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Gas in scattering media absorption spectroscopy

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Gas in scattering media absorption spectroscopy (GASMAS) is an optical technique for sensing and analysis of gas located within porous and highly scattering solids, e.g. powders, ceramics, wood, fruit, translucent packages, pharmaceutical tablets, foams, human paranasal sinuses etc. It was introduced in 2001 by Prof. Sune Svanberg and co-workers at Lund University (Sweden).[1] The technique is related to conventional high-resolution laser spectroscopy for sensing and spectroscopy of gas (e.g. tunable diode laser absorption spectroscopy, TDLAS), but the fact that the gas here is "hidden" inside solid materials give rise to important differences.

Basic Principles

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Free gases exhibit very sharp spectral features, and different gas species have their own unique spectral fingerprints. At atmospheric pressure, absorption linewidths are typically on the order of 0.1 cm−1 (i.e. ~3 GHz in optical frequency or 0.006 nm in wavelength), while solid media have dull spectral behavior with absorption features thousand times wider. By looking for the sharp absorption imprints in light emerging from porous samples, it is thus possible to detect gases confined in solids – even though the solid often attenuates light much stronger than the gas itself.

The basic principle of GASMAS is shown in figure 1. Laser light is sent into a sample with gas cavities, which could either be small pores (left) or larger gas-filled chambers. The heterogeneous nature of the porous material often give rise to strong light scattering, and pathlengths are often surprisingly long (10 or 100 times the sample dimension are not uncommon). In addition, light will experience absorption related to the solid material. When travelling through the material, light will travel partly through the pores, and will thus experience the spectrally sharp gas absorption. Light leaving the material will carry this information, and can be collected by a detector either in a transmission mode (left) or in a reflection mode (right).

In order to detect the spectrally sharp fingerprints related to the gas, GASMAS has so far relied on high-resolution tunable diode laser absorption spectroscopy (TDLAS). In principle, this means that a nearly monochromatic (narrow-bandwidth) laser is scanned across an absorption line of the gas, and a detector records the transmission profile. In order to increase sensitivity, modulation techniques are often employed.

The strength of the gas absorption will depend, as given by the Beer-Lambert law, both on the gas concentration and the path-length that the light has travelled through the gas. In conventional TDLAS, the path-length is known and the concentration is readily calculated from the transmittance. In GASMAS, extensive scattering renders the pathlength unknown and the determination of gas concentration is aggravated. In many applications, however, the gas concentration is known and other parameters are in focus. Furthermore, as discussed in 2.2, there are complementing techniques that can provide information on the optical pathlength, thus allowing evaluation also of gas concentrations.

Challenges

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Diffuse light

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Unknown interaction pathlength

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Optical interference noise

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It is well known that optical interference often is a major problem in laser-based gas spectroscopy.[2][3] In conventional laser-based gas spectrometers, the optical interference originates from e.g. etalon-type interference effects in (or between) optical components and multi-pass gas cells. Throughout the years, great efforts have been devoted to handle this problem. Proper optical design is important to minimize interference from the beginning (e.g. by tilting optical components, avoiding transmissive optics and using anti-reflection coating), but interference patterns can not be completely avoided and are often difficult to separate from gas absorption. Since gas spectroscopy often involves measurement of small absorption fractions (down to 10−7), appropriate handling of interference is crucial. Utilised countermeasures include customized optical design,[4] tailored laser modulation,[5] mechanical dithering,[6][7][8][9] signal post-processing,[10] sample modulation,[8][11][12] and baseline recording and interference subtraction.[13]

In the case of GASMAS, optical interference is particularly cumbersome.[14] This is related to the severe speckle-type interference that originates from the interaction between laser light and highly scattering solid materials.[9] Since this highly non-uniform interference is generated in same place as the utility signal, it cannot be removed by design. The optical properties of the porous material under study determines the interference pattern, and the level of interference is not seldom much stronger than actual gas absorption signals. Random mechanical dithering (e.g. laser beam dithering and/or sample rotation ) has been found effective in GASMAS.[9][15] However, this approach converts stable interference into a random noise that must be averaged away, thus requiring longer acquisition times. Baseline recording and interference subtraction may be applicable in some GASMAS applications, as may other of the methods described above.

Applications

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Medical diagnostics

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See [16][17]

Optical porosimetry

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See [18]

Monitoring of drying processes

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See [19]

Pharmaceutical applications

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See [9][15][18]

Monitoring of food and food packaging

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Much of the food that we consume today is put in a wide variety of packages to ensure food quality and provide a possibility for transportation and distribution. Many of these packages are air or gas tight, making it difficult to study the gas composition without perforation. In many cases it is of great value to study the composition of gases without destroying the package.

