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Cavity-ringdown laser absorption spectroscopy (CRLAS) is an ultrasensitive method to make quantitative absorption measurements of very low concentrations of analytes. The technique uses a laser pulse that is reflected back and forth between two highly reflecting mirrors. This procedure results in a very long path length, to which the measured absorbance is directly proportional as described by the Beer-Lambert law.
An optical detector is placed behind one of the mirrors to detect the small amount of the light that passes through the mirror. With no absorbing analyte present, the laser pulse will decrease in intensity after each round trip due to the loss of light through the monitoring mirror and other losses. When an absorbing species is present between the mirrors, the intensity of the laser pulse decreases more rapidly. The analyte concentration is determined by calibrating this decay time with known concentration of analyte.
CRLAS can be used from the near-UV to the mid-IR. The wavelength range is only limited by instrumental constraints, such as the availability of high reflectivity mirrors and suitable pulsed laser sources. Accessing the near and mid-IR is accomplished with frequency-conversion techniques such as Raman shifting or optical parametric oscillators.
In principle, the analyte can be in a solid, liquid, or gas. In practice, CRALS is primarily used to measure gas-phase species due to scattering losses in solids or liquids.
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