagnosis
13CO2 is nominally % of 12CO2 concentration
Line intensities are ~100x weaker (in same band), not sufficient sensitivity to make measurement near 1.6 µm
2.0 µm CO2 line intensities are ~100x higher than those at 1.6 µm, enabling 13CO2 measurements and reduced averaging time for 12CO2

7. Stable Isotope Ratio Terminology
Carbon Pool δ
13CO2 mole fraction
PDB Standard 0‰
Terrestrial Biosphere
-26‰
Oceans and Atmosphere
-8‰
HITRAN Atmospheric Standard1
-15.8‰
1. P. De Bievre, N.E. Holden, and I.L. Barnes, “Isotopic Abundances and Atomic Weights of the Elements,” J.Phys.Chem.Ref.Data 13,  (1984).

8. Dual wavelength PAS approach
Probe selected transitions of 12CO2 near 2004nm (4990 cm-1) and 13CO2 near 1998nm (5008 cm-1)
Reducing deadtime while tuning, higher duty cycle
Decreased averaging time for 12CO2 if using strong lines near 2004nm (vs 1998nm region or 1570nm)
Ability to probe 12CO2 and 13CO2 lines with same j, similar lower state energy (temperature dependence)

10. 5x

11. 1σ = 0.13 µV
Top pane: Spectrum (symbols) and Galatry profile fit (line) of 384 ppm CO2 in air near 2004nm , measured at 250 torr and 296K
Bottom pane: Residuals of fitted spectrum. Note change of scale
Top pane: Spectrum (symbols) and GP fit (line) of 384 ppm CO2 in air near 1998nm, measured at 250 torr and 296K
Bottom pane: Residuals of fitted spectrum.

12. Measurement precision of 12CO2 near 2004nm, using a lock-in time constant of 30 ms and 0.1 sec averaging
Measurement precision of δ CO2. 12CO2 measured once per 100 seconds for 7.5 seconds, 13CO2 measured using lock-in time constant of 1 second and 7.5 sec averaging

13. Long Term Averaging
Mole fraction of 13CO2 measured with 7.5 s averaging and 1 sec lock-in time constant. Baseline correction (7.5 sec measurement) every 100 seconds.
Concentration determination uses HITRAN2012 line intensities.
Determination of δ CO2. 12CO2 measured once per 100 seconds for 7.5 seconds, 13CO2 measured using lock-in time constant of 1 second and 7.5 sec averaging

14. Dual Wavelength Photoacoustic Spectrometer
Achieves better than 0.01% precision (10 sec averaging) for 12CO2, 7 ppb limit of detection
Achieves better than 0.05‰ precision on δ (13CO2/12CO2) over long term averaging
12CO2 accuracy limited to calibration standards
Future work: calibrate δ CO2 against known 13CO2 standards (IR-MS)

15. Relaxation Dynamics Absorption Excitation Relaxation
PL =peak-to-peak laser power (W)
Bm = microphone response (V Pa-1 )
n=PA resonator response at frequency f
R = efficiency of acoustic relaxation
Detection
Sound Generation
Local Heating

16. Relaxation Dynamics Relaxation Time
Relaxation of excited molecule must be fast compared to acoustic excitation timescale
Relaxation rate varies by molecule, transition wavelength, and bath gas
Relaxation Time
11 µs atm in dry N2
0.1 µs atm in water
G. Wysocki, A.A. Kosterev, F.K. Tittel Appl. Phys. B. 85, 301, 2006

17. QEPAS Relaxation time (fmod = 32768 Hz) ≪ 30 µs
NIST PAS Relaxation time (fmod = 1630 Hz) ≪ µs
20 cm
Relative relaxation efficiency measured at 2004nm of CO2 in N2 as a function of water concentration
Tittle, F. Photonics Spectra, June 2014
G. Wysocki, A.A. Kosterev, F.K. Tittel Appl. Phys. B. 85, 301, 2006

18. Relaxation Dynamics
Normalized Photoacoustic spectrum of 398 ppm CO2 in N2
Simulated spectrum near 2.0 µm of xCO2=400ppm and xwater= 1000ppm, p=250 torr and T=296K

19. Comparison of PA spectra measured in dry N2 (left) and dry air (right)

20. Transition
Frequency
Relative Response (Dry/Wet)
R16
0.97
R18
0.94
R20
0.96
R22
0.80
R24
0.90
R26
0.84
Relative Response <1 indicates suppressed signal in dry air, greater relative enhancement due to water
Calculated water transitions (right panes) and CO2 transitions (left). Water transitions from MARVEL line list.
Tennyson et al, JQSRT 117, 2013, 29-58

21. Conclusions PAS Spectrometer operating near 2.0 µm
Achieves better than 0.01% precision (10 sec averaging) for 12CO2
Achieves better than 0.05‰ precision on δ (13CO2/12CO2) over long term averaging
PA system response shows transition specific relaxation
Calibration must be transition specific
Likely due to overlap with water transitions differentially enhancing lines at low moisture content
Currently unknown why air and N2 exhibit different behaviors
Funding: NIST Greenhouse Gas Measurements and Climate Research Program