Invalidation of the intracavity optogalvanic method for radiocarbon detection

The intracavity optogalvanic spectroscopy (ICOGS) method has been reported to quantify radiocarbon at subambient levels (<1 part per trillion). ICOGS uses a gas sample that is ionized in a low-pressure glow discharge located inside a ¹⁴CO₂ laser cavity to detect changes in the discharge current under periodic modulation of the laser power to determine the ¹⁴CO₂ concentration of the sample. When claims of detection thresholds below ambient levels were not verified by other researchers, we constructed a theoretical analysis to resolve differences between these conflicting reports and built and tested an ICOGS system to establish a lower limit of detection. Using a linear absorbance model of the background contribution of ¹²CO₂ and data from the HITRAN database, we estimate that the limit of detection (3σx) is close to 1.5 ×10⁴ Modern. By measuring a 1.5×10⁴ Modern enriched CO₂ sample in a cavity modulation ICOGS system without a clear signal, we conclude that for this system the limit of detection for ICOGS must be above 1.5 ×10⁴. The implications for previous ICOGS reports are discussed.