Lawrence Livermore National Laboratory

Upon the adoption of Accelerator Mass Spectrometry (AMS) to the measurement of carbon-14, scientists realized the possible implications to biomedical applications. Since this time, AMS has facilitated biomedical studies where sensitive detection of carbon-14 is required. However, AMS’ complexity, cost, and limited throughput have become a bottleneck for biomedical studies.

Together with other scientific applications of carbon-14, biomedical research has driven interest in the development of a laser based method to quantify concentrations of carbon-14. Several groups have explored the possibility of measuring carbon-14 with laser spectroscopy. There have been several efforts to develop alternative means of carbon-14 detection. Cavity ring-down spectroscopy (CRDS) has emerged as a laser-based method capable of carbon-14 measurements. The most sensitive of these systems utilized laser locking to the ring-down cavity and a saturated-absorption cavity ring-down (SCAR) spectroscopic technique. While this demonstrates the efficacy of laser based carbon-14 measurements, the complexity of the laser system used prohibits its ease of distribution to biomedical labs.

We are developing an instrument that balances complexity and sensitivity to enable biomedical studies. Using simple, robust hardware, this laser-based instrument would not replace but would supplement AMS in biomedical studies. It uses robust, mature hardware suitable for turnkey operation. The major components include a quantum cascade laser (QCL), high-finesse optical cavity, mid-IR detector, and a cascade refrigeration system. All of these items were sourced from commercial vendors. The data acquisition and other CRDS specific hardware are from Picarro Inc., a commercial vendor of CRDS systems. Many modifications were made to the measurement and control software, but these changes would not inhibit broad utilization.

Schematic of basic CRDS experimental setup

Schematic of basic CRDS experimental setup, which is representative of the experimental hardware used in the instrument. Starting from the laser, the emission wavelength is determined by an etalon-based wavelength monitor, and the beam is mode matched to the cavity with optical components. Laser light is then injected into the high-finesse cavity. The amount of light in the cavity is monitored by a photodetector positioned along the path of the laser light exiting the cavity. When the light measured by the photodetector exceeds a threshold, the laser is turned off. The decay or “ring-down” of the laser light signal is then measured by the photodetector.

Example ring-down event

Example ring-down event. The black decay represents an empty cavity, while the red represents a decay with additional sample loss. These ring-down times represent the loss in the cavity system. The light measured on the photodetector can be considered equivalent to the amount of light circulating in the cavity. For each round trip in the cavity, the circulating light experiences a fractional loss from the test gas and/or mirrors.

Simple CRDS instruments could enable the proliferation of carbon-14 tracer techniques to aid the biomedical community. Our tests suggest a tabletop-sized CRDS system can now conduct most biological studies previously performed with AMS. Indeed, the availability of simple and inexpensive carbon-14 labeling technologies in labs and hospitals could be transformative. Carbon-14 labeling is a proven method that elegantly and quantitatively traces target species through biochemical systems, and it is highly versatile, as almost all organic compounds can be labeled with carbon-14. Carbon-14 detection with CRDS can provide access to this simple yet powerful technique for tracking organic compounds through biochemical systems.