Cavity ring-down spectroscopy

<h2 id="detailed-description">Detailed description</h2>
Cavity ring-down spectroscopy is a form of <a href="/facts/Laser_absorption_spectroscopy/Mlkdxzu8">laser absorption spectroscopy</a>. In CRDS, a laser pulse is trapped in a highly reflective (typically R > 99.9%) <a href="/facts/Optical_cavity/BITPG3ga">detection cavity</a>. The intensity of the trapped pulse will decrease by a fixed percentage during each round trip within the cell due to <a href="/facts/Absorption_(optics)/xBYKt7yS">absorption</a>, scattering by the medium within the cell, and reflectivity losses. The intensity of light within the cavity is then determined as an <a href="/facts/Exponential_function/ewlYxvR2">exponential function</a> of time.

I
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    {\displaystyle 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 <a href="/facts/Absorbance/Rnv6VMX1">absorbance</a>. 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:

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    {\displaystyle \tau _{0}={\frac {n}{c}}\cdot {\frac {l}{1-R+X}}}

where n is the <a href="/facts/Index_of_refraction/yjkzvgeE">index of refraction</a> within the cavity, c is the <a href="/facts/Speed_of_light/sKSQwdNx">speed of light</a> 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 <a href="/facts/Beer-Lambert_law/nl64rI1C">Beer-Lambert law</a>. Assuming the sample fills the entire cavity,

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

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.

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    {\displaystyle A={\frac {n}{c}}\cdot {\frac {l}{2.303}}\cdot \left({\frac {1}{\tau }}-{\frac {1}{\tau _{0}}}\right)}

Alternatively, the <a href="/facts/Molar_absorptivity/TwglQEAW">molar absorptivity</a>, ε, and analyte concentration, C, can be determined from the ratio of both ring-down times. If X can be neglected, one obtains

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    {\displaystyle {\frac {\tau _{0}}{\tau }}=1+{\frac {\alpha l}{1-R}}=1+{\frac {2.303\epsilon lC}{(1-R)}}}

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.

<h2 id="advantages-of-crds">Advantages of CRDS</h2>
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 <a href="/facts/Analyte/qWzKoY8L">analyte</a>), 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.<a class="footnote-ref" id="fnref:1" href="#fn:1">1</a>
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:

<ul><li>High sensitivity due to the multipass nature (i.e. long pathlength) of the detection cell.</li>
<li>Immunity to shot variations in laser intensity due to the measurement of a rate constant.</li>
<li>Wide range of use for a given set of mirrors; typically, ±5% of the center wavelength.</li>
<li>High throughput, individual ring down events occur on the millisecond time scale.</li>
<li>No need for a <a href="/facts/Fluorophore/wDWBmYhn">fluorophore</a>, which makes it more attractive than <a href="/facts/Laser-induced_fluorescence/aDRRaEjV">laser-induced fluorescence</a> (LIF) or <a href="/facts/Resonance-enhanced_multiphoton_ionization/F5CTN6Po">resonance-enhanced multiphoton ionization</a> (REMPI) for some (e.g. rapidly predissociating) systems.</li>
<li>Commercial systems available.</li></ul>
<h2 id="disadvantages-of-crds">Disadvantages of CRDS</h2>
<ul><li>Spectra cannot be acquired quickly due to the <a href="/facts/Monochromatic/CMCsjH2N">monochromatic</a> laser source which is used. Having said this, some groups are now beginning to develop the use of broadband <a href="/facts/LED/gE8x4MDb">LED</a> or <a href="/facts/Supercontinuum/1NrW5nMY">supercontinuum</a> sources<a class="footnote-ref" id="fnref:2" href="#fn:2">2</a><a class="footnote-ref" id="fnref:3" href="#fn:3">3</a><a class="footnote-ref" id="fnref:4" href="#fn:4">4</a> for CRDS, the light of which can then be dispersed by a <a href="/facts/Diffraction_grating/TUcyt7Do">grating</a> onto a <a href="/facts/Charge-coupled_device/l6xffIcz">CCD</a>, or <a href="/facts/Fourier_transform/XXhKJtc8">Fourier transformed</a> 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.<a class="footnote-ref" id="fnref:5" href="#fn:5">5</a> 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.<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a></li>
<li>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.</li>
<li>Expense: the requirement for laser systems and high reflectivity mirrors often makes CRDS orders of magnitude more expensive than some alternative spectroscopic techniques.</li></ul>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Absorption_spectroscopy/GH7RC14D">Absorption spectroscopy</a></li>
<li><a href="/facts/Laser_absorption_spectrometry/Mlkdxzu8">Laser absorption spectrometry</a></li>
<li><a href="/facts/Noise-immune_cavity-enhanced_optical_heterodyne_molecular_spectroscopy/GcGCL9DG">Noise-Immune Cavity-Enhanced Optical-Heterodyne Molecular Spectroscopy (NICE-OHMS)</a></li>
<li><a href="/facts/TDLAS/hHa6YSq6">Tunable Diode Laser Absorption Spectroscopy (TDLAS)</a></li></ul>

