One of the newer options for analyzing biological samples is a laser-based approach, known as two-color cavity ringdown spectroscopy. This optical detection technique—developed at LLNL—uses two lasers to analyze radiocarbon dioxide (14CO2) concentrations in room-temperature carbon dioxide samples.
Due to its built-in background compensation, the two-color CRDS technique offers higher detection sensitivity, selectivity, and quantitation accuracy than single-laser-based analytical methods, including the saturated absorption cavity ringdown (SCAR) technique. In addition, because 2C-CRDS operates at room temperature, and because it offers superior selectivity as compared to single-laser-based methods, the 2C-CRDS instrument is much smaller, less complex, and less expensive than a biological Accelerator Mass Spectrometry (bioAMS) instrument. Thus, it could potentially pave the way for wider adoption of 2C-CRDS outside a specialized laser laboratory, such as the lab where it is housed at the bioAMS facility.
How it works
2C-CRDS is a highly sensitive, absorption-based detection method that enables accurate quantification of trace-gas concentrations in as little as 5 minutes. Its quantitation accuracy is as low as 4 parts-per-quadrillion (ppq), making it the most sensitive and accurate spectroscopic measurement in the mid-infrared spectral range.
For biological studies utilizing carbon-14, carbonaceous analytes are combusted into carbon dioxide and introduced into the optical cavity. A measurement starts by coupling resonant laser light into the optical cavity that contains the test gas. This light is then interrupted, and an exponential decay, or ring-down, is recorded on an optical detector. Differences between the characteristic decay time of empty and sample-filled cavities are used to quantify the target species.
The instrument’s two lasers (pump and probe) are individually coupled into the optical cavity that contains the test gas. The intracavity pump–probe technique uses a cavity-enhanced pump laser to selectively extract the signal of interest and cancel out instrument drift and unwanted background absorption interference. Because 2C-CRDS integrates the selectivity of pump–probe techniques with the sensitivity of cavity-enhanced detection, the resultant two-color spectrum focuses the sensitivity of the cavity on the species of interest and cancels out drift typical of cavity-enhanced instrumentation. These capabilities are ideal for obtaining trace-gas measurements.
To learn more about how scientists have used the laser-based 2C-CRDS capability in their research, browse our highlighted publications about bioAMS instrumentation and technology.
