Optical Measurement of Radiocarbon below Unity Fraction Modern by Linear Absorption Spectroscopy

Abstract

High-precision measurements of radiocarbon (14C ) near or below a fraction modern 14C of one (F14C ≤ 1) are challenging and costly. An accurate, ultra-sensitive linear absorption approach to detecting 14C would provide a simple and robust bench-top alternative to off-site accelerator mass spectrometry facilities. Here we report the quantitative measurement of 14C in gas-phase samples of CO2 with F14C < 1 using cavity ring-down spectroscopy in the linear absorption regime. Repeated analysis of CO2 derived from the combustion of either biogenic or petrogenic sources revealed a robust ability to differentiate samples with F14C < 1. With a combined uncertainty of 14C /12C = 130 fmol/mol (F14C = 0.11), initial performance of the calibration-free instrument is sufficient to investigate a variety of applications in radiocarbon measurement science including the study of biofuels and bioplastics, illicitly traded specimens, bomb dating, and atmospheric transport.

Laser‐based radiocarbon detection in the laboratory: How soon?

“…A less complex cavity ring‐down system of Genoud et al using a commercially available quantum cascade laser with line width in the tens of MHz range achieved a sensitivity limit of ~40 M, which is of value for monitoring nuclear power plants and other industrial applications. McCartt et al, with a somewhat more complex spectrometer, have achieved a detection limit near 0.2M with 3‐mg sample size by cooling the sample to 250 K, and Fleisher et al have made other improvements to better determine backgrounds using 18‐mg samples at 220 K.…”

“…6 These methods, however, are not well suited to field or online measurements leading to recent research with laser-based techniques. [7][8][9][10][11] The extension of laser-based carbon isotope ratio analysis to radiocarbon, 14 C, was first proposed in 1981 12 but was not viable with the technology available at that time. This was because 14 C has a very low natural abundance, near 1 ppt, requiring both very narrow band lasers for specificity, as well as efficient detection mechanisms for enhanced sensitivity.…”

“…Examples are given in previous studies. 10,11,15 Further, the absorption spectra are based on transitions from the ground state. Interestingly, the CRDS work in 14 CO 2 could only be carried out recently due to technological advances on metrology and stabilization of continuously tunable extremely narrow band lasers over a relatively large wavelength range.…”

Background Noise Reduction in Mid-Infrared Cavity Ring-Down Spectroscopy for Radiocarbon Analysis

“…However, complexity and high cost in AMS may prevent these studies from wide applications. In this context, cavity ring-down spectroscopy (CRDS) using a fundamental vibration absorption line of carbon dioxide in 4.5 μm has been proposed as a substitute for AMS [4][5][6][7][8]. 14 C analysis based on CRDS especially designed for biomedical samples in microdose study has been developed [9].…”

Determining Biogenic Content of Biogas by Measuring Stable Isotopologues 12CH4, 13CH4, and CH3D with a Mid-Infrared Direct Absorption Laser Spectrometer

“…Indeed, in recent years, a number of groups have published spectroscopic approaches for radiocarbon measurement. Detection limits approaching [8] and exceeding [9,10] ambient 14 C levels have been reported. However, at present, these techniques are restricted to laboratory use only.…”