GB2240639A - A gas cell for infrared analysis of gaseous samples - Google Patents

A gas cell for infrared analysis of gaseous samples Download PDF

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Publication number
GB2240639A
GB2240639A GB9101651A GB9101651A GB2240639A GB 2240639 A GB2240639 A GB 2240639A GB 9101651 A GB9101651 A GB 9101651A GB 9101651 A GB9101651 A GB 9101651A GB 2240639 A GB2240639 A GB 2240639A
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United Kingdom
Prior art keywords
gas cell
entry
mirror
windows
cell
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GB9101651A
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GB9101651D0 (en
Inventor
Alfred Hirt
Eberhard Jacob
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MT Aerospace AG
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MAN Technologie AG
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Application filed by MAN Technologie AG filed Critical MAN Technologie AG
Publication of GB9101651D0 publication Critical patent/GB9101651D0/en
Publication of GB2240639A publication Critical patent/GB2240639A/en
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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/01Arrangements or apparatus for facilitating the optical investigation
    • G01N21/03Cuvette constructions
    • G01N21/031Multipass arrangements
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/01Arrangements or apparatus for facilitating the optical investigation
    • G01N21/03Cuvette constructions
    • G01N21/09Cuvette constructions adapted to resist hostile environments or corrosive or abrasive materials
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3504Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing gases, e.g. multi-gas analysis

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  • Physics & Mathematics (AREA)
  • Health & Medical Sciences (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)
  • Optical Measuring Cells (AREA)
  • Investigating Or Analysing Materials By Optical Means (AREA)

Abstract

A gas coil to the analysis of gaseous samples in an infrared spectrometer has at least one focusing mirror 22, 23 by means of which a beam 30, 33 may be focused and reflected to a second focus 26. There is an entry window 25 for the incoming rays and an exit window 26 for the rays 37 leaving the cell, each being positioned in a respective focus. The gas cell may be used at temperatures up to 350 DEG C in conjunction with aggressive materials. The main application for the gas cell is fluorine combustion analysis. <IMAGE>

