Development of a 4.3 µm Quantum Cascade Laser based 13CO2/12CO2 Isotopic Ratio Sensor

2nd International Symposium on Isotopomers

Damien Weidmann, Chad B. Roller, Frank K. Tittel, Robert F. Curl Rice University, 6100 Main Street, Houston, TX, 77005, USA Kiyoji Uehara 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa, 223-8522, Japan Clive Oppenheimer Department of Geography, Downing Place, Cambridge CB2 3EN, U.K. Paolo DeNatale Istituto Nazionale di Ottica Applicata (INOA), LENS, Via N. Carrara, 1, Sesto Fiorentino (FI), Italy

Stresa, Italy November4-7 2003

http://www.ece.rice.edu/lasersci

Motivation for measuring

13CO /12CO 2 2

• Volcano eruption forecasting • Atmospheric studies ƒ Global warming studies ¾Temporal and spatial variations of the isotopic ratios ¾Identification of carbon sources and sinks ƒ Carbon cycle studies • Medical applications • Combustion diagnostics • Biology (Photosynthesis)

Definition of the Delta Precision Value • Isotopic ratio measurements expressed as a δ value

δ=

[13CO2] [12CO2]

[13CO2] [12CO2]

Sample

[13CO2] [12CO2]

• Current standard technique: Isotope Ratio Mass Spectrometry (IRMS) ∆δ~0.01-0.05 0/00 ƒ ƒ ƒ ƒ

Standard

Standard

ƒ Not selective enough

Standard [13C/12C]PDB = 0.011237

• Tunable Laser Absorption Spectroscopy ∆δ~0.2 0/00

CO2 Absorption Line Selection Criteria

• Advantages ƒ ƒ ƒ ƒ ƒ

• Similar strong absorption of

Fundamental bands can be targeted Provide good selectivity ~200-300 MHz Compact and robust mid IR tunable laser No need for cryogenic cooling Less complex than a DFG system

∆T =

2299.642

2.093E-20

1339

13CO2

2299.795

3.10E-20

197

2314.304

9.15E-20

942

13CO2

2314.408

1.99E-21

917

CO2 Line Selection

12CO 2

overlapping region at 2300 cm-1

∆δ kT 2 ∆E

1) Erdélyi et al. Appl. Phys. B 75, 289, 2002 2) This work 3) McManus et al., Spectr. Acta A 58, 2465, 2002

• Avoid presence of other interfering atmospheric trace gas species

Water vapor interference is depicted in pink

• Both absorption lines must lie in a laser frequency scan window

Sensitivity to Experimental Parameters

Dual path length approach

Calculations for CO2 present in volcanic fumarolic gases

• Concentration retrieval by line integration C=

∆T = 6mK !

McManus et al. 12CO2

and

lines

Focus on the 2311 cm-1 CO2 Lines

Erdelyi et al. and NCAR biocomplexity line selection

12CO2

2

ƒ The lower levels of line transition must be similar

CO2 Line Selection Comparison of different strategies Lower State Energy (cm-1)

13CO

• Temperature stability requirement

ƒ Dominant noise is pulse-to-pulse intensity fluctuations ƒ Larger laser linewidths due to thermal chirp

Intensity (cm-1/molec.cm-2)

and

13CO 2

ƒ Lead salt laser ƒ Difference Frequency Generation

ƒ Dual path length approach

• Drawbacks due to pulse mode operation

Frequency (cm-1)

12CO 2

Small mass difference problem Not real time Not field deployable Complex sample preparation and sample destruction

• Fourier Transform Spectroscopy ∆δ~0.1-0.2 0/00

• The standard is the Pee Dee Belemnite dolomite

• Required precision ∆δ = 0.1 0/00 and ∆13C~[13CO2] ∆δ

Quantum Cascade Laser: Advantages and Drawbacks

Isotopic Ratio Measurement Techniques

∫ αdν ⋅ R ⋅ T

L⋅S ⋅ NA ⋅P

∆L=2.4 µm

„

Path length tolerance

„

Pressure tolerance

„

Water vapor collision broadening

∆T = 250mK ∆P>5 mT

Our choice 12CO2

2311.105566

4.731E-19

704.3005

13CO2

2311.398738

4.936E-21

704.3340

∆T → ∞

• Dual path length approach is required • Insensitive to temperature variations

Dual path length gas cell design

• 2.4 cm path length Measurement of [12CO2]

• 2.4 m path length

⇒ Need for a water trap

Measurement of [13CO2]

QC laser based Isotopic Ratio Sensor Layout TEC IR detector

Short cell

Herriott cell

Conclusions and Future Outlook • The line pair selected for this work is insensitive to temperature variations

Short cell

Herriott cell

Ultimately the spectrometer noise itself will limit ∆δ

• Dual path length approach was selected • Compact and LN2 free-instrument • 0.1 0/00 delta precision is technically QCL

Bread board: 12x18” (30x45 cm) The sensor must be operated in a dry nitrogen atmosphere

challenging

• Future: Volcanic gas emission studies in US and Italy