Cavity ring-down spectroscopy

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.^[1] It has been widely used to study gaseous samples which absorb light at specific wavelengths, and in turn to determine mole fractions 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 that is used to illuminate a high-finesse optical cavity, which in its simplest form consists of two highly reflective mirrors. When the laser is in resonance with a cavity mode, intensity builds up in the cavity due to constructive interference. 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. CRDS is traditionally conducted with monochromatic laser sources resulting in restricted spectral coverage. Recent efforts have demonstrated the use of broadband laser frequency comb sources for significantly broader spectral coverage.^[2]

If a light-absorbing material is now placed in the cavity, the mean lifetime 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.

Cavity ring-down spectroscopy is a form of laser absorption spectroscopy. In CRDS, a laser pulse is trapped in a highly reflective (typically ${\displaystyle R>99\%}$) detection cavity. The intensity of the trapped pulse will decrease by a fixed percentage during each round trip within the cell due to absorption, 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.

${\displaystyle I(t)=I_{0}\exp \left(-t/\tau \right)}$

El principio de funcionamiento se basa en la medición de una tasa de decaimiento en lugar de una absorbancia absoluta . Esta es una de las razones de la mayor sensibilidad con respecto a la espectroscopia de absorción tradicional, ya que la técnica es inmune a las fluctuaciones del láser disparo a disparo. La constante de decaimiento,${\displaystyle \tau }$, que es el tiempo que tarda la intensidad de la luz en disminuir a${\displaystyle 1/e}$La constante de decaimiento de la intensidad inicial se denomina tiempo de decaimiento y depende del mecanismo o mecanismos de pérdida dentro de la cavidad. Para una cavidad vacía, la constante de decaimiento depende de la pérdida por reflexión y de diversos fenómenos ópticos como la dispersión y la refracción.

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

dónde${\displaystyle n}$es el índice de refracción dentro de la cavidad,${\displaystyle c}$es la velocidad de la luz en el vacío,${\displaystyle l}$es la longitud de la cavidad,${\displaystyle R}$es la reflectividad del espejo, y${\displaystyle X}$tiene en cuenta otras pérdidas ópticas diversas. Esta ecuación utiliza la aproximación de que${\displaystyle \ln(1+x)\approx x}$para${\displaystyle x}$cerca de cero, que es el caso en condiciones de decaimiento de la cavidad. A menudo, las pérdidas diversas se tienen en cuenta en una pérdida efectiva del espejo para simplificar. Una especie absorbente en la cavidad aumentará las pérdidas de acuerdo con la ley de Beer-Lambert . Suponiendo que la muestra llena toda la cavidad,

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

dónde${\displaystyle \alpha }$es el coeficiente de absorción para una concentración específica de analito en la longitud de onda de resonancia de la cavidad. La absorbancia decádica,${\displaystyle A}$, debido a que el analito se puede determinar a partir de ambos tiempos de decaimiento.

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

Alternativamente, la absortividad molar ,${\displaystyle \epsilon }$y concentración del analito,${\displaystyle C}$, se puede determinar a partir de la relación de ambos tiempos de decaimiento. Si${\displaystyle X}$puede ser ignorado, se obtiene

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

Cuando el objetivo analítico es la relación entre las concentraciones de las especies, como por ejemplo en las mediciones de carbono-13 a carbono-12 en el dióxido de carbono, la relación de los tiempos de decaimiento medidos para la misma muestra en las frecuencias de absorción relevantes se puede utilizar directamente con extrema exactitud y precisión.

El sistema CRDS presenta dos ventajas principales sobre otros métodos de absorción:

En primer lugar, no se ve afectado por las fluctuaciones en la intensidad del láser. En la mayoría de las mediciones de absorción, se debe asumir que la fuente de luz permanece constante entre el blanco (sin analito ), el estándar (cantidad conocida de analito) y la muestra (cantidad desconocida de analito). Cualquier deriva (cambio en la fuente de luz) entre mediciones introducirá errores. En CRDS, el tiempo de decaimiento no depende de la intensidad del láser, por lo que las fluctuaciones de este tipo no representan un problema. La independencia de la intensidad del láser hace que CRDS no requiera calibración ni comparación con estándares. ^{[ 3 ]}

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, which makes it more attractive than laser-induced fluorescence (LIF) or resonance-enhanced multiphoton ionization (REMPI) for some (e.g. rapidly predissociating) systems.
• Commercial systems available.

• Spectra cannot be acquired quickly if monochromatic laser source is used. To resolve this issue, latest research has demonstrated the use of broadband frequency comb source for significantly larger spectral coverage.

• 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.
• Dynamic range is limited. As the absorber concentration increases, the ring-down time decreases, thus detectors must be fast enough to record sufficient data points for a desired uncertainty.

• Absorption spectroscopy
• Laser absorption spectrometry
• Noise-Immune Cavity-Enhanced Optical-Heterodyne Molecular Spectroscopy (NICE-OHMS)
• Tunable Diode Laser Absorption Spectroscopy (TDLAS)

• 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. Bibcode:1988RScI...59.2544O. doi:10.1063/1.1139895. S2CID 6033311.
• 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. Bibcode:1995JChPh.102.2708Z. doi:10.1063/1.468647.
• Giel Berden; Rudy Peeters; Gerard Meijer (2000). "Cavity ring-down spectroscopy: Experimental schemes and applications". International Reviews in Physical Chemistry. 19 (4): 565–607. Bibcode:2000IRPC...19..565B. doi:10.1080/014423500750040627. S2CID 98510055.
• Crosson, E. R. (2008). A cavity ring-down analyzer for measuring atmospheric levels of methane, carbon dioxide, and water vapor. Applied Physics B, 92(3), 403–408.
• Chen, H., Winderlich, J., Gerbig, C., Hoefer, A., Rella, C. W., Crosson, E. R., et al. (2010). High-accuracy continuous airborne measurements of greenhouse gases (CO₂ and CH₄) using the cavity ring-down technique. Atmospheric Measurement Techniques, 3, 375–386.
• Gherman, T., & Romanini, D. (2002). Mode-locked cavity-enhanced absorption spectroscopy. Optics Express, 10(19), 1033–1042.
• Furlani, C., Liu, D. Y., & Worsnop, D. R. (2018). Ammonia detection by cavity ring-down spectroscopy for agricultural emissions monitoring. Atmospheric Environment, 182, 331–339.
• Thoma, E. D., et al. (2009). Development of mobile laboratory CRDS instruments for environmental and industrial monitoring. Environmental Science & Technology, 43(15), 5628–5634.
• Stromer, B., Bednar, A., Janjic, M., Becker, S., Kylloe, T., Allen, J., Trapani, M., Hargrove, J., & Hargrove, J. (2021, August). Trace explosives detection by cavity ring-down spectroscopy (CRDS) (ERDC/EL TR-21-5). U.S. Army Engineer Research and Development Center. https://doi.org/10.21079/11681/41520^[1]
• Zhao, W., & O'Keefe, A. (2001). Espectroscopia de decaimiento de cavidad de disociación térmica para la detección sensible de NO ₂ y vapores explosivos. Applied Physics B, 72(7), 843–847.
• Gmachl, C., Capasso, F., Sivco, DL, & Cho, AY (2002). Avances recientes en láseres de cascada cuántica y aplicaciones para la detección química. Reports on Progress in Physics, 64(11), 1533–1601.
• Thorpe, MJ, Moll, KD, Jones, RJ, Safdi, B., & Ye, J. (2006). Espectroscopia de decaimiento de cavidad de banda ancha para la detección molecular sensible y rápida. Science, 311(5767), 1595–1599.