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 joseph.hodges@nist.gov International Symposium on Molecular Spectroscopy, 72nd meeting June 19-23, 2017, Champaign-Urbana, IL

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 = 14.27 decays/(min g) Definition of 1 “Modern” unit of specific radioactivity AMod = 0.95*AOX-I = 13.56 decays/(min g) 1. Karlen et al. Arkiv For Geofysik, 4(22), 465-571, (1964) C2H2O4 Oxalic Acid

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 = 1.2933(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.

ur (specific radioactivity) = 0.5 % OX-I

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

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 12.0109 g mol-1 = 1/e decay time of 14C = 8267 yr : relative uncertainty = 0.7 %. NA = Avogadro Constant

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) = (0.52 + 0.72)1/2 = 0.86 % *x14C,Mod = 0.95AOX-It MC/NA= 1.18×10-12

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

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

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

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

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

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

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

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)

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, 270802. 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, 385-388. 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, 1342-1345. 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, 8714-8719. 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

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