MULTIPASS CELL AND ANALYSIS METHOD

Abstract: A multipass cell (1) for optical analyses, such as spectrometry, of a fluid; the cell comprises a plurality of mirror means (20) each having an essentially planar light-reflecting mirror face (210); the mirror means are arranged in an essentially circular pattern having a center (P); the planar mirror faces (210) are facing the center of cell (1).

Description: MULTIPASS CELL AND ANALYSIS METHOD Field of the Invention The present invention generally relates to devices for optical analysis of fluids as well as to methods for performing such analyses, and specifically to a multipass cell and an analytical method using such a cell. Background of the Invention A feature common to many opto-analytical methods is that the value measured not only depends upon specific physical parameters of the substance of interest, or analytical sample, and the optical radiation used in the synthesis but also, inter alia, from the length of the path of the radi¬ ation in the analytical sample. Typical opto-analytical methods where that path length influences the values measured and which, hence, are of interest for the invention are methods where the absorption of a more or less defined optical radiation by the sample is measured. Such methods are well known and include, but are not restricted to, normal absorption spectroscopy, fluorescence spectroscopy, Raman spectroscopy, as well as so-called heterometric methods, such as nephelometry (measuring turbidity) and the like. Measuring or monitoring the concentration of contaminants in exhaust gases is a specific and preferred example. Depending upon the absorptive power (optical absorbency) of a sample, two opposite approaches can be envisaged: in highly absorptive samples a relatively short path of the radiation in the sample would be preferably. On the other hand, in the case of low absorptive power of the sample, e.g. caused by a small absorptive power or by dilution of the substance of interest contained therein, a relatively long path is preferred because this will normally increase the sensitivity of the analysis by spectral absorption. Determination of traces of a particular substance, such as a pollutant, in a medium of interest, such as ambient air e.g. because of environmental or other reasons, by optical methods is an analytical field of growing interest, and a conventional means to increase reliability (i.e. reproduc- ibility), specifity and sensitivity of the particular opto- analytical method is the use of what is called a "multipass cell" because the length of the path of the analytic radi¬ ation in and through the analytical sample contained in or passing through the cell is prolonged by arranging mirrors within the cell such that the path length within the cell is prolonged by a number of reflections within the cell. Multipass cells for qualitative and quantitative methods of analyzing optically transparent fluids are known and have been described, for example, in DE-A-29 37 352 (the dis¬ closure of which specification is incorporated herein for all purposes by way of reference). Such prior art multipass cells generally comprise a chamber in the form of a cylindrical container associated with a number of mirrors for reflecting radiation that passes into the chamber through an optical entry and leaves the chamber through an optical exit. The mirrors of prior art multipass cells have been arranged either within or outside of the chamber. Generally, in prior art multipass cells substantially all radiation that passes through the chamber forms a bundle of rays that pass longitudinally or diagonally through the chamber. Consequently, the mirrors must be situated at or near the chamber ends and concave mirrors must be used to avoid focusing problems. According to the best knowledge of applicants, no prior art multipass cells are available which use planar mirrors. Obviously, concave mirrors for use in a multipass cell require a relatively high degree of optical precision and, hence, are costly. Positioning concave mirrors in known multipass cells re¬ quires means for adjusting the mirrors in order to compen¬ sate for mechanical and/or thermal deviations which may and generally do occur in the course of analytical operation. This is particularly important if the mirrors are arranged on different portions of the cell, for example on separate flanges or rear walls of a cylindrical chamber. A further disadvantage of known multipass cells is that they usually permit only one single predetermined length of the optical path. For analyzing different components of the same fluid, however, a variety of optical paths of differing length may be needed which, in turn, would require using several different cells or an extremely sophisticated structure for varying the path length of a given multipass cell. Further, in the practice of using prior art multipass cells the number of reflections tends to be limited while posi¬ tional control of the mirrors for operation and maintenance is complicated and requires skilled operators. Objects. Summary, and Terms of the Invention Accordingly, it is a main object of the present