Intracavity optogalvanic spectroscopy: Is there any evidence of a radiocarbon signal?

Abstract

In 2008, the first report of an ultrasensitive method for ro-vibrational spectrometry of radiocarbon dioxide was published. The method, called intracavity optogalvanic spectroscopy (ICOGS), claimed a sensitivity and limit-of-detection comparable to accelerator mass spectroscopy. ICOGS was claimed to utilize the isotope-dependent ro-vibrational absorption lines of carbon dioxide in the infrared spectrum. In order to facilitate unambiguous detection of radiocarbon, the sample was placed inside the cavity of a radiocarbon dioxide laser. This intracavity approach was claimed to increase the sensitivity by seven orders of magnitude compared with traditional optogalvanic methods. However, despite the methodical and thorough efforts of several research groups worldwide, these claims have not been possible to reproduce. Instead, we have previously reported serious deviations from the original results, where we found that ICOGS suffers from considerable problems with the stability and reproducibility of the optogalvanic signal, and that misinterpretations of these uncertainties likely are the explanation for the claimed sensitivity in the first reports. Having identified the stability and reproducibility of the detection as major concerns, we decided to improve the setup by with state-of-the-art plasma source technology. Deploying a custom-made stripline split-ring resonator optogalvanic detector, we have now investigated the applicability of ICOGS to radiocarbon detection even further. Measurements have been made with a wide range of parameters including different gas mixtures at various pressures and wavelengths.We have also conducted measurements with gas flowing through the sample cell to investigate the effect of plasma induced decomposition of the sample. Still, we have seen no indications of a significant radiocarbon signal in a concentration range between 0.29 Modern and 9.7 Modern, i.e., the range of interest to the radiocarbon community. Hence, our conclusions after four years of working in this field, is that ICOGS is not a viable method for radiocarbon detection.

Optogalvanic spectroscopy with microplasma sources—current status and development towards a lab on a chip

“…Through the years, we have employed several different designs, materials and fabrication methods when making our split-ring resonator plasma sources [8][9][10][12][13][14]. For example, both microstrip and stripline waveguides have been fabricated on, e.g., silicon and Pyrex, using standard MST processes such as sputtering, electroplating, and wet and reactive ion etching.…”

“…In this paper, we present results obtained using the 12 CO 2 laser, more precisely a Merit SL laser from Access Lasers Inc. Because of the CO 2 lasers, the experiments have focused at spectroscopic studies of pure CO 2 or mixtures of CO 2 and N 2 . Moreover, the experiments have been conducted at pressures in the 10 to 1000 Pa range with ( [12]) or without ( [5][6][7]) a flow through the SSRR. In this paper, results from both configurations are presented.…”

Comment on “Intracavity OptoGalvanic Spectroscopy Not Suitable for Ambient Level Radiocarbon Detection”

“…Most importantly, it fills a void in the aggregated literature on ICOGS [1][2][3][4][5][6] , namely the study of highly enriched 14 CO 2 samples. Hence, it is an important contribution to the field, and well worthy of publication in Analytical Chemistry.…”

Response to “Comment on ‘Invalidation of the Intracavity Optogalvanic Method for Radiocarbon Detection’” by Daniel E Murnick

“…His first criticism is that we should have demonstrated unequivocally that his earlier results could not be reproduced under identical conditions. We felt that this had already been accomplished in two different laboratories, at Groningen University and at Uppsala University (Persson et al 2013; Paul and Meijer 2015; Persson and Salehpour 2015), but he arbitrarily disregards their findings. For what reason should we also disregard the invalidating results from other laboratories when no evidence is produced to discredit them?…”