Description

1 4 9 A Gas Cell for the Analysis of Materials 2:2.10 C=,;3,9 The
invention relates to a gas cell more particularly for the analysis of gaseous materials in an infrared spectrometer having an entry and an exit window for the infrared radiation.
Gas cells of this type are used for the analysis of materials In a procedure in which the cell is firstly evacuated, then filled with the gas to be analyzed and finally placed in an infrared spectrometer.
As a general rule cylindrical gas cells are employed, whose end walls form windows transparent to infrared radiation. There are however problems due to leakage, more particularly at high temperatures and in conjunction with aggressive gases. Furthermore the resistance to corrosion of known Infrared windows Is very low. Such windows rapidly become opaque to infrared radiation.
Accordingly one object of the present invention Is to provide a gas cell of the type Initially mentioned which Is highly vacuum- tight even at high temperatures.
In order to achieve this and/or other objects the present Invention provides a gas cell for the analysis of a gaseous sample In an infrared spectrometer, said cell having an entry window for infrared radiation, an exit window for the infrared radiation and internal mirrors with which the infrared radiation in a beam is able to be focused, said entry window and said exit window being respectively positioned in a respective focus.
owing to the provision of focal points and the location of 1 the infrared radiation windows at such points it is possible to provide windows which are very small in relation to the size and more specifically to the diameter of the gas cell, such windows rendering possible the production of highly effective vacuum-tight joints between the windows and the body of the cell. These connections are furthermore resistant to elevated temperatures.
This furthermore means that simultaneously the memory effect in a trace element analysis Is reduced owing to the smaller window areas. When performing trace element analysis using conventional gas cells there is namely a chance of memory effects, as for instance in the case of gas cells equipped with silver chloride windows when used for gases containing fluorine or fluorides at temperatures in excess of 3011 C. This is because a porous protective film Is then formed on the surface of the silver chloride, which although having the effect of protecting the underlying material against attack, as compared with the metallic surface of the body of the gas cell has a significantly greater absorption for gases. This effect Is negligible in the gas cells in accordance with the invention owing to the very small size of the windows. Furthermore owing to reaction with fluorine or fluorides silver chloride rapidly becomes opaque to infrared radiation.
A further advantage offered by the gas cell in accordance with the invention Is that the internal reflection means that the infrared radiation passes through the gas to be analysed at least twice so that the necessary optical path length may be achieved with a substantially smaller size of gas cell.
The mirrors used are preferably metallic ones and in accordance with a simple design of cell having one focussing So mirror, the mirror Is located at one end of the gas cell, and the entry and exit windows are at opposite ends.
In accordance with a further particularly useful feature of the Invention two mirrors are provided at opposite ends of the gas cell, one such mirror being preferably a plane mirror and the other -1 one being a focusing mirror. Coming from the plane mirror or, respectively, the entry window a conical radiation beam will Impinge on the focusing mirror, which will reflect the rays back as a parallel beam to the plane mirror. The parallel ray beam reflected by the plane mirror Is finally caused to converge by the focusing mirror to leave the gas cell via the exit mirror arranged at the focus.
This design offers the advantage that there is a complete symmetry of the ray path with reference to the axis of symmetry of the gas cell. This facilitates the necessary adjustment. Furthermore the infrared radiation passes through the gas contained in the gas cell four times.
The windows have a diameter of under 10 mm and preferably between 4 and 6 mm and they are placed with alignment with the optic axis in the plane mirror, the windows being placed of the two sides of the optic axis relatively close to each other. In order to facilitate the optical coupling of such gas cells in the spectrometer, there is the proposal in accordance with the invention to provide a prism associated with the gas cell In front of the entry and the exit windows in order to so deflect the beams that they are shone in different directions.
In the case of a gas cell with one plane and one spherical mirror the geometrical configuration of the gas cell is such that the end face with the plane mirror is circular and the opposite end face has an oval form so that the inner circumferential face of the gas cell surrounds the beams formed between the two mirrors as far as possible without any intermediate clearance. The effect of this Is that the optically effective volume of the gas cell Is nearly completely the same as the spatial volume of the gas cell, that is to say optically the space In the gas cell is used practically completely.
The gas cell in accordance with the invention is particularly suitable for inorganic element analysis, In which the sample is combusted with fluorine. The possible use of very small windows in the gas cell permits the utilization of very expensive materials, as for instance diamond, a material which resists the attack of fluorine and gaseous fluorides at temperatures up to 300,0 C and is substantially transparent for the complete infrared range.
Furthermore diamond reduces the memory effect in trace element analysis. The use of fluorine-resistant metallic materials, such as nickel for the gas cell and of diamond windows has for the first time opened up the possibility of recording infrared spectra of aggressive compounds such as CIF3, CIF5, BrF5, IF7# XeF4, XeF61 AsF5, SbF5, ReF7, and IrF6 at temperatures up to 3500 C.
In this respect further significant advantages are achieved for trace element analysis by the possibility of working at temperatures between 30 and 3500 C and of increasing the range of elements which may be assayed (because they form moderately volatile fluorides), since the working temperature is increased to between 200 and 30011 C.
The diamond windows are preferably mounted in nickel and brazed in the gas cell using titanium and/or zirconium as active components.
A further possible window material resistant to elevated temperatures and fluorine which may be used is lanthanum trifluoride, which however has a long wave cutoff at 800 cm-1, which reduces the range of measurement.
Further features and advantages of the Invention will be gathered from the ensuing detailed description of several embodiments thereof referring to the accompanying diagrammatic drawings.
Figure 1 shows a first working embodiment In accordance with the invention.
Figures 2 to 4 are respective views of a second working embodiment of the Invention.