invention to provide for a multipass cell that operates reliably and sensitively with ordinary (i.e. planar) mirrors, permits a high number of passes, and greatly facilitates problems of positioning the mirrors. This object and further advantages which will become appar¬ ent as the specification proceeds are achieved according to a first general embodiment of the invention by a multipass cell for optical analysis of a fluid comprising a plurality of mirror means each having an essentially planar light- reflecting mirror face and, in a preferred embodiment, a backface parallel to the mirror face; the mirror means are arranged in an essentially circular pattern having a cent¬ er, and the planar mirror faces face that center. The term "light-reflecting mirror face" or an abbreviation of this term as used herein generally refers to a normally solid surface capable of reflecting light of the type used in the particular analytical use of the cell. The term "light" is used herein in its broadest sense to include polychromatic and/or monochromatic electromagnetic radiation of any wave length that permits analytical oper¬ ation by known methods and means, e.g. of focusing, reflecting and detection. A preferred type of light is polychromatic or monochromatic radiation within the wave length region extending from ultraviolet (UV) to infrared (IR) as understood in the optical art. The term "reflection" is not limited herein to a particular high degree of reflection, or - conversely - low degree of absorption. A degree of reflection of only about 50% (mean¬ ing that only half of the impinging radiation is reflected while the other half is allowed to pass) may be sufficient. However, for many purposes of the invention a bright sur- face such as conventionally used for optical mirrors will be satisfactory. Metal deposits, e.g. made by vacuum depo¬ sition of suitable metals including noble metals, such as gold, platinum or alloys, or by precipitation of silver, covered or not by a protective layer, as well as highly polished surfaces of metals, such as silver, noncorrosive steel, gold and other noble metals can be used for mirrors in cells according to the present invention. The term "planar" is used herein as generally understood in the art and refers to the geometric shape of a mirror face suitable according to the invention. A simple test of planarity is generation of an undistorted and identically sized mirror image of an object. In view of the obvious advantages of planar mirrors over concave mirrors it is surprising that prior art does not disclose multipass cells with planar mirrors. It is believed that the problems of positional control and place- ment of planar mirrors has prevented use of such mirrors in multipass cells; according to a general aspect, the present invention teaches a simple mirror arrangement for a multi¬ pass cell and a method of manufacturing a multipass cell. Brief Description of Preferred Embodiments In the course of the studies leading to the present inven¬ tion we have found that the problem of arranging a suffi¬ ciently large number of planar mirrors while maintaining a high degree of positional control can be solved in a sur¬ prisingly simple and efficient manner; both the mirror arrangement as well as manufacture of a multipass cell, according to the present invention, are predicated to a large extent, at least, upon the commercial availability of relatively simple structures with a high degree of dimen¬ sional precision (deviation from nominal dimension typical- ly less than ± 1 μm and preferably not more than ± 0.1 μm) which, according to the invention, can be used effectively for positioning of planar mirrors. Preferred positioning means according to the invention include two types of complemental elements of which the first or "recessed" type serves to support the mirrors along an outer or inner peripheral surface of a circular body (also termed "mirror support" herein). Preferred examples of the first type are circular bodies with a plu¬ rality of regularly spaced and regularly shaped peripheral recesses formed by surfaces that extend parallel to the axis of rotation of the circular body and are arranged along an outer peripheral face of a generally disk-shaped or wheel-shaped circular body, or along an inner peripheral face of an annular body. Preferably, the recesses should be spaced equidistantly around the periphery. Also, it is preferred that the mirror support should have an even number of recesses for receiv¬ ing a plurality of distancing and positioning elements of the type explained in more detail below. The recesses may but need not have a symmetrical shape relative to a theor¬ etical radial line through the "center" of each recess. Typical preferred examples of this first type of position¬ ing means are cog wheels (recesses at an external periph¬ ery) and ring gears (recesses at an internal periphery) both of which should be of the "straight toothed" type and consist of a structural material, such as a metal selected from steel, aluminum and other metals capable of high precision moulding or machining. Preferred examples of the second or "protruding" type of positioning means are cylindrical elements (also termed "cylinders" herein for brevity), such as normally used in cylinder bearings, i.e where the mechanical connection between the outer and inner shell of the bearing is formed by a number of steel cylinders having uniform diameters and being arranged equidistantly between the shells of the bearing. The present invention teaches the combination of a mirror support, such as a cog wheel or ring gear, with a suffi¬ cient number (i.e. corresponding with the number of recesses) of cylinders, all having a uniform diameter some- what larger than the depth of the recesses ( "matching diameters" herein) so that each recess securely defines the position of a cylinder, and that two adjacent (and all other) cylinders protrude from the recesses for defining the position of a corresponding planar mirror in a mirror arrangement for use in a multipass cell. A "corresponding" or "coordinated" mirror is that particular mirror which is in contact with, and held in a precisely determined posi¬ tion by, two adjacent cylinders which, in turn, are those "coordinated" to a particular mirror. From this general teaching it will be apparent that shape and dimensions (width and depth) of the recesses should match with the cylinders such that each cylinder can be held snugly in a recess of the mirror support and is pre- vented from any displacement in the direction of the recessed periphery of the mirror support. Further details will be discussed below in connection with the drawings. In sum, the present invention provides a novel type of multipass cell having a plurality of planar mirror surfaces in a circular arrangement, geometrically positioned in a simple and exact manner by means of elements which are commercially available and have a high degree of precision while permitting any desired chamber volume, and wherein a very high number of reflections is possible as well as a variation of the length of the optical path in one and the same multipass cell. Generally, the cell according to the invention comprises a chamber means for holding the mirror arrangement and re¬ ceiving a fluid probe. The term "fluid" is intended to encompass substances which, at the operating temperature, are liquids, gases or mixtures of gases and liquids; obvi¬ ously, the substance of interest could be either fluid or solid if it is dispersed in a medium which is to be optically analyzed according to the invention. The chamber may have any desired shape and volume and may consist of any material capable of receiving and retaining the sample of analytic interest. The chamber may but need not consist of a material that is transparent to the optical radiation used. However, for many purposes of the invention the chamber will comprise a housing made essen¬ tially of a metal or synthetic plastic and will have an optical entry and exit for the radiation used in the analy¬ tic method in which the cell according to the invention is employed. Preferably, the chamber means of a multipass cell according to the invention will have a single optical port only, which serves both as an entry port and an exit port, i.e. when the optical radiation used is reflected out of the chamber after having passed the mirror arrangement. The optical port may be positioned either on a peripheral por¬ tion of the cell or on top of the latter if a deflector is arranged within the cell for directing the path of the radiation into a plane which is normal (90°) to the central axis of the circular mirror arrangement according to the invention. Generally, the chamber will be a housing which receives and holds the circular mirror arrangement according to the invention. Such an arrangement will have a central axis defined by the point that is equidistant from all mirrors of the mirror arrangement. For the purpose of definition, the plane that intersects perpendicularly both with the central axis as well as with the longitudinal extension of all mirrors can be considered to constitute the "main plane" of the cell. Since the mirror arrangement comprises an essentially circular mirror support, the chamber may have a generally circular outer shape as well, it being understood that the outer shape of the chamber or housing is a matter of choice, and that the circular mirror support may be sur- rounded by an element which need not be circular. A pre- ferred type of housing will be in the form of a cylinder or disk. According to another and generally preferred embodiment, each mirror has a peripheral position defined essentially by a line (termed "S" hereinbelow) which extends perpen¬ dicularly (i.e. "normal") relative to its planar mirror face, and a majority, at least (generally meaning all mirrors, or all mirrors but one), of the mirrors is posi¬ tioned such that the angle (α) enclosed between lines (S) of mutually adjacent mirror faces satisfies the condition 360°/n, in which n is an even-numbered integer and at least equal to four. The term "360°" is intended herein to refer to a full circle (as opposed to the alternative definition of a full circle having 400°). The number of mirrors provided in a given