Cavity-Ringdown Laser Absorption Spectroscopy (CRLAS)
Introduction
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.
Instrumentation
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.