In figure 1 a gas cell 10 is shown, which consists of a cylindrical metal container 11 with an entry opening 13 In its 1 1 bottom wall 12 and an exit opening 14 for the Infrared radiation 15. The second end wall of the cylindrical container 11 has a spherical mirror 16 which Is so designed that It reflects the beam 17 entering through the entry window 13 and focuses It on the exit window 14. The mirror 16 may have a reflective metallic coating, which Is applied to the Inner face of a concave end wall 18 of the gas cell 10.
The windows 13 and 14 have a very small diameter of under mm. In the case of such dimensions It Is possible to use windows transparent to Infrared with a very high sealing effect.
For low temperatures, between 25 and 301> C windows 13 and 14 of silver chloride are used for the Infrared transmission range of 5000 to 450 cm-1. Calcium fluoride Is a further conventional window material which at temperatures up to 25W C Is completely resistant to fluorine and resistant to most fluorides. However, calcium fluoride Is very brittle and It Is therefore very difficult to connect it with the container 11 with a superior sealing effect and In a manner resistant to elevated temperatures with the gas cell 11. This difficulties are however reduced owing to the small dimensions. Nevertheless calcium fluoride will only be used If only materials with an Infrared transmission In the range between 5000 and 930 cm-1 are to be expected in the gas sample.
The Internal reflection In the gas cell 10 means that the optical path Is twice as long as In conventional gas cells with two opposite windows. This leads to a reduction In the dimensions of the gas cell 10. A further factor relevant for the configuration of the body of the gas cell 11 Is the need to take Into account the opticaJly effective volume within the container 11. That Is to say, volume 20 of the container which is not pervaded by the Infrared radiation 15, 17 and 19 should be avoided. In the Illustrated working embodiment of figure 1 the volume 20 within a conical ring part Is not pervaded by the Infrared radiation 15 and 17. In this case It would be possible to design the container with conical configuration In place of a cylindrical one and then it Is possible to reduce the volume 20 which is not optically used.
Figure 2 shows a working embodiment of a gas cell with a plame mirror 22 and a focusing mirror 23, which are placed so as to be opposite to each other and on a common optic axis 24. The entry window 25 and the exit window 26 are provided In the plane mirror 22 with mutual allgnment with the optic axis 24 and a line 27, the mirrors being arranged on either side of the optic axis 24 with a clearance 1 in relation to each other.
Figure 2 also diagrammatically indicates the ray path between the two mirrors 22 and 23. A ray 30 focused In the sample chamber of an FTIR (Fourier transform infrared) spectrometer is deflected by a mirror coated prism 31 through 900 and through the entry window 25 into the gas cell cavity 32. The entering conical beam 33 impinges on the focusing mirror 23. In the illustrated working embodiment of figure 2 the focusing mirror 23 Is in the form of a mirror with spherical curvature, whose radius Is twice as great as the length h of the gas cell. Therefore the focus of the spherical mirror 23 will be located in the plane of the plane mirror 22.
The ray cone 33 entering through the window 25 Is accordingly reflected back by the mirror 23 as a cylindrical, parallel beam 34. Owing to the off-center position of the entry window 25 and therefore of the position of the ray cone 33 the cylindrical beam 34 will be Inclined at an angle of a In relation to optic axis line 24. This beam will be reflected back by the plane mirror 22 at the angle a. In order to make the drawing more straightforward the cylindric al beam, which is reflected back, is indicated with short dashes. The parallel rays 25 are then focused by the spherical mirror 23 at a point (the ray cone 36 being indicated in long dashes), the focus being located with bilateral symmetry about the optic axis 24 with respect to the entry focus.
It is at this point that the exit window 26 Is positioned. The collimated beam 27 emerging from the exit window 26 is finally deflected by the prism 31 through 9011 and passes to the analyzer.
1 -c t i 1 i i The arrangement described here offers the advantage that there is complete symmetry of the ray path with respect to the axis of symmetry of the gas cell which corresponds to the optic axis 24. The adjustment of such a gas cell and, respectively, of the mirrors is accordingly very simple.
on examining the ray paths It will be seen that starting with the circular outline at the plane mirror 22 the ray beam will be offset In the vicinity of the curved mirror 23 by an amount 1 so that at the end of the gas cell with the curved mirror 23 the rays will extend over an oval zone 351 and 331. Accordingly the rays occupy a frustoconical volume. For spectral analysis it Is desirable for the volume of the gas cell to be pervaded as completely as possible by the infrared rays. Therefore for an arrangement in accordance with figure 2 a gas cell container 40 Is provided, which is illustrated in section in figure 3 and in plan in figure 4, and has the form of the frustum of a cone, whose one end wall is circular in accordance with the plane mirror 22, while the opposite end wall has an oval cross section 351 and 331. Such an inner space 32 In the gas cell may for instance be produced by drilling two holes 34 and, respectively, 35, in round stock and inclined at the angles +a and -a In relation to the axis in accordance with the parallel ray beams 34 and 35. The sharp edges 41 then produced may be additionally smoothed off. The result is then an interior space 32 in the gas cell which is used optically to a substantially complete extent.
The material for the windows 13 and 14 and, respectively, 25 and 26 will be selected in accordance with the individual application and the temperature of operation for the spectrometer measurements. The gas cell container 11 and 40 will as a rule be manufactured of a metal, the mirrors also being made of metal.
The advantages of the gas cell in accordance with the invention make themselves more particularly felt in the case of Inorganic element analysis in methods In which the sample Is burnt with elementary fluorine followed by gas FTIR analysis on the products of combustion. owing to the small dimensions of the windows It is more readily feasible to fit the gas cells with expensive materials such as for Instance diamond for the windows, since conventional window, materials such as silver chloride only have a limited field of application for such windows. Silver chloride corrodes in the presence of fluorine and any fluorides even over 300 C and although at temperatures up to 25011 C calcium fluoride Is completely resistant to fluorine and to most fluorides, it is not transparent for the complete range of infrared measurement. owing to the suppression of the memory effect due to the use of infrared windows with small dimensions and manufactured of diamond, trace element analysis and working temperatures up to 35011 C become possible. This increases the number of elements which may be assayed, the further elements included being those whose fluorides are moderately volatile. The diamond windows In nickel mounts are set in the gas cell and brazed in a vacuum-tight manner. Furthermore the metal mirrors are also connected with the casing of the gas cell in a vacuum-tight manner, the edges of the mirrors serving as sealing edges if desired.
t