arrangement according to the invention is limited by practical rather than theoretical parameters, namely by the size of the cell. Typically, the number of mirrors will be in the range of from 6 to 24 but neither the lower nor the upper limit is believed to be critical per se. Further, normal lines (S) of the reflecting mirror surfaces will intersect at a common point (P) within said cell. Preferably, point P coincides with the point that defines the intersection of the central axis mentioned above with the main plane. Each mirror will be in operative physical contact with two coordinated cylinders as explained above; depending upon the choice of the mirror support, the cylinders may contact either the mirror face of each mirror or its back face. Preferably, the physical contact is as direct as feasible but intermediate elements might be used between each mirror and its coordinated cylinders if this is required for specific purposes. The peripheral recesses or grooves of the mirror support may have any cross-sectional shape (as viewed in the main plane); preferably, all recesses of a given mirror support have the same shape and size, such as typically the teeth or cogs of a cog wheel or ring gear. Such grooves may be V- or U-shaped, or have a curved trapezoidal shape formed by two symmetrical flanks which may be straight or curved, and which form an opening angle in the direction of preferably 60° to 120°. For positioning a mirror, one cylindrical element is arranged in each of two neighbouring grooves and each mirror is made to be held in operative contact with two cylinders. Accordingly, the contacts between the mirror support and the cylindrical elements, on the one hand, and between the cylinders and the mirrors, on the other hand, will have the form of well defined lines. The length of the cylindrical element is not believed to be overly critical if the line of contact with the corresponding recess and mirror will have a sufficient length, i.e. sufficient for securely defining the position of the mirror. The cylindri¬ cal elements may extend from the bottom face of the mirror support up to the top face of the mirror support, or beyond either face, or have a smaller length then the contacting faces of the recess of the mirror support. If the cylinders are situated in a position between the corresponding mirror and the center of the arrangement, i.e. in the case of a mirror support where the recesses are arranged at an inter¬ nal periphery of the support, the cylinders should not extend to that part of the reflecting mirror surface where a beam of radiation that passes the cell will be reflected. Typically, each cylindrical element will have an axial length that corresponds to at least half the "height" (length in axial direction) of the corresponding recess or groove of the mirror support. The diameters of the cylin¬ ders are chosen such that the coordinated mirror does not contact the peripheral face of the mirror support. If a mirror is positioned symmetrically on two adjacent cylin¬ ders having the same diameter, the corresponding mirror will be aligned normally (at 90°) with regard to a line that passes through the center point P explained above. If a given mirror is to be positioned such that its reflecting face is aligned other than normally (e.g. for double passage of the light through the cell), the contact¬ ing cylinders will have a differing diameter. In other words, the diameters of the cylindrical elements can be used to define the angular position of the coordinated mirror. As mentioned briefly above, commercial¬ ly available and mass-produced bearing rollers are made to meet extremely stringent requirements as to diameter toler- ances and are available at very low prices. Similar con¬ siderations apply as to the tolerances of the diameters and recess dimensions of commercially available cog wheels and ring gears. As will be easily understood, each mirror will have a length sufficient to provide firm contact with the cylin¬ ders and to extend upwards from the mirror support for a sufficient length to provide an operative reflecting face portion of sufficient length, e.g. extending beyond the upper face of the mirror support by at least about 30% of the length of the recess in the mirror support. If the mir¬ rors are arranged along the inner peripheral surface of a mirror support, such as a ring gear, then the backside - faces of the mirrors should be planar as well and, prefer¬ ably, parallel to the mirror faces because in this case the cylindrical element will be in contact with the back faces of the mirrors. In order to secure and hold the mirrors in their positions in contact with the cylinders and safely connected to the mirror support, conventional clamps or tensioning means can be used. According to a first preferred embodiment of the invention, the mirrors are releasably mounted and, for example, secured by a clamping bracket. According to a second preferred embodiment, the mirrors are permanently connected with the cylinders and secured, for example, by means of a polymer adhesive such as a thermoplastic polymer or a reactive resin of the epoxy type. The mirror