Cavity Ring-down Spectrometry
   HOME

TheInfoList



OR:

Cavity ring-down spectroscopy (CRDS) is a highly sensitive optical spectroscopic technique that enables measurement of absolute optical extinction by samples that scatter and absorb light. It has been widely used to study gaseous samples which absorb light at specific
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s, and in turn to determine
mole fraction In chemistry, the mole fraction or molar fraction, also called mole proportion or molar proportion, is a quantity defined as the ratio between the amount of a constituent substance, ''ni'' (expressed in unit of moles, symbol mol), and the to ...
s down to the parts per trillion level. The technique is also known as cavity ring-down laser absorption spectroscopy (CRLAS). A typical CRDS setup consists of a
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...
that is used to illuminate a high-finesse
optical cavity An optical cavity, resonating cavity or optical resonator is an arrangement of mirrors or other optical elements that confines light waves similarly to how a cavity resonator confines microwaves. Optical cavities are a major component of lasers, ...
, which in its simplest form consists of two highly reflective
mirror A mirror, also known as a looking glass, is an object that Reflection (physics), reflects an image. Light that bounces off a mirror forms an image of whatever is in front of it, which is then focused through the lens of the eye or a camera ...
s. When the laser is in
resonance Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...
with a cavity
mode Mode ( meaning "manner, tune, measure, due measure, rhythm, melody") may refer to: Arts and entertainment * MO''D''E (magazine), a defunct U.S. women's fashion magazine * ''Mode'' magazine, a fictional fashion magazine which is the setting fo ...
,
intensity Intensity may refer to: In colloquial use * Strength (disambiguation) *Amplitude * Level (disambiguation) * Magnitude (disambiguation) In physical sciences Physics *Intensity (physics), power per unit area (W/m2) *Field strength of electric, m ...
builds up in the cavity due to
constructive interference In physics, interference is a phenomenon in which two coherence (physics), coherent waves are combined by adding their intensities or displacements with due consideration for their phase (waves), phase difference. The resultant wave may have ...
. The laser is then turned off in order to allow the measurement of the exponentially decaying light intensity leaking from the cavity. During this decay, light is reflected back and forth thousands of times between the mirrors giving an effective path length for the extinction on the order of a few kilometers. If a light-absorbing material is now placed in the cavity, the
mean lifetime A quantity is subject to exponential decay if it decreases at a rate proportional to its current value. Symbolically, this process can be expressed by the following differential equation, where is the quantity and ( lambda) is a positive ra ...
decreases as fewer bounces through the medium are required before the light is fully absorbed, or absorbed to some fraction of its initial intensity. A CRDS setup measures how long it takes for the light to decay to 1/''e'' of its initial intensity, and this "ringdown time" can be used to calculate the concentration of the absorbing substance in the gas mixture in the cavity.


Detailed description

Cavity ring-down spectroscopy is a form of laser absorption spectroscopy. 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
absorption Absorption may refer to: Chemistry and biology *Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which su ...
, scattering 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 Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)". Alternatively, for samples which scatter light, absorbance may be defined as "the negative log ...
. This is one reason for the increased sensitivity over traditional absorption spectroscopy, as the technique is then immune to shot-to-shot laser fluctuations. The decay constant, τ, which is the time taken for the intensity of light to fall to 1/e of the initial intensity, 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 \cdot \frac where ''n'' is the
index of refraction In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...
within the cavity, ''c'' is the
speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant exactly equal to ). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time i ...
in vacuum, ''l'' is the cavity length, ''R'' is the mirror reflectivity, and ''X'' takes into account other miscellaneous optical losses. This equation uses the approximation that ln(1+''x'') ≈ ''x'' for ''x'' close to zero, which is the case under cavity ring-down conditions. Often, the miscellaneous losses are factored into an effective mirror loss for simplicity. An absorbing species in the cavity will increase losses according to the Beer-Lambert law. Assuming the sample fills the entire cavity, :\tau = \frac \cdot \frac where α is the absorption coefficient for a specific analyte concentration at the cavity's resonance wavelength. The decadic absorbance, ''A'', due to the analyte can be determined from both ring-down times. :A = \frac \cdot \frac \cdot \left ( \frac - \frac \right) Alternatively, the
molar absorptivity In chemistry, the molar absorption coefficient or molar attenuation coefficient () is a measurement of how strongly a chemical species absorbs, and thereby attenuates, light at a given wavelength. It is an intrinsic property of the species. The ...
, ε, and analyte concentration, ''C'', can be determined from the ratio of both ring-down times. If ''X'' can be neglected, one obtains :\frac =1+ \frac = 1+\frac When a ratio of species' concentrations is the analytical objective, as for example in carbon-13 to carbon-12 measurements in carbon dioxide, the ratio of ring-down times measured for the same sample at the relevant absorption frequencies can be used directly with extreme accuracy and precision.