Claims (14)

Claims
1 A gas cell for the analysis of a gaseous sample In an infrared spectrometer, said cell having an entry window for Infrared radiation, an exit window for the Infrared radiation and an internal mirror with which the infrared radiation in a beam is able to be focused, said entry window and said exit window being respectively positioned In a respective focus.
2 The gas cell as claimed in claim 1, wherein at least one focusing metal mirror Is provided.
3 The gas cell as claimed In claim 1 or in claim 2, wherein said mirrors are a plane mirror and the focusing mirror which are centered on a common optic axis opposite to each other.
4 The gas cell as claimed in claim 3, wherein said focusing mirror is a spherically curved mirror, whose radius is twice as large as the distance between the mirrors along the common optic axis.
The gas cell as claimed In claim 3 or claim 4, wherein the entry and the exit windows for the Infrared radiation are contained In the plane mirror.
6 The gas cell as claimed In claim 5, comprising a deflecting prism for the entry and exit radiation in front of windows of the gas cell.
7 The gas cell as claimed In any one of the preceding claims, wherein the entry and the exit windows are positioned on a line which Is aligned with the optic axis on the two sides of the axis and close to the same.
8 The gas cell as claimed in any one of the preceding claims, wherein the said gas cell has a volume less than 400 ml and preferably of 30 to 200 ml and is so designed that the optically active volume Is equal to a major part of the volume of the gas cell and Is preferably equal to more than 90% thereof.
9 The gas cell as claimed In claim 3, wherein the configuration is made corresponding to the outline of the Infrared beams formed by simple and multiple reflection.
The gas cell as claimed in any one of the preceding claims, wherein such cell is used for the assay of inorganic elements by combustion with fluorine.
11 The gas cell as claimed in claim 10, wherein the entry and exit windows consist of diamond.
12 The gas cell as claimed in claim 11, wherein the said diamond windows are borne in nickel mounts which are brazed in the gas cell container In high vacuum using high temperature brazing materials with titanium and/or zirconium as active components.
13 The gas cell as claimed In claim 11, wherein the entry and exit windows consist of lanthanum trifluorlde.
14 A gas cell as claimed in claim 1, substantially as described hereinbefore with reference to and as Illustrated figure 1 of the accompanying drawings.
A gas cell as claimed In claim 1, substantially as described hereinbefore with reference to and as Illustrated in figures 2 through 4 of the accompanying drawings.
Published 1991 at The Patent Office. State House. 66/71 High Holborn, lAmdon WCIR 47P. Further copies may be obtained from Sales Branch. Unit 6. Nine Mile Point, Cwmfelinfach. Cross Keys, Newport. NPI 7HZ. Printed by Multiplex techniques ltd, St Mary Cray. Kent.
-1 Z
GB9101651A 1990-01-27 1991-01-25 A gas cell for infrared analysis of gaseous samples Withdrawn GB2240639A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
DE4002436A DE4002436A1 (en) 1990-01-27 1990-01-27 GAS CUVETTE FOR MATERIAL ANALYSIS

