Carson et al. (2016) have measured the optogalvanic response of an intracavity cell discharge containing carbon dioxide enriched in radiocarbon in a 14CO2 laser, and compared same to an unenriched sample. The measurement was carried out by modulating the laser wavelength while slowly tuning through the laser gain profile. The results of the measurements are claimed to “invalidate the optogalvanic method for radiocarbon detection.” A broadband linear absorption model is presented in support of this hypothesis. In fact, the experimental design was such as to minimize any possibility for 14C detection, and the model presented is not relevant to their experiment. Crucial control measurements were not carried out and the model used did not differentiate between broadband absorption spectroscopy and intracavity optogalvanic spectroscopy (ICOGS) with a narrow-band single-mode CO2 laser.

Our results, when taken alongside those of two other research groups, appear to invalidate the early positive results for intracavity optogalvanic spectroscopy (ICOGS) and its detection of radiocarbon. Daniel Murnick, whose lab published those early results, has directed a comment setting out the perceived shortcomings of our article and the reasons that the results do not constitute invalidation, as our article claims. Absent support from his own findings or published literature, his various counterclaims regarding our experimental procedure appear to constitute only his considered professional opinion and the largest confounder highlighted by our research is not even mentioned. The preponderance of documented evidence regarding ICOGS clearly supports the null hypothesis and in our opinion, nothing in Murnick’s comment indicates otherwise.