Cavity ring down spectroscopy

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Cavity Ring Down Spectroscopy (CRDS) is a form of laser absorption spectroscopy, also known as Cavity Ring-down Laser Absorption Spectroscopy (CRLAS). In CRDS, a laser pulse is trapped in a highly reflective (typically R > 99.9%) detection cavity. The intensity of the trapped pulse will decrease by a fixed percentage during each round trip within the cell due to both absorption by the medium within the cell and reflectivity losses. The intensity of light within the cavity is then determined as an exponential function of time.

$I(t) = I_0 \exp \left (- t / \tau \right)$

The principle of operation is based on the measurement of a decay rate rather than an absolute absorbance. This is one reason for the increased sensitivity over traditional absorption spectroscopy. The decay constant, τ, is called the ring-down time and is dependent on the loss mechanism(s) within the cavity. For an empty cavity, the decay constant is dependent on mirror loss and various optical phenomena like scattering and refraction:

$\tau_0 = \frac{n}{c} \cdot \frac{l}{1-R+X}$

where n is the index of refraction within the cavity, c is the speed of light in vacuum, l is the cavity length, R is the mirror reflectivity, and X is the miscellaneous optical losses. Often, the miscellaneous losses are factored into an effective mirror loss for simplicity. An absorbing species in the cavity will add another losses mechanism similar to the Beer-Lambert law. Assuming the sample fills the entire cavity,

$\tau = \frac{n}{c} \cdot \frac{l}{1-R+X+ \alpha l }$

where α is the absorption coefficient for a specific analyte concentration. The absorbance, A, due to the analyte can be determined from both ring-down times.

$A = \frac{n}{c} \cdot \frac{l}{2.303} \cdot \left ( \frac{1}{\tau} - \frac{1}{\tau_0} \right)$

Alternately, the molar absorptivity, ε, and analyte concentration, C, can be determined from the ratio of both ring-down times.

$\frac{\tau}{\tau_0} = \frac{ \alpha l }{1-R} = \frac{ \epsilon l C}{2.303(1-R)}$

1 Advantages of CRDS include:
2 Disadvantages of CRDS include:
3 See also
4 External links

[edit] Advantages of CRDS include:

High sensitivity due to the multipass nature (i.e. long pathlength) of the detection cell.
Immunity to shot variations in laser intensity due to the measurement of a rate constant.
High throughput, individual ring down events occur on the millisecond time scale.
No need for a fluorophore, which makes it more attractive than LIF or REMPI for some (e.g. rapidly predissociating) systems.

[edit] Disadvantages of CRDS include:

Spectra cannot be acquired quickly due to the monochromatic laser source which is used. Having said this, some groups are now beginning to develop the use of broadband LED sources for CRDS, which can then be dispersed by a grating onto a CCD, or Fourier transformed.
CRDS mirrors often work only over a narrow wavelength range (e.g. 415nm +-5nm)
Analytes are limited both by the availability of tunable laser light at the appropriate wavelength and also the availability of high reflectance mirrors at those wavelengths.
Expense: the requirement for laser systems and high reflectivity mirrors often makes CRDS orders of magnitude more expensive than some alternative spectroscopic techniques.