Intracavity Optogalvanic Spectroscopy. An Analytical Technique for ¹⁴C Analysis with Subattomole Sensitivity

Abstract

We show a new ultrasensitive laser-based analytical technique, intracavity optogalvanic spectroscopy, allowing extremely high sensitivity for detection of (14)C-labeled carbon dioxide. Capable of replacing large accelerator mass spectrometers, the technique quantifies attomoles of (14)C in submicrogram samples. Based on the specificity of narrow laser resonances coupled with the sensitivity provided by standing waves in an optical cavity and detection via impedance variations, limits of detection near 10(-15) (14)C/(12)C ratios are obtained. Using a 15-W (14)CO2 laser, a linear calibration with samples from 10(-15) to >1.5 x 10(-12) in (14)C/(12)C ratios, as determined by accelerator mass spectrometry, is demonstrated. Possible applications include microdosing studies in drug development, individualized subtherapeutic tests of drug metabolism, carbon dating and real time monitoring of atmospheric radiocarbon. The method can also be applied to detection of other trace entities.

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

“…Given that none of these claims were challenged, we must assume that Murnick has conceded them. This again challenges his own claims (Murnick et al 2008), which make no mention of these confounding factors.…”

“…Our theoretical basis is an extension of the model he espoused in his own peer-reviewed publications. Compare what we present against his own model (Murnick et al 2008). We expanded one of his products into an integral and placed rough bounds on his own otherwise unspecified term K, which "is a corresponding optogalvanic proportionality constant that depends on the details of the electric discharge" (Murnick et al 2008).…”

“…Compare what we present against his own model (Murnick et al 2008). We expanded one of his products into an integral and placed rough bounds on his own otherwise unspecified term K, which "is a corresponding optogalvanic proportionality constant that depends on the details of the electric discharge" (Murnick et al 2008). If our model is invalid because of the reasonable caveats of Bachor et al (1982), should this not also challenge his own published model?…”

Improved optogalvanic detection with voltage biased Langmuir probes

“…where I 0 is the laser intensity,  L is the absorption cross-section of the photon-molecule interaction, lr 2 is the cylindrical volume in the plasma that is exposed to the laser beam, and K is a proportionality parameter accounting for the effects that the physics and chemistry of the plasma have on the signal [13]. Studies have shown that K is not a constant and depends on a variety of parameters [19,20], wherefore Eq.…”

“…Instead, old technology has been used, and these attempts failed to spark any significant scientific or commercial interest [13,14].…”