<ul><li>Anthony O'Keefe; David A.G. Deacon (1988). "Cavity ring-down Optical Spectrometer for absorption measurements using pulsed laser sources". Review of Scientific Instruments. 59 (12): 2544. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1988RScI...59.2544O">1988RScI...59.2544O</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1063%2F1.1139895">10.1063/1.1139895</a>. <a href="/facts/S2CID_(identifier)/ldJsHa2Y">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:6033311">6033311</a>.</li>
<li>Piotr Zalicki; Richard N. Zare (15 February 1995). "Cavity ring-down spectroscopy for quantitative absorption measurements". The Journal of Chemical Physics. 102 (7): 2708–2717. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1995JChPh.102.2708Z">1995JChPh.102.2708Z</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1063%2F1.468647">10.1063/1.468647</a>.</li>
<li>Giel Berden; Rudy Peeters; Gerard Meijer (2000). <a href="https://www.differ.nl/node/754">"Cavity ring-down spectroscopy: Experimental schemes and applications"</a>. International Reviews in Physical Chemistry. 19 (4): 565–607. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2000IRPC...19..565B">2000IRPC...19..565B</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1080%2F014423500750040627">10.1080/014423500750040627</a>. <a href="/facts/S2CID_(identifier)/ldJsHa2Y">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:98510055">98510055</a>.</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">Soran Shadman; Charles Rose; Azer P. Yalin (2016). "Open-path cavity ring-down spectroscopy sensor for atmospheric ammonia". Applied Physics B. 122 (7): 194. Bibcode:2016ApPhB.122..194S. doi:10.1007/s00340-016-6461-5. S2CID 123834102. <a href="/wiki/Applied_Physics_B" target="_blank">/wiki/Applied_Physics_B</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">K. Stelmaszczyk; et al. (2009). "Towards supercontinuum cavity ring-down spectroscopy". Applied Physics B. 94 (3): 369. Bibcode:2009ApPhB..94..369S. doi:10.1007/s00340-008-3320-z. S2CID 120500308. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">K. Stelmaszczyk; et al. (2009). "Cavity ring-down absorption spectrography based on filament-generated supercontinuum light". Optics Express. 17 (5): 3673–8. Bibcode:2009OExpr..17.3673S. doi:10.1364/OE.17.003673. PMID 19259207. S2CID 21728338. <a href="https://doi.org/10.1364%2FOE.17.003673" target="_blank">https://doi.org/10.1364%2FOE.17.003673</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">W. Nakaema; et al. (2011). "PCF-Based Cavity Enhanced Spectroscopic Sensors for Simultaneous Multicomponent Trace Gas Analysis". Sensors. 11 (2): 1620–1640. Bibcode:2011Senso..11.1620N. doi:10.3390/s110201620. PMC 3274003. PMID 22319372. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3274003" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3274003</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">"Review paper Cavity Ring Down spectroscopy (CRDS)". mbp.science.ru.nl. Retrieved 2021-03-19. <a href="http://mbp.science.ru.nl/giel_berden/review.html" target="_blank">http://mbp.science.ru.nl/giel_berden/review.html</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">Truong, G.-W.; Douglass, K. O.; Maxwell, S. E.; Zee, R. D. van; Plusquellic, D. F.; Hodges, J. T.; Long, D. A. (2013). "Frequency-agile, rapid scanning spectroscopy". Nature Photonics. 7 (7): 532–534. Bibcode:2013NaPho...7..532T. doi:10.1038/nphoton.2013.98. S2CID 123428749. <a href="https://zenodo.org/record/1233473" target="_blank">https://zenodo.org/record/1233473</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
</ol>

Cavity ring-down spectroscopy open-in-new

Cavity ring-down spectroscopy