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

High-precision measurements of radiocarbon (C-14) near or below a fraction modern C-14 of 1 ((FC)-C-14 <= 1) are challenging and costly. An accurate, ultrasensitive linear absorption approach to detecting C-14 would provide a simple and robust benchtop alternative to off-site accelerator mass spectrometry facilities. Here we report the quantitative measurement of C-14 in gas-phase samples of CO2 with (FC)-C-14 < 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 (FC)-C-14 < 1. With a combined uncertainty of C-14/C-12 130 fmol/mol ((FC)-C-14 = 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.