cavity ring down spectroscopy - crds: principle, instrumentation and results

Haseeb Hakkim

MS Student

IISER Mohali

Cavity Ring Down Spectroscopy : Principle,

Instrumentation and Results

OUTLINE

DEFINITION

LASERS

HISTORY

WORKING PRICNCIPLE

NEED FOR SUCH A TECHNIQUE ?

APPLICATIONS

CRDS @ IISER MOHALI

CONCLUSION & REFERENCES

© Haseeb Hakkim , IISER MOHALI

Cavity Ring Down Spectroscopy

Cavity ringdown spectroscopy (CRDS) is a sensitive technique that makes use of

an optical cavity to measure absorptions on the order of parts per million and beyond. By

choosing appropriate tunable light sources, we can measure the spectroscopy and kinetics

of the chemical systems.

It was originally developed as a tool for determining the high reflectivity of mirrors and has

since been widely employed as widely employed in measuring weak spectroscopic

transitions and concentrations of trace gases.


A laser is a device that emits light through a process of optical amplification based on

the stimulated emission of electromagnetic radiation.

A laser is constructed from three principal parts:

• An energy source /pump source.

• A gain medium or laser medium, and

• Two or more mirrors that form an optical

resonator.

LASER


The term "laser" originated as an acronym for “Light Amplification by Stimulated

Emission of Radiation.

E1 < E2 < E3; that is, the energy of level 2 lies between that of the ground state and level 3.

The pump source provides energy to the laser system.

Initially, the system of atoms is at thermal equilibrium, and the majority of the atoms will be in the ground state, i.e., N1 ≈ N, N2 ≈ N3 ≈ 0.

If we continue pumping the atoms, we will excite an appreciable number of them into level 3, such that N3 > 0. These excited atoms to quickly decay to level 2.

An atom in level 2 may decay by spontaneous emission to the ground state, releasing a photon of frequency ν12 given by E2 – E1 = hν12.

If the lifetime of this transition, τ21 >> τ32 , a population inversion (N2 > N1 ) has thus been achieved between level 1 and 2, and optical amplification at the frequency ν21 can be obtained.

A resonant cavity supports only modes that meet the resonance condition :

Nλ = 2/ (cavity length)

λ - laser wavelength N - mode number

The output of a laser is defined by the overlap of the gain bandwidth and these resonant cavity modes.


Examples of pump sources :

• Electrical discharges • Flashlamps• Arc lamps

The type of pump source used principally depends on the gain medium, and this also determines how the energy is transmitted to the medium. A helium–neon (HeNe) laser uses an electrical discharge in the helium-neon gas mixture, a Nd:YAG laser uses either light focused from a xenon flash lamp or diode lasers, and excimer lasers use a chemical reaction.

Examples of different gain media:

• Liquids, such as dye lasers. These are usually organic chemical solvents, such as methanol, ethanol or ethylene glycol, to which are added chemical dyes such as coumarin, rhodamine, and fluorescein.

• Gases, such as carbon dioxide, argon, krypton and mixtures such as helium–neon. • Solids, such as crystals and glasses. The solid host materials are usually doped with an impurity such as

chromium, neodymium, erbium or titanium ions.• Semiconductors.

• Light from another laser, • chemical reactions• Explosive devices.


• The beginnings of cavity ring-down spectroscopy (CRDS) came in the early 1980s with the idea that mirror reflectivity could be determined with very high precision by measuring the phase shift of a continuous wave (CW) laser exiting an “ optical cavity” between two mirrors. This method, developed by J.M. Herbelin, is known as the Cavity Attenuated Phase Shift method or CAPS method. The difference in the CAPS method is that the photon lifetime (τ) in the cavity is not measured directly; instead, it is calculated from the shift in phase angle (α) based on the following relationship:

tan α = 4πfτ

• The next development came from Anderson, Frisch, and Masser in 1984. This group devised a technique to measure the photon lifetime in the cavity directly by switching off the CW laser and monitoring the decay of light intensity at the detector over time. This decay time became known as the “ring-down time”. A Pockel’s cell is used to switch the laser off very fast – on the order of the cavity round trip time. The main limitation to using Anderson’s CRDS technique to record an absorption spectrum is the difficulty in continuous scanning of the laser wavelength .

J. J. Scherer, Chem. Rev. 1997, 97, 25-51© Google

HISTORY

This drawback lead to the development, in 1988, of a pulsed laser CRDS method by O’Keefe

and Deacon.


Typical experimental set-up of pulsed CRDS

based on O’Keefe and Deacon design.

Mirror reflectometer of Anderson et al. : Instead of measuring the phase shift, the intensity decay of the cavity is measured after the light is quickly shut off with a Pockels cell.

