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.
Mid-infrared laser absorption spectroscopy utilizing a high-finesse optical cavity enables high precision trace analysis of gas molecules. In particular, optical detection of radiocarbon (14C) based on cavity ringdown spectroscopy using a quantum cascade laser (QCL) is gaining attention as an alternative to accelerator mass spectrometry. This paper reports a compact-packaged narrow-linewidth QCL system utilizing resonant optical feedback from an external V-shaped cavity. Based on frequency noise analysis, the derived laser linewidth is 44 kHz for 100 μs integration time with the capability to perform seamless frequency scanning around 10 GHz. We installed this laser system within a table-top cavity ringdown spectrometer for 14CO2. A single-shot detection limit of 1.2 × 10−9 cm−1 Hz−1/2 leading to a detectable abundance evaluated from a noise analysis of 0.2 in fraction modern 14C for a 10-s averaging time was achieved. This capability of rapid analysis for 14CO2 is suitable for various applications requiring trace 14C analysis.
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