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, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
s, and in turn to determine
mole fraction
In chemistry, the mole fraction or molar fraction (''xi'' or ) is defined as unit of the amount of a constituent (expressed in moles), ''ni'', divided by the total amount of all constituents in a mixture (also expressed in moles), ''n''tot. This ex ...
s down to the
parts per trillion
In science and engineering, the parts-per notation is a set of pseudo-units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they ...
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" is an acronym for "light amplification by stimulated emission of radiation". The fir ...
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 forms a cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and provi ...
, which in its simplest form consists of two highly reflective
mirror
A mirror or looking glass is an object that Reflection (physics), reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the ...
s. When the laser is in
resonance
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
with a cavity
mode
Mode ( la, modus 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 ...
,
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, ma ...
builds up in the cavity due to
constructive interference
In physics, interference is a phenomenon in which two waves combine by adding their displacement together at every single point in space and time, to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive ...
. 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 Proportionality (mathematics), proportional to its current value. Symbolically, this process can be expressed by the following differential equation, where is the quantity and ...
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 ...
, scattering by the medium within the cell, and reflectivity losses. The intensity of light within the cavity is then determined as an
exponential function
The exponential function is a mathematical function denoted by f(x)=\exp(x) or e^x (where the argument is written as an exponent). Unless otherwise specified, the term generally refers to the positive-valued function of a real variable, a ...
of time.
:
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 lo ...
. 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:
:
where ''n'' is the
index of refraction
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
within the cavity, ''c'' is the
speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
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,
:
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.
:
Alternatively, the
molar absorptivity Molar may refer to:
*Molar (tooth), a kind of tooth found in mammals
*Molar (grape), another name for the Spanish wine grape Listan Negro
*Molar (unit), a unit of concentration equal to 1 mole per litre
*Molar mass
*Molar volume
*El Molar, Tarragon ...
, ε, and analyte concentration, ''C'', can be determined from the ratio of both ring-down times. If ''X'' can be neglected, one obtains
:
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), or chemical species is a substance or chemical constituent that is of interest in an analytical procedure. The purest substances are referred to as analytes, such as 24 karat gold, NaCl, water, etc. ...
), 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
Resonance-enhanced multiphoton ionization (REMPI) is a technique applied to the spectroscopy of atoms and small molecules. In practice, a tunable laser can be used to access an excited intermediate state. The selection rules associated with a ...
(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 color scheme, palette is composed of one color (or lightness, values of one color). Images using only Tint, shade and tone, shades of grey are called grayscale (typically digital) or Black and wh ...
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 Electronics, device that Light#Light sources, emits light when Electric current, current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy i ...
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 perpendicul ...
onto a
CCD, or
Fourier transform
A Fourier transform (FT) is a mathematical transform that decomposes functions into frequency components, which are represented by the output of the transform as a function of frequency. Most commonly functions of time or space are transformed, ...
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 refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating fie ...
*
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
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{{Lasers
Spectroscopy