carrier with the cylinders and the mirrors can be mounted on a flange which closes off or seals the multipass cell. Conventional optical means for entry and exit of the radiation into and from the cell can be arranged on the chamber wall or at the flange. Details concerning the entry and exit optics, the ports for passing the fluid sample into or through the cell as well as flushing means and other components for operation of multipass cells are known in the art and need no further discussion. In a preferred embodiment of the inventive multipass cell all mirrors are arranged equally and evenly so that the lines S of the mirror surfaces intersect with the axis of the mirror support and are arranged equidistantly with regard to such axis. The angles between adjacent lines S are equal and, preferably, amount to 360°/n, wherein n is an even integer and at least 4. The number of mirrors, on the other hand, need not be even and if one particular mirror in an otherwise circular arrangement would interfere with the port of entry and exit of the radiation used, this mirror can be omitted and the resulting arrangement will still be considered "essentially circular" in the sense of the invention. In order to obtain the greatest possible number of reflections and an optical path of maximum length, the entering beam of radiation preferably is not introduced centrically, i.e. aimed at the diagonally opposite mirror face, but at one of the mirror faces next to the diagonally opposed mirror face. Due to the symmetry of the mirror arrangement, and with such an angle of entry, the incident beam of radiation is reflected n-1 times and exits from the cell after n pass¬ ages through the chamber via the exit optics. If n is a relatively large even integer, preferably having even divisors, a different number of reflections may be obtained. For example, the following entry angles will result in a different number of passages of the light ray for n=24 and its even numbered divisors 12, 8, 6 and 4: i.e. 45° for 4, 30° for 6, 22.5° for 8, 15° for 12 and 7.5° for 24 reflections. In this manner, a plurality of beams can be made to pass optical paths of different length without the need to change the position of the mirrors. Mirror arrangements according to the invention that deviate from a strictly uniform symmetry can be provided easily, and will provide for specific benefits, if the pair of cylinders coordinated with of one or more mirrors have differing diameters. Optimal arrangements of the mirrors for any given purpose and the corresponding dimensions of mirror support and cylinders can be calculated easily by those experienced in the optical art. For example, the optical path can be doubled by turning the last reflecting mirror by α/4 at the base point of line S so that the reflected beam passes through the same optical path twice but in opposite directions. The preferred arrangement of mirrors in an even and essen¬ tially isomorph pattern on a symmetrical mirror support is advantageous for operational safety, in particular because additional means for adjusting and handling the mirrors are not required. Upon a homogenous thermal expansion or con¬ traction of the mirror support due to changes of the ambi¬ ent temperature, the length of the optical paths changes to a minor extent but the angles or the reflections remain constant. Brief Explanation of the Drawings The invention will be better understood when reference is made to the accompanying drawings in which: Figure 1 is a schematic top view illustration of a prior art multipass cell; Figure 2 is a schematic top view of a circular arrangement of a plurality of mirrors in a multi¬ pass cell according to the invention; Figure 3 is a more detailed yet still schematic top view of a multipass cell according to the inven¬ tion; Figure 4A is a schematic top view of an arrangement of a plurality of planar mirrors positioned on a circular mirror support; Figure 24 is a schematic side view along line A-A of Figure 4A; and Figure 5 is a schematic top view of a segment of a mirror support to show the effect of cylinder pairs of the same and of differing diameters. Detailed Discussion of the Invention and Examples The prior art cell 1 shown diagrammatically in Figure 1 comprises a housing or chamber 15 containing three concave mirrors 14,16,18. Radiation enters at port 11 and leaves at port 12. Due to reflection of the radiation by the concave mirrors, the length of the path of the radiation will be three times that of chamber dimension L. Minor deviations of the path of radiation passing cell 1 will be compensated by the concave shape of the mirrors. While it is possible to increase the number of reflection and, hence, the path length, e.g. by controlled motion of the concave mirrors, it will be apparent that such motion requires rather soph¬ isticated arrangements for actuation and control. In contrast, a circular arrangement 2 of planar mirrors 20 according to the invention as depicted in top view in Figure 2 provides a much large number of passes without any motion of the mirrors relative to each other. Obviously, such an arrangement will provide substantial advantages