WO2012142549A1 - Système à multiples passages laser contenant une cellule pour effectuer des mesures spectroscopiques - Google Patents

Système à multiples passages laser contenant une cellule pour effectuer des mesures spectroscopiques Download PDF

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Publication number
WO2012142549A1
WO2012142549A1 PCT/US2012/033709 US2012033709W WO2012142549A1 WO 2012142549 A1 WO2012142549 A1 WO 2012142549A1 US 2012033709 W US2012033709 W US 2012033709W WO 2012142549 A1 WO2012142549 A1 WO 2012142549A1
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WO
WIPO (PCT)
Prior art keywords
cell
raman
pass
approximately
concave mirrors
Prior art date
Application number
PCT/US2012/033709
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English (en)
Inventor
Jacek Borysow
Manfred Fink
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IsoSpec Technologies, LP
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Publication date
Application filed by IsoSpec Technologies, LP filed Critical IsoSpec Technologies, LP
Publication of WO2012142549A1 publication Critical patent/WO2012142549A1/fr

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
    • G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
    • G01N21/65—Raman scattering
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/01—Arrangements or apparatus for facilitating the optical investigation
    • G01N21/03—Cuvette constructions
    • G01N21/031—Multipass arrangements
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
    • G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
    • G01N21/65—Raman scattering
    • G01N2021/651—Cuvettes therefore