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GB2240639A true GB2240639A (en) 1991-08-07

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH0949793A (en) * 1995-08-07 1997-02-18 Fuji Electric Co Ltd Multi-reflection sample cell
WO2003002454A3 (en) * 2001-06-29 2003-05-30 Showa Denko Kk High-purity fluorine gas, production and use thereof, and method for analyzing trace impurities in high-purity fluorine gas
WO2016074773A1 (en) * 2014-11-10 2016-05-19 Dräger Safety AG & Co. KGaA Optical gas sensor comprising an led emitter for the emission of light of a narrow bandwidth

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DE4203166C2 (en) * 1992-02-05 1995-05-04 Erno Raumfahrttechnik Gmbh Device for analyzing gases or gas mixtures, in particular trace gases, by means of infrared spectroscopy
DE19926121C2 (en) * 1999-06-08 2001-10-18 Cs Halbleiter Solartech Analyzer
US6486474B1 (en) * 1999-08-13 2002-11-26 Regents Of The University Of Minnesota Infrared spectrometer for the measurement of isotopic ratios
DE10216047A1 (en) * 2002-04-11 2003-10-23 Univ Albert Ludwigs Freiburg Spectrophotometer multiple pass, monolithic optical cell for gas analysis and concentration measurements, provides extended path length without use of moveable mirror or other linkages
DE20301081U1 (en) 2002-05-24 2003-04-10 Dräger Safety AG & Co. KGaA, 23560 Lübeck Optical gas sensor
DE102018115200B4 (en) * 2018-06-25 2020-02-13 Lisa Laser Products Gmbh Method and device for optically measuring a sample arranged in a sample tube with a conical bottom
CN109406402B (en) * 2018-09-05 2020-12-11 浙江省海洋水产研究所 Universal cuvette device for absorbing fluorescence and measurement method

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GB1236926A (en) * 1967-09-29 1971-06-23 Perkin Elmer Ltd Gas analysis cells
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Cited By (6)

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Publication number Priority date Publication date Assignee Title
JPH0949793A (en) * 1995-08-07 1997-02-18 Fuji Electric Co Ltd Multi-reflection sample cell
WO2003002454A3 (en) * 2001-06-29 2003-05-30 Showa Denko Kk High-purity fluorine gas, production and use thereof, and method for analyzing trace impurities in high-purity fluorine gas
US6955801B2 (en) 2001-06-29 2005-10-18 Showa Denka K.K. High-purity fluorine gas, production and use thereof, and method for analyzing trace impurities in high-purity fluorine gas
WO2016074773A1 (en) * 2014-11-10 2016-05-19 Dräger Safety AG & Co. KGaA Optical gas sensor comprising an led emitter for the emission of light of a narrow bandwidth
CN107148566A (en) * 2014-11-10 2017-09-08 德尔格安全股份两合公司 The optical gas sensor of LED emitters with the light for launching narrow bandwidth
US10190974B2 (en) 2014-11-10 2019-01-29 Dräger Safety AG & Co. KGaA Optical gas sensor comprising an LED emitter for the emission of light of a narrow bandwidth

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DE4002436A1 (en) 1991-08-01
GB9101651D0 (en) 1991-03-06

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