over a multipass cell with concave mirrors if placement of the individula mirrors in the arrangement can be effected with the required degree of precision. Figure 3 shows in a diagrammatic manner an inventive multi¬ pass cell 3 having a chamber 19 with an entry port 110 and an exit port 120 for the sample fluid, as well as entry and exit optics 13 for optical radiation. Chamber 19 contains a an essentially circular arrage ent of a plurality of mir- rors 20 arranged in rotational symmetry on a mirror support 30 so that the light-reflecting planar mirror faces 210 are orientated towards the rotational axis P of support 30 and are spaced therefrom by equal distances so that their "theoretical perpendiculars" or "normal" lines S, intersect at point P. Lines S of two adjacent mirrors (20a, 20b) intersecting at P form the sides of angle α which preferab¬ ly is 360°/n, wherein n is an even numbered integer greater than, or equal to, 4. Back faces 201 of mirrors 20 may but need not extend in parallel to reflecting mirror faces 210. As will be apparent to those experienced in the optical art, the length of the path in a cell according to the invention can be varied depending upon the angle of inci¬ dence of radiation into the cell which, in turn, will determine the first mirror which reflects the radiation. In a preferred embodiment of using an inventive cell, the source of radiation and its path into the cell will be maintained constant while the angular position of the cell or chamber is set for the desired path length. This is but a matter of convenience, however, and is possible as well to maintain constant the position of the cell and varying the path of the radiation into the cell. Figure 4A illustrates in a diagrammatic manner and for brevity in one and the same drawing both mirror supports with external as well as with internal peripheral faces: on the left side of the drawing of Figure 4A mirror support 41 is of the cog wheel type while mirror support 42 on the right hand side of Figure 4A is of the ring gear type. In reality, a mirror arrangement according to the invention would normally include either a mirror support with an external peripheral face, or a mirror support with an internal peripheral face since a combination of both types would complicate the structure without compensatory addi¬ tional advantages. Both types of mirror support are shown in Fig. 4A with the preferred symmetrical arrangements of mirrors 20, either along external peripheral face 410 of mirror support 41, or along inner peripheral face 420 of mirror support 42. With either type of support, mirrors 20 are arranged adjac- ent to and in contact with pairs of cylinders 30 which, in turn, are positioned in matching recesses 43 of the outer or inner peripheral faces 410, 420 of mirror supports 41 and 42, respectively. Each mirror 20 is dimensioned and positioned such that it is located adjacent to and in contact with two cylindrical elements 30a and 30b but does not contact mirror support 41 or 42; recesses 43 and elements 30 are dimensioned so as to satisfy this require¬ ment. Figure 4B shows the sectional view along line A-A of Figure 4A; again, the left side of the drawing illustrates the cog wheel type mirror support 41 with its external peripheral face 410, while the right hand side of the drawing shows the ring gear type mirror support 42 with its internal peripheral face 420. Because all reflecting mirror faces 21 are oriented toward (i.e. face) rotational axis P, mirror faces 21 (as shown in Figure 4B) must extend above cylinders 30 and the upper face of support 41 when using the outer peripheral face 410 of mirror support. Contrarily, when arranging the mirrors at the internal periphery 420 of a gear ring type mirror support 42, mirrors 20 need not extend beyond top face 411 of mirror support 42 and the cylinders 30 could well extend above mirrors 20 if this would be desired for specific reasons. On the other hand, when using the internal periph¬ eral face 420 of a gear ring type mirror support 42 for positioning mirrors 20 the back faces 201 of such mirrors 20 should be essentially parallel to the mirror faces 21 so that the cylinders 30 with the same diameters can be used. However, the use of generally prismatic mirrors (not shown), i.e. with an enclosed angle between the mirror face and the back face) is not excluded if cylinders of differ- ing diameters are to be used. Figure 4 illustrates a diagrammatic top view of a broken- off segment 5 of mirror support 51. Grooves 55 serve to receive cylinder pairs 53a,b;54a,b. The cylinders of cylin¬ der pair 53a and 53b have the same diameter "d" in line with one of the generally preferred embodiments; as a consequence, normal line S of mirror 20 (as well all other mirrors not shown) will intersect with point P at the center of mirror support 51. All mirror faces are turned towards center point P. However, when using cylinders 54a, 54b of differing diam¬ eters "d." and "d 2 ", normal line S of mirror 58 will not intersect with point P. As will be explained below, the diameters of cylinders 54a, 54b will have a difference of diameters such that line S of