Definitions

  • a Raman spectral analyzer to measure the scattered light from a multi-pass Raman cell containing a vessel for containing the substance to be analyzed.
  • Raman scattering is a type of inelastic scattering of electromagnetic radiation, such as visible light, discovered in 1928 by Chandrasekhara Raman.
  • electromagnetic radiation such as visible light
  • Raman scattering When a beam of monochromatic light is passed through a substance some of the radiation will be scattered. Although most of the scattered radiation will have the same frequency as the incident radiation (“Rayleigh” scattering), some will have frequencies above (“anti-Stokes” radiation) and below (“Stokes” radiation) that of the incident beam.
  • This effect is known as Raman scattering and is due to inelastic collisions between photons and molecules that lead to changes in the vibrational and/or rotational energy levels of the molecules. This effect is used in Raman spectroscopy for identifying and investigating the vibrational and rotational energy levels of molecules.
  • Raman spectroscopy is the spectrophotometric detection of the inelastically scattered light.
  • “Stokes” emissions have lower energies (lower frequencies or a decrease in wave number (cm 1 )) than the incident laser photons and occur when a molecule absorbs incident laser energy and relaxes into an excited rotational and/or vibrational state.
  • Each molecular species will generate a set of characteristic Stokes lines that are displaced from the excitation frequency (Raman shifted) whose intensities are linearly proportional to the density of the species in the sample.
  • Anti-Stokes emissions have higher frequencies than the incident laser photons and occur only when the photon encounters a molecule that, for instance, is initially in a vibrational excited state due to elevated sample temperature. When the final molecular state has lower energy than the initial state, the scattered photon has the energy of the incident photon plus the difference in energy between the molecule's original and final states. Like Stokes emissions, anti-Stokes emissions provide a quantitative fingerprint for the molecule involved in the scattering process. This part of the spectrum is seldom used for analytical purposes since the spectral features are weaker. However, the ratio of the Stokes to the anti- Stokes scattering can be used to determine the sample temperature when it is in thermal equilibrium.
  • the Stokes and anti-Stokes emissions are collectively referred to as spontaneous Raman emissions. Since the excitation frequency and the frequency of the Stokes (and anti- Stokes) scattered light are typically far off the excitation of any other component in the sample, fluorescence in near infrared (NIR) wavelengths is minimal. The sample is optically thin and will not alter the intensities of the Stokes emissions (no primary or secondary extinctions), in stark contrast to infrared spectroscopy.
  • NIR near infrared
  • Raman spectroscopy is a well-established technology to determine the presence of trace compounds down to very low (e.g. n mol/liter) levels. With Raman analysis, absolute densities can be determined, the sparse spectra minimize interferences, and overtones and combination lines are strongly suppressed.
  • Multi-pass cells are used in various scientific spectroscopy experiments as well as in industrial environments and in medical applications. Multi-pass cells are particularly important in absorption measurements of weakly absorbing species with low concentrations and Raman spectroscopy due to the extremely small cross sections. Frequently White cells are used, which typically use spherical mirrors; and Herriott cells, also known as an off axis resonator. Numerous variants of the latter one have been reported in the literature with either spherical mirrors or astigmatic mirrors. Many multi-pass absorption cells are available commercially. New designs of multi-pass cells for absorption spectroscopy are still being reported in the literature.
  • Raman Spectroscopy presents further challenges.
  • a spectrometer analyzes the spectral components of the scattered light in the acceptance cone. Therefore, an advantageous property for a multi-pass configuration is that the excitation light source (e.g. laser) passes through the same very small scattering region. This is a main reason why the relatively large volume multi-pass cells developed for absorption measurements tend to be unsuitable for Raman spectroscopy without major modification.
  • One of the first multi-pass Raman tubes was reported by Waber et al.
  • a design of multi-pass cell capable of producing a very high flux of laser light at small focal region imaged later on to the spectrometer slit was demonstrated by Hartley and Hill.
  • Their light trapping system used an ellipsoidal mirror and a flat mirror positioned such that the laser light bouncing between two mirrors eventually collapsed on the major axis of the ellipsoidal mirror.
  • This system requires custom manufacturing of ellipsoidal mirrors and tends to be difficult to align.
  • the geometry of the present disclosure is relatively simply to align, can be built from off-the-shelf components, and offers very large signal gains (up to ⁇ 50) in comparison to single pass cell configuration.
  • the major components of an embodiment of the presently disclosed Raman multi-pass cell are:
  • the nominal reflectivity of the mirrors at normal incidence is better than 99.99%.
  • a small cylindrical vessel (5 cm long and 3.5 cm in diameter) containing gas or liquid samples placed between the mirrors.
  • the vessel has 3mm thick windows made out of BK7 glass mounted on each side. The windows have anti-reflection coating to minimize losses.
  • the alignment of the multi-pass cell without the sample vessel inside is
  • FIG. 1A shows a multi-pass system built from a pair of 50.2 mm diameter concave mirrors with 100.0 mm radius of curvature, separated by a distance of about 200 mm with a laser beam inside, without a sample cell inside;
  • FIG. IB shows the multi-pass system of FIG. 1A with a sample cell placed between the spherical mirrors, with the windows in the sample cell made out of BK7 glass of index of refraction equal to 1.5 and 3 mm thickness;
  • FIG. 1C shows shows the multi-pass system of FIG. IB after alignment correction according to the present disclosure
  • FIG. 2 shows displacement of the laser beam passing through a glass window of thickness d
  • FIG. 3 shows correction to alignment of a multi-pass cell with the glass window inside
  • FIG. 4 shows a graph of correction (horizontal shift of the spherical mirror) to the alignment of the multi-pass system as a function of the thickness of the windows of the sample cell.
  • a Raman apparatus in accordance with the present disclosure is designed to address how to modify the aligned multi-pass laser system after a cell containing a sample is placed in the middle between the two reflecting mirrors. Computations were done using the optical design program Zemax to find the changes in the optical paths. The addition of two windows alters significantly the alignment of the multi-pass configuration; this is seen by a comparison between FIG. 1A and FIG. IB.
  • FIG. 1A shows the laser beam travelling between two spherical mirrors crossing each time at one of the two very tight spots in the middle.
  • FIG. IB shows trajectories of the laser beam travelling between the mirrors after the sample cell with 3 mm thick windows has been inserted. The end windows cause sufficient deviation from the original trajectories that the reflected laser beams don't cross the same spot inside the sample cell anymore, and thus the major principle of a multi-pass cell is lost.
  • the scattering volume created by laser beams in the multi-pass system was greatly increased and the photon density in the scattering volume for the spectral analysis has become unacceptably small due to the insertion of the glass cell, which is apparent in FIG. IB.
  • FIG. 2 shows the laser beam displacement caused by the sample cell window of thickness d.
  • this window may be made of BK7 glass.
  • Ay (d / cos(P)) sin(a - ⁇ ) (1)
  • d is the thickness of the window
  • a is the angle of incidence of the laser beam
  • ⁇ is the angle of the refracted beam computed from Snell's law
  • n is the index of refraction of BK7 window at 780 nm (or whatever laser wavelength is appropriate).
  • FIG. 1C The simulations show that moving the mirror horizontally will restore the alignment. This is shown in FIG. 1C.
  • the ray tracing in that figure is the result of the Zemax calculations and was obtained by increasing the separation between the spherical mirrors until the laser beams travelling inside the cell crossed the same spots again.
  • Eq. 2 After using the result of Eq. 1, Eq. 2 becomes:
  • the laser beam arrives at the mirror at normal incidence and is reflected on itself and then displaced by the windows in the cell in such a way that it will arrive undisturbed at the right mirror of the multi-pass cell.
  • Eq. 3 For small incident angles Eq. 3 can be written as:
  • n index of refraction of the windows assuming that Snell's law for small angles was used.
  • the proposed alignment procedure for sample cell windows inside the multi-pass cell as thick as 6 mm was experimentally verified.
  • Eq. 4 is modified as follows:
  • L is the length of the sample and 3 ⁇ 4 is the index of refraction of the sample.
  • the present disclosure provides a simple solution to the seemingly very complex problem of making corrections to the alignment of a multi-pass laser system after placing a glass vessel with flat windows in the multi-pass path.