Advantages of CRDS

There are two main advantages to CRDS over other absorption methods: First, it is not affected by fluctuations in the laser intensity. In most absorption measurements, the light source must be assumed to remain steady between blank (no
analyte An analyte, component (in clinical chemistry), titrand (in titrations), or chemical species is a substance or chemical constituent that is of interest in an analytical procedure. The remainder of the sample is called the matrix. The procedure ...
), standard (known amount of analyte), and sample (unknown amount of analyte). Any drift (change in the light source) between measurements will introduce errors. In CRDS, the ringdown time does not depend on the intensity of the laser, so fluctuations of this type are not a problem. Independency from laser intensity makes CRDS needless to any calibration and comparison with standards. Second, it is very sensitive due to its long pathlength. In absorption measurements, the smallest amount that can be detected is proportional to the length that the light travels through a sample. Since the light reflects many times between the mirrors, it ends up traveling long distances. For example, a laser pulse making 500 round trips through a 1-meter cavity will effectively have traveled through 1 kilometer of sample. Thus, the advantages 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. *Wide range of use for a given set of mirrors; typically, ±5% of the center wavelength. *High throughput, individual ring down events occur on the millisecond time scale. *No need for a
fluorophore A fluorophore (or fluorochrome, similarly to a chromophore) is a fluorescent chemical compound that can re-emit light upon light excitation. Fluorophores typically contain several combined aromatic groups, or planar or cyclic molecules with se ...
, which makes it more attractive than
laser-induced fluorescence Laser-induced fluorescence (LIF) or laser-stimulated fluorescence (LSF) is a spectroscopic method in which an atom or molecule is excited to a higher energy level by the absorption of laser light followed by spontaneous emission of light. It was f ...
(LIF) or resonance-enhanced multiphoton ionization (REMPI) for some (e.g. rapidly predissociating) systems. *Commercial systems available.


Disadvantages of CRDS

*Spectra cannot be acquired quickly due to the
monochromatic A monochrome or monochromatic image, object or palette is composed of one color (or values of one color). Images using only shades of grey are called grayscale (typically digital) or black-and-white (typically analog). In physics, mon ...
laser source which is used. Having said this, some groups are now beginning to develop the use of broadband
LED A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (corresp ...
or
supercontinuum In optics, a supercontinuum is formed when a collection of nonlinear processes act together upon a pump beam in order to cause severe spectral broadening of the original pump beam, for example using a microstructured optical fiber. The result is ...
sources for CRDS, the light of which can then be dispersed by a
grating A grating is any regularly spaced collection of essentially identical, parallel, elongated elements. Gratings usually consist of a single set of elongated elements, but can consist of two sets, in which case the second set is usually perpendicu ...
onto a CCD, or
Fourier transform In mathematics, the Fourier transform (FT) is an integral transform that takes a function as input then outputs another function that describes the extent to which various frequencies are present in the original function. The output of the tr ...
ed spectrometer (mainly in broadband analogues of CRDS). Perhaps more importantly, the development of CRDS based techniques have now been demonstrated over the range from the near UV to the mid-infrared. In addition, the frequency-agile rapid scanning (FARS) CRDS technique has been developed to overcome the mechanical or thermal frequency tuning which typically limits CRDS acquisition rates. The FARS method utilizes an electro-optic modulator to step a probe laser side band to successive cavity modes, eliminating tuning time between data points and allowing for acquisition rates about 2 orders of magnitude faster than traditional thermal tuning. *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.


See also

*
Absorption spectroscopy Absorption spectroscopy is spectroscopy that involves techniques that measure the absorption of electromagnetic radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons ...
*
Laser absorption spectrometry Laser absorption spectrometry (LAS) refers to techniques that use lasers to assess the concentration or amount of a species in gas phase by absorption spectrometry (AS). Optical spectroscopic techniques in general, and laser-based techniques in ...
* Noise-Immune Cavity-Enhanced Optical-Heterodyne Molecular Spectroscopy (NICE-OHMS) * Tunable Diode Laser Absorption Spectroscopy (TDLAS)


References

* * * {{Lasers Spectroscopy