mirror 58 is "offset" rela¬ tive to a normal and intersecting line S by an angle of α/4. Example 1 This example illustrates an arrangement of mirrors 20 in accordance with the embodiment of Fig. 4 to provide for twenty-four passages of the light ray. The angle α between two perpendiculars S of neighbouring mirrors 20a and 20b was 360°/24, i.e. equal to 15°. Mirror support 41 is a circular disk with a radius of 45.2746 mm, a height of 16 mm, and has V-shaped grooves 43 in its outer peripheral face 410. As a matter of practice, mirror support 41 of this example was a cog wheel with forty-eight equally shaped and dimen¬ sioned "teeth" or recesses 43 in equidistant distribution around the external face of the cog wheel, i.e. each with an angular displacement of the recesses of 7.5°; each recess has straight and symmetrical side faces arranged at an outwardly diverging angle of 90°. The depth of all recesses 43 is 2828.4 micrometers (μm) . A cylinder 30 with a length of 16 mm and a diameter of 4 mm is placed into each recess 43. Twenty-three mirrors 20 with plane mirrors faces 21 are each arranged on adjacent pairs of cylinders 30. One theoretically possible mirror location is left out to permit placement of the optical port for entry and exit of radiation. All mirror faces 21 are orientated towards the rotational axis P of support 41 and have a height of 24 mm, i.e. they project above support 41 by 1/3, or 6 mm. This "free" portion of mirror face 21 is used for reflecting the radia- tion. Alternatively, the mirror support is an annulus 42 having a circular internal peripheral face 420 and dimensions to provide for a mirror arrangement essentially as explained above. Various means can be used for securing the mirrors and cylinders on the mirror support. Resilient steel brackets were used in this eample and be secured (in a manner not illustrated) to the bottom face of support 41. Alternative¬ ly, a flange or ring (not illustrated) can be used to press the mirrors with a defined force or spring tension against the cylinders. All remaining cell components, i.e. the ports for passing sample fluid into and out of the cell, the optical port for the radiation including selection of suitable materials will be apparent to those experienced in the art of con¬ structing and using prior art multipass cells. Example 2 The cell of this example was similar to that of Example 1 except that the last reflecting mirror face of the path was arranged at an angle different from 90° as explained in connection with Figure 5. Mirror support, recesses and mirrors were the same as in Example 1 except that the cylinders of pair 54a, 54b in contact with mirror face 52 had diameters different from those of the other cylinders. The diameter of cylinder 54a was 4167.7 μm and that of cylinder 54b was 3846.0 μm. All other cylinders had a diameter of 4000.0 μm. Compared with the symmetrical arrangement of mirror faces 21 in Example 1 , this results in an angular displacement of mirror face 58 at the origin or base point of line S d of α/4 which corresponds to 3.75°. The direction of the angular dis¬ placement relative to point P preferably is selected such that the radiation reflected by mirror face 58 impinges upon the diagonally opposite mirror face (not shown in Fig. 3) so that it will be passed back again along the same path along which it had travelled before being reflected on mirror face 58. This results in a doubling of the optical path in the cell which, however, can again be varied by the angle of the radiation at the point of entry into the cell as explained above. It is to be emphasized that the above illustrations and examples of the invention are meant to illustrate and not to limit the invention. Various modifications will be apparent to those skilled in the art of making and using multipass cells for spectroscopy and related purposes. For example, optically transparent fluids of interest for the present invention include gases and liquids which are at least partially permeable in the wave length range of ultraviolet (UV) to infrared (IR). The chamber enclosing the circular mirror arrangement of the invention can be made of glass, a ceramic material, stainless steel, structural synthetic polymer or of a sheet metal coated if necessary with at least one protective layer to resist chemically aggressive probe media under the conditions of measurement, preferably ambient temperatures and pressures normally used in conventional multipass cell chambers for receiving the fluid probe. Filling and dis¬ charging the probe into and from the cell and any surround¬ ing chamber can be effected batch-wise or continuously, and appropriate openings are provided for this purpose as is conventional in prior art multipass cells. Positioning, mounting and securing of the mirrors along an external or internal peripheral circular face of the mirror support (which, in turn, can but need not be circular as well) can be carried out as described as above using the recesses at the periphery of the mirror support as a kind of gauge or master and then securing the mirrors by flex¬ ible brackets, or by embedding their lower ends into a