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  • Health & Medical Sciences (AREA)
  • Physics & Mathematics (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Biochemistry (AREA)
  • General Health & Medical Sciences (AREA)
  • General Physics & Mathematics (AREA)
  • Immunology (AREA)
  • Pathology (AREA)
  • Nuclear Medicine, Radiotherapy & Molecular Imaging (AREA)
  • Investigating, Analyzing Materials By Fluorescence Or Luminescence (AREA)

Abstract

La présente invention concerne un système à multiples passages laser, comprenant deux miroirs sphériques et, au centre, une cellule pour échantillon à fenêtres plates. Pour une fenêtre ayant une épaisseur d et un indice de réfraction n, un ajustement de précision de la séparation des miroirs de ~ 2d(l-[l/n]) permet d'aligner le faisceau laser et d'effectuer un traçage. Cet agencement peut être utile dans des expériences de spectroscopie Raman et dans d'autres applications.
PCT/US2012/033709 2011-04-15 2012-04-15 Système à multiples passages laser contenant une cellule pour effectuer des mesures spectroscopiques WO2012142549A1 (fr)

Applications Claiming Priority (2)

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US201161476166P 2011-04-15 2011-04-15
US61/476,166 2011-04-15

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WO2012142549A1 true WO2012142549A1 (fr) 2012-10-18

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9523638B2 (en) 2014-09-07 2016-12-20 Unisearch Associates Inc. Gas cell assembly and applications in absorption spectroscopy
US10113955B2 (en) 2015-11-25 2018-10-30 Unisearch Associates Inc. Gas cell for absorption spectroscopy
CN110530848A (zh) * 2019-09-27 2019-12-03 国网重庆市电力公司电力科学研究院 一种检测装置及检测方法
WO2023079155A1 (fr) * 2021-11-08 2023-05-11 Helmut-Schmidt-Universität/Universität Der Bundeswehr Hamburg Agencement à passages multiples et dispositif d'élargissement spectral de rayonnement laser

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4127329A (en) * 1976-12-21 1978-11-28 Northeast Utilities Service Company Raman scattering system and method for aerosol monitoring
US4953976A (en) * 1989-03-20 1990-09-04 Spectral Sciences, Inc. Gas species monitor system
US5786893A (en) * 1993-04-15 1998-07-28 Board Of Regents, The University Of Texas System Raman spectrometer
US6795177B2 (en) * 2001-11-01 2004-09-21 Axiom Analytical, Inc. Multipass sampling system for Raman spectroscopy

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4127329A (en) * 1976-12-21 1978-11-28 Northeast Utilities Service Company Raman scattering system and method for aerosol monitoring
US4953976A (en) * 1989-03-20 1990-09-04 Spectral Sciences, Inc. Gas species monitor system
US5786893A (en) * 1993-04-15 1998-07-28 Board Of Regents, The University Of Texas System Raman spectrometer
US6795177B2 (en) * 2001-11-01 2004-09-21 Axiom Analytical, Inc. Multipass sampling system for Raman spectroscopy

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9523638B2 (en) 2014-09-07 2016-12-20 Unisearch Associates Inc. Gas cell assembly and applications in absorption spectroscopy
US9739705B2 (en) 2014-09-07 2017-08-22 Unisearch Associates Inc. Gas cell assembly and applications in absorption spectroscopy
US10113955B2 (en) 2015-11-25 2018-10-30 Unisearch Associates Inc. Gas cell for absorption spectroscopy
CN110530848A (zh) * 2019-09-27 2019-12-03 国网重庆市电力公司电力科学研究院 一种检测装置及检测方法
WO2023079155A1 (fr) * 2021-11-08 2023-05-11 Helmut-Schmidt-Universität/Universität Der Bundeswehr Hamburg Agencement à passages multiples et dispositif d'élargissement spectral de rayonnement laser

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