The perhaps best example is studies of the amount of oxygen in food packages. Oxygen is naturally present in most food and food packages as it is a major component in air. However, oxygen is also one of the great causes or needs for aging of biological substances, due to its source for increase of chemical and microbiological activity. Today, methods like [Modified atmosphere] (MAP) and [Controlled atmosphere] packaging (CAP) are implemented to reduce and control the oxygen content in food packages to prolong [shelf life] and ensure safe food. To assure the effectiveness of these methods it is important to regularly measure the concentration of oxygen (and other gases) inside these packages. GASMAS provides the possibility of doing this non-intrusively, without destroying any food or packages. The two main advantages of measuring the gas-composition in packages without perforation is that no food is wasted in the controlling process and that the same package can be controlled repeatedly during an extended time period to monitor any time-dependence of the gas composition. The studies can be used to guarantee the tightness of packages but also to study food deterioration processes.

Much food itself contains free gas distributed in pores within. Examples are fruit, bread, flour, beans, cheese, etc. Also this gas can be of great value to study to monitor quality and maturity level (see e.g.[20] and [21]).

Spectroscopy of gas confined in nanoporous materials

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See [22][23][24]

References

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  1. ^ Sjöholm, M.; Somesfalean, G.; Alnis, J.; Andersson-Engels, S.; Svanberg, S. (2001-01-01). "Analysis of gas dispersed in scattering media". Optics Letters. 26 (1). The Optical Society: 16–8. Bibcode:2001OptL...26...16S. doi:10.1364/ol.26.000016. ISSN 0146-9592. PMID 18033492.
  2. ^ Silver, Joel A. (1992-02-20). "Frequency-modulation spectroscopy for trace species detection: theory and comparison among experimental methods". Applied Optics. 31 (6). The Optical Society: 707–17. Bibcode:1992ApOpt..31..707S. doi:10.1364/ao.31.000707. ISSN 0003-6935. PMID 20720674.
  3. ^ Svensson, Tomas (2008). "Fighting optical interference" (PDF). Pharmaceutical and Biomedical Applications of Spectroscopy in the Photon Migration Regime (PhD thesis). Lund University, Sweden. Sect. 4.3.4.
  4. ^ McManus, J. Barry; Kebabian, Paul L. (1990-03-01). "Narrow optical interference fringes for certain setup conditions in multipass absorption cells of the Herriott type". Applied Optics. 29 (7). The Optical Society: 898–900. Bibcode:1990ApOpt..29..898M. doi:10.1364/ao.29.000898. ISSN 0003-6935. PMID 20562931.
  5. ^ Reid, J.; El-Sherbiny, M.; Garside, B. K.; Ballik, E. A. (1980-10-01). "Sensitivity limits of a tunable diode laser spectrometer, with application to the detection of NO2 at the 100-ppt level". Applied Optics. 19 (19). The Optical Society: 3349–53. Bibcode:1980ApOpt..19.3349R. doi:10.1364/ao.19.003349. ISSN 0003-6935. PMID 20234619.
  6. ^ Webster, Christopher R. (1985-09-01). "Brewster-plate spoiler: a novel method for reducing the amplitude of interference fringes that limit tunable-laser absorption sensitivities". Journal of the Optical Society of America B. 2 (9). The Optical Society: 1464–1470. Bibcode:1985JOSAB...2.1464W. doi:10.1364/josab.2.001464. ISSN 0740-3224.
  7. ^ Silver, Joel A.; Stanton, Alan C. (1988-05-15). "Optical interference fringe reduction in laser absorption experiments". Applied Optics. 27 (10). The Optical Society: 1914–6. Bibcode:1988ApOpt..27.1914S. doi:10.1364/ao.27.001914. ISSN 0003-6935. PMID 20531678.
  8. ^ a b Fried, Alan; Drummond, James R.; Henry, Bruce; Fox, Jack (1990-03-01). "Reduction of interference fringes in small multipass absorption cells by pressure modulation". Applied Optics. 29 (7). The Optical Society: 900–2. Bibcode:1990ApOpt..29..900F. doi:10.1364/ao.29.000900. ISSN 0003-6935. PMID 20562932.
  9. ^ a b c d Svensson, Tomas; Andersson, Mats; Rippe, Lars; Johansson, Jonas; Folestad, Staffan; Andersson-Engels, Stefan (2007-12-21). "High sensitivity gas spectroscopy of porous, highly scattering solids". Optics Letters. 33 (1). The Optical Society: 80–2. doi:10.1364/ol.33.000080. ISSN 0146-9592. PMID 18157265.
  10. ^ Riris, Haris; Carlisle, Clinton B.; Warren, Russell E.; Cooper, David E. (1994-01-15). "Signal-to-noise ratio enhancement in frequency-modulation spectrometers by digital signal processing". Optics Letters. 19 (2). The Optical Society: 144–146. Bibcode:1994OptL...19..144R. doi:10.1364/ol.19.000144. ISSN 0146-9592. PMID 19829572.