J. J. Scherer, Chem. Rev. 1997, 97, 25-51

www.pnas.org/cgi/doi/10.1073/pnas.0904230106http://www.picarro.com

• Light from a tunable semiconductor diode laser is directed into a small (35 cc) optical resonator cavity containing the analyte gas.

• When the build-up is complete, the laser is shut off

• Light circulates in the cavity ~100,000 times, traveling ~20 km or more. The high precision of CRDS comes from this incredibly long path length providing parts-per-trillion detection levels for some gases.

• There are three partially reflective mirrors on the cavity; a small amount of light leaks out from the third mirror with each pass. A photo detector is positioned behind this mirror, measuring the light intensity at each pass.

• The energy decays from the cavity, through loss mechanisms, exponentially in time. This energy decay is measured, as a function of time, on the photo detector – and is known as a “ring down”

• In an empty cavity, the only loss mechanisms are the mirrors and the ring-down time (the exponential decay time) is long.

WORKING PRICNCIPLE OF A CRDS

SPECTROMETER


• When gas is present, the rotational vibrational modes provide additional loss mechanisms and the ring-down times get shorter, in proportion to gas concentration.

• The ring down time measurement is continuously repeated (~100 times per second) at several different well-controlled points in wavelength as the laser is tuned across the molecular signature of the analyte gas .

• The ring down profiles are transformed into an absorption curve with a well defined line shape

• The gas concentrations or isotope ratios are determined by a multi-parameter fit to this lineshape and are proportional to the area under the curve.


If the only loss process for light is leakage through the mirrors, the ring down is characterized by

tr - roundtrip time for light in the cavityR - mirror reflectivityLopt - distance between the two mirrorsc - speed of lightT0 - 1/e decay time of the light ( ring down lifetime).

A - absorbanceI0 - initial light intensityI - transmitted light intensity σ - absorption cross sectionLabs - path length of light through the absorberN - number density of absorber molecules (per unit volume).

The absorbance can be calculated by the equation :

Sprague et al.,31 Copyright 2012 American Chemical Society

For molecules with small cross sections or for trace gases , a detectable absorbance can be attained by two methods :

• Increasing the concentration of absorber molecules

• Or by increasing the physical absorption path length using long pass absorption spectroscopy.

Eric Corosson, www.picarro.com

Some of the most popular absorption spectroscopy techniques :

• Laser-Induced Fluorescence (LIF)• Resonance enhanced Multiphoton Ionization (REMPI) • Photoacoustic Spectroscopy (PAS)

These three methods are all sensitive laser-based methods, which ‘indirectly’ measure absorption features of the analyte. In other words, some other physical phenomenon (fluorescence, ionization, and pressure waves, respectively) are detected and then related back to absorption in each of these methods.

WHY DO WE NEED A NEW TECHNIQUE ?

• 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.• Compact, less expensive and more portable.

Advantages of CRDS


Isotope-ratio mass spectrometry (IRMS) is a specialization of mass spectrometry, in which mass spectrometric methods are used to measure the relative abundance of isotopes in a given sample .

In the most general terms the instrument operates by ionizing the sample of interest, accelerating it over a potential in the kilo-volt range, and separating the resulting stream of ions according to their mass-to-charge ratio (m/z). Beams with lighter ions bend at a smaller radius than beams with heavier ions. The current of each ion beam is then measured using a 'Faraday cup' or multiplier detector.

ISOTOPE-RATIO MASS SPECTROMETRY (IRMS)

Not suitable for field measurements.

Carbon isotope ratios are usually expressed in parts per thousand (or per mil, ‰) relative to a standard, using the common notation

ä13C = [(Rsample – Rstandard) /Rstandard ]x 1000

whereR =13C/12C

is the ratio of the heavier to the lighter stable isotope of carbon. The common reference for ä13C was obtained from a cretaceous marine fossil, Belemnitella americana, from the Pee Dee formation in South Carolina.

This material had an anomalously high 13C:12C ratio (0.0112372), and was established as δ13C value of zero. All original supplies of PDB have been essentially exhausted and replaced by secondary carbonate standards calibrated against those prepared by the U.S. NationalInstitute of Standards and Technology (NIST) .

Determination of 13C/12C for Carbon Dioxide in Human Breath

Breath tests based on ä13C were used to assess liver function and disease, gastric emptying, bacterial overgrowth, and pancreatic function.

