Optical ¹⁴C Tracing for Biological and Pharmaceutical Applications Using Two-Color Cavity Ringdown Spectroscopy

Laser-based ¹⁴C quantitation has been proposed as a more affordable, higher-throughput, table-top alternative to accelerator mass spectrometry (AMS). Here, we demonstrate the feasibility of a mid-IR ¹⁴C detector based on two-color cavity ringdown spectroscopy (2C-CRDS) for low-level ¹⁴C isotope tracing in biological studies. The 2C-CRDS technique quantifies the sample ¹⁴C content by measuring the ¹⁴CO₂ absorption signals from the combusted samples with mid-IR lasers. With 2C-CRDS, we previously demonstrated the most sensitive and accurate optical measurements of ¹⁴CO₂. The current detection sensitivity and quantitation accuracy of the instrument, at a few parts per quadrillion (where a quadrillion = 10¹⁵) ¹⁴C/C mole fraction, is competitive against AMS. Here, by applying the 2C-CRDS ¹⁴C sensor to two applications relevant to ¹⁴C-labeled biochemical analysis and pharmaceutical studies, we demonstrate sub-fCi level (where 1 fCi = 10^-15 Ci) quantitation of sample ¹⁴C activity, with a minimum sample-size requirement of 3 mg of carbon. The current measurement throughput, ∼25 min/sample, is largely limited by the sampling efficiency of the online combustion and CO₂ processing interface to the 2C-CRDS instrument. The possibility of a significantly improved measurement throughput of a few minutes per sample is suggested by the results of a flow-through ¹⁴CO₂ sampling scheme. This improved measurement efficiency, combined with the relatively low cost and compact size of a 2C-CRDS sensor, could potentially revolutionize high-sensitivity ¹⁴C tracing in biological, pharmaceutical, and clinical studies.