cross-linkable polymer composition, curing the latter and then removing the gauge. Further, conventional means for measuring and/or correcting operational parameters such as temperature, pressure etc. can be arranged within the cell. As explained above, preferred distancing elements between the mirrors and the mirror support are high-precision cylinders of the bearing type because of the commercial availability of such mass-produced high-precision items. However, the scope of the present invention encompasses use of distancing elements other than cylinders, e.g. with a polygonal cross-section, if such elements are or become available commercially and at relatively low costs. By the same token, the cylinders or other distancing elements with the same function in the invention need not be made of steel but could consist, at least in part, of other ma- terials that, when shaped into cylindrical or other types of distancing elements according to the invention, provide for a sufficient degree of dimensional stability and main¬ tenance of sufficiently small tolerances. Accordingly,

Claims: What we claim is 1. A multipass cell for optical analysis of a fluid; said cell comprising a plurality of mirror means each having an essentially planar light-reflecting mirror face; said mirror means being arranged in an essentially cir¬ cular pattern having a center, and said planar mirror faces facing towards said center. 2. The cell of claim 1 comprising a chamber means for receiving a fluid probe; said chamber means further comprising an arrangement of said mirror means in said circular pattern within said chamber means; said cham¬ ber means having a main plane, and each of said planar mirror faces having a longitudinal extension which is normal to said main plane. 3. The cell of claim 2 wherein each mirror means comprises a back face extending parallel to said planar mirror face. 4. The cell of claims 2 or 3, wherein: (i) each mirror means in said arrangement has a posi- tion defined essentially by a line (S) normal rela¬ tive to said planar mirror face of said mirror means; (ii) a majority, at least, of said mirrors means in said arrangement is positioned such that the angle (α) enclosed between said normal lines (S) of mutually adjacent mirror faces satisfies the condition 360°/n, in which n is an even integer and at least equal four, and 360° defining a full circle; and (iii) said normal lines (S) of said majority of reflec- ting surfaces intersect at a common point (P) with¬ in said cell. 5. The cell of claim 4, wherein n equals the number of mirror means in said arrangement. 6. The cell of claim 4, wherein said point (P) is spaced equidistantly from said reflecting mirror faces and defines an axis which extends parallel to said longitu¬ dinal extensions of said reflecting mirror faces. 7. The cell of any of claims 1 - 6, wherein each of said mirror means is in contact with two cylindrical ele¬ ments, each of which is positioned and held in a peri- pheral recess of a circular body having an axis of rotation, and each said recess being defined by planes extending parallel to said axis of rotation. 8. The cell of claim 7, wherein said circular body is a straight-toothed cog wheel having an even number of teeth. 9. The cell of claim 7, wherein said circular body is a straight-toothed ring gear having an even number of teeth. 10. The cell of claim 7, wherein said cylindrical elements are bearing cylinders. 11. A mirror arrangement for a multipass cell comprising: a plurality of mirror means each having a planar mirror face and a planar back face extending par¬ allel to said mirror face; each of said back face being in contact with two cylindrical elements; each of said cylindrical elements being posi¬ tioned and held in a peripheral recess of a circular body having an axis of rotation; each recess being defined by planes exten¬ ding parallel with said axis of rotation. 12. The mirror arrangement of claim 11, wherein said cir¬ cular body (40) is a straight-toothed element having an even number of teeth and is selected from the group consisting of cog wheels and rim gears. 13. A method of producing a multipass cell comprising the steps of: providing a mirror arrangement as claimed in claims 11 or 12; mounting said mirror arrangement within an essentially fluid-tight cell having an optical passage; providing means for passing a fluid sample into said cell; and providing means for passing light through said cell. 14. A method of optically analyzing a fluid sample compri- sing the step of: providing a multipass cell as claimed in any of claims 1 - 10; feeding a sample fluid into said cell; passing a beam of light through said cell and measuring a physical parameter of said fluid which is dependent upon the length of passage of light through said sample fluid. 15. The method of claim 14 wherein said beam of light is a laser beam.

Inventors: PELLAUX JEAN-PAUL, WEBER EUGEN, PELLAUX, JEAN-PAUL, WEBER, EUGEN
Applicants: ORBISPHERE LAB, PELLAUX JEAN PAUL, WEBER EUGEN, ORBISPHERE LABORATORIES NEUCHATEL S.A., PELLAUX, JEAN-PAUL, WEBER, EUGEN
Application year: 1996
Main classification: G01N21
IPC codes: G01N21