A cavity ring-down spectrometer for study of biomedical radiocarbon-labeled samples

A compact cavity ring-down spectrometer aimed at the detection of trace amounts of radiocarbon (14C) in biomedical samples is demonstrated. Rapid sampling, as required for large scale studies, is made possible by modifications to a CHNS elemental analyzer. After conversion of the carbon content of a sample into CO2, spectral analysis using a high-finesse cavity in conjunction with a quantum cascade laser system reveals the ratio of 14CO2 in relation to the stable isotopologues of 12C and 13C. Cooling and temperature stabilization of the cavity is achieved by a combination of liquid cooling and thermo-electric elements. The system is studied in terms of reproducibility, linearity, and sensitivity as well as contamination and memory effects of the sampling process. While the performance of the system is still limited mainly due to etaloning and other systematic effects, first spectra of biomedical samples, such as urine and feces, have been analyzed at 14C concentration levels above ten times natural abundance. Results are compared with those from a traditional liquid scintillation counter system. Possibilities for improvement of the sensitivity are discussed.