Cavity Ringdown Laser Absorption Spectroscopy

Abstract

Cavity 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 −6 inside 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.

An Optical Offgas Sensor Network Incorporating a HG Cavity Ringdown Spectrometer and IR Diode Lasers

“…The objective of this research project was the writing, implementation and testing of the Labview software to be used in Cavity Ring-down experiments for duel channel monitoring. Our overall conclusion, from both this work and our previous studies, [4][5][6][7][8][9][10][11] is that mercury CRDS remains the most promising technique to provide real-time monitoring of elemental mercury in exhaust gas streams. However, the UV linewidth of tunable light source must be sufficiently narrow to allow the differentiation of the mercury from the SO 2 background and the repetition rate must be sufficiently high (~50 Hz) to provide real-time measurements.…”

“…CRDS is a sensitive absorption technique that was first developed by O'Keefe and Deacon in 1988 1 and its application to atomic analytical problems dates from work began by the PI in 1997. 9 This technique injects a pulse of light into a stable optical cavity formed by two highly reflecting mirrors. The light reflects back and forth in the cavity giving extremely long effective pathlengths.…”

Cavity Ringdown Laser Absorption Spectroscopy

“…(70) Miller and Winstead have reviewed atomic and analytical applications of CRDS. (71) Wagner et al have reviewed early CRDS studies in the infrared spectral region. (72) The application of CRDS for kinetics studies has been reviewed by Atkinson.…”

“…(76) Very recently, Wang has reviewed plasma cavity ringdown spectroscopy (P-CRDS) for elemental and isotopic measurements ( Figure 1). (77) The purpose of this article is to provide an update of the previous review (71) on the application of CRDS in analytical and atomic spectroscopy by Miller and Winstead in 2000. The discussion attempts to cover a wide range of applications of CRDS with a new emphasis in this article on environmental monitoring, the latest developments in P-CRDS for elemental and isotopic measurement, and combustion flame chemistry and plasma diagnostics.…”

Diode laser microwave induced plasma cavity ringdown spectrometer: Performance and perspective

“…Following its introduction, 1 cavity ringdown spectroscopy ͑CRDS͒ was rapidly implemented for the study of numerous species in a variety of environments. [2][3][4][5][6][7] Research and applications relevant to CRDS have expanded from the initial concept to include other light sources, [8][9][10][11] optical cavities, [12][13][14][15][16] methods of cavity excitation and shutoff, [17][18][19][20][21][22][23] and combinations with other traditional instruments. 24 -29 Although the development of ringdown-based commercial instrumentation is still in its early stages, this technology has already shown much promise in many application areas such as semiconductor manufacturing, 30 environmental monitoring, 31,32 and medical diagnostics.…”