Optical detection and quantification of radiocarbon dioxide (

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Optical detection and quantification of radiocarbon dioxide (




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Presentations text content in Optical detection and quantification of radiocarbon dioxide (

Slide1

Optical detection and quantification of radiocarbon dioxide (14CO2) at and below ambient levels

D. A. Long, A. J. Fleisher, Q. Liu, and J. T. HodgesNational Institute of Standards and TechnologyGaithersburg, Maryland

HDR Architecture, Inc.

Slide2

Long-lived isotope of carbon (half-life 5,730 years)Used in carbon datingPresent abundance is 1.2 parts-per-trillion (ppt)Fossil fuels and their products are nearly entirely depleted in 14C

14C

Slide3

BioplasticsBiofuels and biofuel blendsIdentify illicitly traded specimensAtmospheric CO2 source apportionmentRadiocarbon dating

Why measure 14C?

Slide4

Current method – AMS

Measurements of

14

C are extremely difficult due to low natural abundance (~1.2

ppt

)

AMS uses an accelerator to mass separate the

analyte

Then analyzed using mass spectrometry

High precision (generally 0.2-3%)Disadvantages:-Expensive-Requires a large facility and highly trained staff -Very limited number of facilities-Can’t be performed in situ-Sample destructive

image from LLNL

Slide5

14CO2 transitions are shifted relative to 12CO2Allows for spectroscopic measurements of 14CO2 in the mid-infrared

Optical measurements have the potential for rapid, high precision, in situ measurements without complicated sample preparations.12CO

214CO2

14

CO

2

Zoom in

60,000,000,000X

Optical measurements of

14CO

2

Slide6

The maximum 14CO2 concentration we will observe is 1.2 ppt.At our sample pressure of 7 Torr this corresponds to only 4×106 14CO2 molecules in the laser beam.Over a single pass of our 1.5 m path at most 2.7×10

-8 of the light is absorbed by the 14CO2.In addition, there are spectral interferences from a wide range of other CO2 isotopes and N2O. Finally, the mid-infrared laser technology is significantly less advanced than in the more common telecom region (i.e., near 1.5 µm).Due to these challenges, optical detection at ambient levels has been demonstrated by only three research groups (CNR-Italy, NIST, LLNL).

Why is this so challenging?

Slide7

InSb PD

Cavity ring-down spectroscopy

Optical Cavity

Wavemeter

AOM

DFB

QCL

λ/2

Lens

L3

TEM

00

A. J. Fleisher et al.,

Phys. Rev. A,

(2016).

D. A. Long et al.,

Opt. Lett

., (2016).

Pol.

Pol.

Lens

Slide8

The need for low temperatures

13CO214CO2

Slide9

M2

PR

1.5 m long2.5 L gas volume

stabilized against temperature drifts via 4 invar rods

MHz in 1 hour

 

CLR

Slide10

D. A. Long et al., Opt. Lett. 41, 1612 (2016)

CRDS performance

Slide11

14CO2

13CO2

13CO2 andN2O

Biogenic CO

2

Petrogenic CO

2

CO

2

source identification

Slide12

Robust sample identification

Slide13

±6σ confidence bounds360 total spectra recorded over 14 daysBiogenicF14C = 0.74 ± 0.08

PetrogenicF14C = −0.31 ± 0.17Distribution functions

Slide14

Combined F14C = 8.1 %N = 40 scansT = 47 minutes14C/12C = 96 fmol/

molor96 parts-per-quadrillion (ppq)Uncertainty budget and sensitivity

Slide15

Developed a low temperature (200 K) cavity ring-down spectrometer in the mid-infrared.Have measured the 14C content of fossil fuel and biofuel samples.After 1 hour can reach a measurement precision < 0.1 ppt.

Instrument cost is < $100k.Conclusions

Slide16

Roger van Zee, Keith Gillis (NIST)Charles Miller (JPL)NIST Innovation in Measurement Science (IMS) AwardNIST Greenhouse Gas Measurements and Climate Research ProgramAcknowledgements

Slide17

Slide18

System improvementsImproved supermirrors with lower absorption and transmission lossesLengthen averaging time and increase acquisition rate by improving laser frequency stability

Improved thermal isolation of cold cell from the optical tableNew design of the cold cell with a significant reduction in the observed radial heat transferGas separation methods to remove N2O interference


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