Cavity Ringdown Laser Absorption Spectroscopy

Abstract

AbstractCavity ringdown laser absorption spectroscopy (CRDS), a distinctly new variant of classical absorption spectroscopy, is based upon the time required for the intensity of a light pulse in an optical cavity to decay. The sensitivity of the technique stems from the large number of passes that the light pulse makes within the cavity. For example, mirrors of 99.99% reflectivity in a 1‐m long cavity yield a ringdown time of approximately 33.4‐µs for an empty cavity. This is equivalent to a 10‐km pathlength traveled during the first time constant! The presence of a sample having an absorbance of 1 × 10−6inside the cavity will yield a readily measurable change in ringdown time of approximately 1%. When compared with traditional single‐pass absorption spectroscopy, which typically detects absorbances of the order of 1 × 10−3, the potential of CRDS becomesclear. As for any absorption technique, the conditions for Beer's law behavior must be fulfilled. One primary advantage of the cavity ringdown method is to allow the use of the extensive wavelength operating range of pulsed lasers for absorption spectroscopy. Because CRDS depends solely on the decay time, it is unaffected by the inherent shot‐to‐shot power fluctuations of pulsed lasers. Although still in the early stages of development, CRDS has been used to study chemical species in environments ranging from molecular beams and gas cells to atmospheric pressure flames and plasmas, in spectral regions ranging from the ultraviolet (UV) to the infrared (IR). It has only been very recently, with first couplings of CRDS and atomization sources (inductively coupled plasma (ICP), graphite furnace), that the technique has broadened to include analytical atomic spectroscopy. The potential of cavity ringdown for atomic spectroscopy has been demonstrated with the determination of mercury detection limits (DLs), in air, of less than 1 part per trillion.At the time of writing (1999), CRDS is very much still in its infancy, especially with regard to analytical applications. For a possible glimpse of the future, it should be noted that as the wavelength coverage of diode lasers continues to grow, the possibilities for constructing small, ultrasensitive CRDS spectrometers for analytical atomic and molecular spectrometry continue to increase. However, any inherent advantage of CRDS over other analytical techniques will be determined by the quality of mirrors used for cavity construction, the stability of the baseline ringdown time measurement, the atomization source, and the fluorescence characteristics of the element under consideration. Although the technology associated with the ringdown technique continues to advance rapidly, CRDS is as yet a novel approach for many analytical measurements, and is sure to continue to find new areas of application. The next few years should prove very exciting as new instrumentation becomes available and the technique's full potential is explored.