H. pylori,14 is the leading cause of ulcers. It is estimated that about two-thirds of the world’s population has H. pylori. 5 The test is based on the absence of urease in the healthy human gastrointestinal tract. Individuals having H. pylori will exhibit an increase in the concentration of 13CO2 in their breath following ingestion of the 13C-labeled urea. H. Pylori has a high urease activity, and converts urea (CO(NH2)2) into HCO3- and NH4+. The HCO3- finds its way through the blood stream to the lungs and is exhaled as CO2. Hence, Results are reported as the difference over baseline (DOB).

A DOB value less than 2-3 generally indicates the absence of H. pylori whereas a DOB value greater

than 2-3 is strongly associated with the presence of H. pylori.

The output of a tunable external-cavity diode passes through an isolator, preventing back reflections from reaching the diode laser. The beam is then focused into an acoustooptic modulator (AOM) capable of rapidly deflecting the beam from the optical cavity. Following the AOM, a beam splitter directs a small fraction of the laser radiation onto an etalon used to precisely measure the wavelength of the laser radiation. The remaining light passes though mode-matching optics before coupling into a three-mirror high-finesse ring-down cavity (RDC).

Analytical Chemistry, Vol. 74, No. 9, May 1, 2002 2007

INSRUMENTATION

The two absorption features chosen to measure the relative abundances of 12C16O16O and 13C16O16O arise from different rotational/vibrational transitions of the CO2 molecule (R(62) line of the 12C16O16O (3,0°,13) r (0,0°,1) band and R(30) line of the 13C16O16O (3,0°,12)r(0,0°,1) band).

The peaks at 6261.83 and 6262.25 cm-1 attributed to absorption by 13C16O16O and 12C16O16O, respectively, were chosen to measure the relative abundance of the twocarbon isotopes.

Analytical Chemistry, Vol. 74, No. 9, May 1, 2002 2007

A spectroscopic approach for making high precision CSIA measurements of the 13C/12C isotope ratio of organic compounds, which is less expensive, does not require trained personnel, and is portable.

The technique relies on the chromatographic separation of a mixture into individual organic compounds, the combustion of each organic compound into carbon dioxide, water, and other oxidation products, and the precise measurement of the 13C/12C isotope ratio in the carbon dioxide gas, using the absorption

method of cavity ring-down spectroscopy.

13C/12C Isotope Ratios In Organic Compounds At Natural Abundance

Cavity ring-down line shape profiles of the R(36)line of 12C16O16O and the R(12) line of 13C16O16O for the(3,00,1) – (0,00,0) combination band of carbon dioxidenear 6251 cm-1.

The sample cavity is surrounded by layers of thermally insulating material to provide a high degree of passive thermal stability. The cavity is further actively stabilized by means of a solid-state heating system locked to the output of a thermal sensor.

The sample pressure in the cavity is sensed using a high-linearity pressure transducer. The system computer uses this pressure data in a feedback loop to control the proportional valves which adjust the inlet and outlet gas flow of the cavity.

CRDS @ IISER MOHALI

• The Cavity Ring Down Spectrometer at Atmospheric Chemistry was installed on 07-10-2014.

• The G2508 model is the latest Picarro analyzer that measures N2O concentration along with CH4, CO2, NH3 and H2O simultaneously .

• The system includes an analyzer, a vacuum pump ,a multiport valve and GUI interface.

• We have been measuring 24 x 7 data since 20-10-2014 .

Picarro G2508 CRDS @IISER


The CRDS can be also used for closed soil chamber measurements. Gas emitted from the soil is captured by the soil chamber. It then flows to the G2508 cavity ring down spectrometer (CRDS) where the gas concentrations are measured. The gas is drawn through the system by the low-leak external vacuum pump. The gas exiting the pump returns to the soil chamber.


• Environmental monitoring,

• Emissions monitoring,

• Greener automotive engine development,

• Semiconductor fabrication plant

• Cleanroom technology

• Bio-pharmaceutical process monitoring.

All these applications can benefit from a turnkey analyzer that provides real-time

speed, high precision and sensitivity to parts per billion and beyond which is exactly

what CRDS offers.

Also applications requiring trace gas analysis include :

CONCLUSION


REFERENCES

• Cavity Ring Down Spectroscopy: History, Fundamentals, and Applications - Brian LaFranchi

• www.pnas.orgcgidoi10.1073pnas.0904230106

• Isotopes in Environmental and Health Studies - Vol. 42, No. 1, March 2006, 21–35

• Analytical Chemistry, Vol. 74, No. 9, May 1, 2002 2003

• http://www.picarro.com

• http://www.rp-photonics.com

• dx.doi.org/10.1021/jp406691e | J. Phys. Chem. A 2013, 117, 13399−13411


http://www.picarro.com/

http://www.rp-photonics.com/

THANK YOU


YNWA

cavity ring down spectroscopy - crds: principle, instrumentation and results

Education

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