1. SI-Traceable Scale for Measurements of Radiocarbon Concentration
Joseph T. Hodges, Adam J. Fleisher, Qingnan Liu, Abneesh Srivastava and David A. Long
Chemical Sciences Division
National Institute of Standards and Technology
Gaithersburg, MD
International Symposium on Molecular Spectroscopy, 72nd meeting June 19-23, 2017, Champaign-Urbana, IL

2. Absolute International Standard Activity (AISA) for 14C
The internationally accepted radiocarbon dating reference value is 95 % of the radioactivity, in AD 1950, of the National Bureau of Standard (NBS) oxalic acid (OX-I), normalized to d13C = -19 ‰ with respect to Vienna Pee Dee Belemnite (VPDB).
specific radioactivity in 1950 of NBS oxalic acid (OX-I) 1
AOX-I = decays/(min g)
Definition of 1 “Modern” unit of specific radioactivity
AMod = 0.95*AOX-I = decays/(min g)
1. Karlen et al. Arkiv For Geofysik, 4(22), , (1964)
C2H2O4
Oxalic Acid

3. Physical Basis for the Modern 14C Scale
AOX-I Radioactivity was compared to pre-industrial-era wood:
Oxalic acid source was fermented beets grown in 1955:
standard made in 1958 and nearly exhausted by 1978:
also known as NBS Standard Reference Material
(4990, 4990A, 4990B):
A 1-lb sample remains, the original lot cannot be reproduced:
AOX-II made from fermented French beet molasses grown in 1977:
455 kg lot was made and characterized:
also known as NIST SRM (4990C):
Its specific radioactivity and d13C content were measured and compared to AOX-I
AOX-II/AOX-I = (4) (d13C)AOX-II - (d13C)AOX-I = 1.49(5) ‰
Approximately 30 % (300 lb) of the AOX-II remains. At the current consumption rate, it is projected last for 40 years.

4. ur (specific radioactivity) = 0.5 %
OX-I

5. NIST Oxalic Acid Samples
OX-I
SRM 4990 B
1 lb remaining
OX-II
SRM 4990 C
50 lb remaining

6. Mole-Fraction-Equivalent
of 1 “Modern” 14C unit
x14C,Mod = 0.95AOX-It MC/NA= 1.18×10-12
AOX-I = Specific radioactivity of NBS Oxalic Acid (OX-I) in 1950 AD
MC = Avg atomic weight of OX-I sample for d13C = -19 ‰, given by g mol-1
= 1/e decay time of 14C = 8267 yr : relative uncertainty = 0.7 %.
NA = Avogadro Constant

7. Combined uncertainty in x14C,Mod scale*
ur (AOX-I) = 0.5 % specific radioactivity
ur (t) = 0.7 % 1/e decay time of 14C
ur(x14C,Mod) = ( )1/2 = 0.86 %
*x14C,Mod = 0.95AOX-It MC/NA= 1.18×10-12

8. Limitations of Existing x14C Scale
Absolute uncertainty of 0.86 %
Based on a reference material which is nearly consumed.
Need to correct for time based on t
Will need to link future-generation reference materials to AOX-I and/or AOX-II, which could lead to scale variability
Direct dating is subject to systematic uncertainty in the 14C decay rate (0.7 %) although values are calibrated against tree-ring and coral/shell historical records

9. An Alternative Approach
for a 14C Standard
Method:
Measure 14C16O2 P20 n3-band peak area using cavity
ring-down spectroscopy (CRDS) in the linear regime, with x & y
spectrum axes directly linked to measurements of time & frequency.
Use ab initio calculated value of line intensity to “scale” the measured
peak area, thus precluding the need for calibration against a sample
of known mole fraction

10. Frequency-stabilized cavity ring-down spectroscopy (FS-CRDS)
cw probe laser
freq-stab. ref las ca
cavity stabilization servo
pzt
Ring-down cavity
decay signal
1/(c t) = a0 + a(n)
time
frequency
frequency
absorption spectrum
stabilized
resonant
frequencies
FSR

11. Methodology for ab initio line parameter calculations
L. McKemmish, University College London

12. Linking CRDS to the international system of units (SI)
Ring-down cavity
Probe laser
Optical frequency comb
Cs Clock
Probe laser servo
I2-stabilized HeNe laser
Cavity length servo
Photoreceiver
& digitizer
Primary pressure standards
Primary temperature
standards
Calibrated thermometers
Calibrated manometers
Frequency ctr t n

13. Spectroscopic Measurement of Absorber Concentration & Mole Fraction
Calculated line intensity
Fitted area
absolute
concentration
(molec. vol-1)
mole fraction
(mol mol -1)
Peak area obtained by fitting line profile to measured spectrum

14. Conversion from 14C16O2 mole fraction to x14,C
Isotopologue (x/x646)HITRAN
646: 16O14C16O 1
648: 16O14C18O 4.01x10-3
647: 16O14C17O 7.46x10-4
848: 18O14C18O 4.02x10-7
748: 17O14C18O 1.49x10-7
747: 17O14C17O 1.39x10-7
From spectroscopic
area measurement
< 0.005
Conversion is weakly dependent on d18O value of the CO2 sample

15. Advantages of Spectroscopic 14C Quantification
Absolute scale, linked to measurements of time, frequency,
temperature and pressure and quantum-chemical calculations of
dipole moment of probed 14C16O2 transition.
Independent of the definition of “Modern” background 14C
Realizable from laboratory-to-laboratory without the need for
reference samples.
Lower combined relative uncertainty than that of “Modern” scale
Independent of the half-life of 14C and could be used to reduce
the uncertainty of t
Increased dynamic range (enriched mixtures to ‰-level
depleted samples)

16. CRDS Measurements of 14CO2
Galli, I., Bartalini, S., Borri, S., Cancio, P., Mazzotti, D., De Natale, P., Giusfredi, G., Molecular Gas Sensing Below Parts Per Trillion: Radiocarbon-dioxide Optical Detection. Phys. Rev. Lett. 2011, 107,
Galli, I., Bartalini, S., Ballerini, R., Barucci, M., Cancio, P., De Pas, M., Giusfredi, G., Mazzotti, D., Akikusa, N., De Natale, P., Spectroscopic Detection of Radiocarbon Dioxide at Parts-per-quadrillion Sensitivity. Optica 2016, 3,
Genoud, G., Vainio, M., Phillips, H., Dean, J., Merimaa, M., Radiocarbon Dioxide Detection Based on Cavity Ring-down Spectroscopy and a Quantum Cascade Laser. Opt. Lett. 2015, 7,
McCartt, A. D., Ognibene, T. J.,Bench, G., Turteltaub, K. W. ,Quantifying Carbon-14 for Biology using Cavity Ring-down Spectroscopy. Anal. Chem. 2016, 88,
Fleisher A.J., Long D.A., Liu Q., Hodges J.T., Towards the Robust Detection of Radiocarbon via Linear Absorption Spectroscopy, CLEO 2017, San Jose CA, May 2017.
Long D.A., Fleisher A.J. Liu Q., Hodges, J.T., Optical Detection and Quantification of Radiocarbon Dioxide (14CO2) at and below Ambient Levels. Talk MJ10, ISMS 2017.
Non-linear (SCAR)
<< 1 Modern
>> 1 Modern
>> 1 Modern
Linear < 1 Modern

17. Thank you for your attention!