  11. ^ Liger, Vladimir; Zybin, Alexander; Kuritsyn, Yurii; Niemax, Kay (1997). "Diode-laser atomic-absorption spectrometry by the double-beam—double-modulation technique". Spectrochimica Acta Part B: Atomic Spectroscopy. 52 (8). Elsevier BV: 1125–1138. Bibcode:1997AcSpe..52.1125L. doi:10.1016/s0584-8547(97)00029-3. ISSN 0584-8547.
  12. ^ Werle, P.; Lechner, S. (1999). "Stark-modulation-enhanced FM-spectroscopy". Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy. 55 (10). Elsevier BV: 1941–1955. Bibcode:1999AcSpA..55.1941W. doi:10.1016/s1386-1425(99)00067-0. ISSN 1386-1425.
  13. ^ Werle, P.; Mücke, R.; Slemr, F. (1993). "The limits of signal averaging in atmospheric trace-gas monitoring by tunable diode-laser absorption spectroscopy (TDLAS)". Applied Physics B: Photophysics and Laser Chemistry. 57 (2). Springer Science and Business Media LLC: 131–139. Bibcode:1993ApPhB..57..131W. doi:10.1007/bf00425997. ISSN 0721-7269. S2CID 120472037.
  14. ^ Svensson, Tomas (2008). "Gas in scattering media absorption spectroscopy" (PDF). Pharmaceutical and Biomedical Applications of Spectroscopy in the Photon Migration Regime (PhD thesis). Lund University, Sweden. Ch. 5.
  15. ^ a b Svensson, T.; Andersson, M.; Rippe, L.; Svanberg, S.; Andersson-Engels, S.; Johansson, J.; Folestad, S. (2008-01-18). "VCSEL-based oxygen spectroscopy for structural analysis of pharmaceutical solids". Applied Physics B. 90 (2). Springer Science and Business Media LLC: 345–354. Bibcode:2008ApPhB..90..345S. doi:10.1007/s00340-007-2901-6. ISSN 0946-2171. S2CID 123165703.
  16. ^ Persson, Linda; Andersson, Mats; Cassel-Engquist, Märta; Svanberg, Katarina; Svanberg, Sune (2007). "Gas monitoring in human sinuses using tunable diode laser spectroscopy". Journal of Biomedical Optics. 12 (5). SPIE-Intl Soc Optical Eng: 054001. Bibcode:2007JBO....12e4001P. doi:10.1117/1.2777189. ISSN 1083-3668. PMID 17994889. S2CID 46590102.
  17. ^ Lewander, Märta; Guan, Zuguang; Svanberg, Katarina; Svanberg, Sune; Svensson, Tomas (2009-06-15). "Clinical system for non-invasive in situ monitoring of gases in the human paranasal sinuses". Optics Express. 17 (13). The Optical Society: 10849–63. Bibcode:2009OExpr..1710849L. doi:10.1364/oe.17.010849. ISSN 1094-4087. PMID 19550485.
  18. ^ a b Svensson, Tomas; Alerstam, Erik; Johansson, Jonas; Andersson-Engels, Stefan (2010-05-17). "Optical porosimetry and investigations of the porosity experienced by light interacting with porous media". Optics Letters. 35 (11). The Optical Society: 1740–2. Bibcode:2010OptL...35.1740S. doi:10.1364/ol.35.001740. ISSN 0146-9592. PMID 20517400.
  19. ^ Andersson, Mats; Persson, Linda; Sjöholm, Mikael; Svanberg, Sune (2006). "Spectroscopic studies of wood-drying processes". Optics Express. 14 (8). The Optical Society: 3641–53. Bibcode:2006OExpr..14.3641A. doi:10.1364/oe.14.003641. ISSN 1094-4087. PMID 19516511.
  20. ^ L. Persson, B. Anderson, M. Andersson, M. Sjöholm and S. Svanberg, "Studies of Gas Exchange in Fruits Using Laser Spectroscopic Techniques ", FRUITIC-05, Symposium on Information and Technology for Sustainable Fruit and Vegetable Production (2005). [1]
  21. ^ Lewander, M.; Guan, Z. G.; Persson, L.; Olsson, A.; Svanberg, S. (2008-09-30). "Food monitoring based on diode laser gas spectroscopy". Applied Physics B. 93 (2–3). Springer Science and Business Media LLC: 619–25. Bibcode:2008ApPhB..93..619L. doi:10.1007/s00340-008-3192-2. ISSN 0946-2171. S2CID 73566631.
  22. ^ Svensson, Tomas; Shen, Zhijian (2010-01-11). "Laser spectroscopy of gas confined in nanoporous materials". Applied Physics Letters. 96 (2): 021107. arXiv:0907.5092. Bibcode:2010ApPhL..96b1107S. doi:10.1063/1.3292210. ISSN 0003-6951. S2CID 53705149.
  23. ^ Svensson, Tomas; Lewander, Märta; Svanberg, Sune (2010-07-21). "Laser absorption spectroscopy of water vapor confined in nanoporous alumina: wall collision line broadening and gas diffusion dynamics". Optics Express. 18 (16). The Optical Society: 16460–73. Bibcode:2010OExpr..1816460S. doi:10.1364/oe.18.016460. ISSN 1094-4087. PMID 20721033.
  24. ^ Yang, Lin; Somesfalean, Gabriel; He, Sailing (2014-01-29). "Laser absorption spectroscopy of oxygen confined in highly porous hollow sphere xerogel". Optics Express. 22 (3). The Optical Society: 2584–94. Bibcode:2014OExpr..22.2584Y. doi:10.1364/OE.22.002584. ISSN 1094-4087. PMID 24663551.
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