OPTICAL FLOW CELL FOR USE IN SPECTRAL ANALYSIS

Abstract: An optical flow cell (1) for use in spectral analysis comprises a housing (2) formed with intersecting first and second bores (4 and 3) forming between them a fluid cavity (5). The first and second bores (4 and 3) serve, respectively, for fluid throughflow and radiation transmission. The second bore (3) holds in a fluid tight fit and releasable fashion two radiation transmitting optical probes (6 and 7) having each an outer end portion and an inner end portion with an inner end face (13). The outer end portion of one probe (6) is linked to a radiation source and the outer end portion of another probe (7) is linked to a spectrometer, the inner end faces (13) of the probes facing the fluid cavity (5) and being spaced from each other. Each of the optical probes (6 and 7) is in the form of an optical fiber cable (10) having a ferrule (11) mounted on the inner end portion of the cable and firmly held in an associated radiation transmitting bore section.

Description: OPTICAL FLOW CELL FOR USE IN SPECTRAL ANALYSIS FIELD OF THE INVENTION The invention refers to an optical flow cell for use in spectral analysis. BACKGROUND OF THE INVENTION The invention relates to optical flow cells for use in spectral analysis in association with suitable spectrometers, for example in liquid and gas chromatography. In the course of such analysis a stream of a monitored fluid is continuously flown though the flow cell and a radiation beam is transmitted across the flowing fluid and the exiting beam is detected and analyzed. Different designs of such optical flow cells are disclosed, for example, in US 4,540,280, 4,588,893, 5,078,493 and 5,151,474. Many known optical flow cells of the kind specified comprise a housing with two intersecting throughgoing bores, one for the flow of a fluid to be analyzed and the other for the transmission of radiation which latter is fitted with means for directing the radiation towards and away from the bore. At their intersection the two throughgoing bores form a fluid cavity and in order to ensure a desired length of an optical path across the fluid passing through the fluid cavity and to prevent the fluid from penetrating into the radiation transmitting bore, two oppositely located transparent windows are provided which separate the fluid cavity from the adjacent portions of the radiation transmitting bore. The windows are either mounted directly in the housing of the flow cell, such as for example in US 4,588,893, or they constitute the end faces of optical probes inserted in the radiation transmission bore through both ends thereof, such as for example in US 5,078,493. Thus, the radiation arriving at one end of the radiation transmit¬ ting bore is directed via a first transparent window to a fluid sample located inside the fluid cavity whilst radiation exiting from the sample across a second window is transmitted to a spectrometer. In order to ensure in such prior art devices a high accuracy of measurement which in turn depends on the degree by which the dimensions of the fluid cavity correspond to design values, the windows must be appropriately manufactured and positioned so as to maximize the accuracy of compliance of the distance between the windows, known in the art as end separation, with the design value, and to minimize transverse and angular misalignments of the windows. Furthermore, the flow cell design must provide for a high degree of repeatability on each reassembly. In addition, specific measures must be taken for the windows to withstand extreme conditions, e.g. high pressure and/or high temperature, under which flow cells of the kind specified most often operate. The above problems become critical in flow cells where extremely small dimensions of the fluid cavity and very high accuracy of measurements are required. This is particularly the case with measurements in the infra-red (IR) range because, with the absorption coefficients of most of materials in the IR range being very high, a high sensitivity of measure¬ ments can be achieved only if volumes of samples to be analyzed are very small and the optical path in the sample is extremely short. SUMMARY OF THE INVENTION It is the object of the present invention to provide a new flow cell in which the use of windows is eliminated and which provides for high accuracy in the establishment of the dimensions of a fluid cavity required for analysis of small volumes of fluid with a high optical sensitivity. In accordance with the invention, there is provided an optical flow cell for use in spectral analysis of the kind that comprises a housing formed with intersecting first and second bores forming between them a fluid cavity, which first and second bores serve, respectively, for fluid throughflow and radiation transmission, said second bore holding in a fluid tight fit and releasable fashion two radiation transmitting optical probes having each an outer end portion and an inner end portion with an inner end face, the outer end portion of one probe being linked to a radiation source and the outer end portion of another probe being linked to a spectrometer, the inner end faces of the probes facing said fluid cavity and being spaced from each other; characterized in that each of said optical probes is an optical fiber cable having a ferrule mounted on the inner end portion of the cable and firmly held in an associated radiation transmitting bore section. Thus, the flow cell according to the present invention has a relatively simple construction in which the end faces of the fiber optic cables of the optical probes are in direct contact with the analyzed fluid whereby the need for windows is obviated. This enables mutually engaging surfaces of the ferrule and the housing, which are involved in setting of a required distance between the end faces of the optical probes, to be disposed relatively remote from the fluid cavity, whereby practically no fluid or at least no particles, if any, comprised therein can reach the engaging surfaces. With such a design, accurate setting of the distance between the end faces of the fiber optic probes is ensured by appropriate machining, e.g. polishing, of the end faces of the probes and the mutually engaging surfaces of the ferrules and housing. Transverse and angular alignments of the inner end faces of the optical fiber probes are obtained by properly selecting the diametric dimensions of the optical fiber cable and its ferrule, and the required high accuracy of production is ensured by the very nature of fiber optic technology. Thus, in a flow cell designed according to the present invention, there is no need for any ad hoc alignment or adjustment and the relative positions of the constituent member remains essentially unchanged upon taking apart and re-assembly, which is extremely advantageous, because flow cells of the kind concerned by the present invention must frequently be taken apart for cleaning purposes. The ferrules are selected from among materials, e.g. metals, which provide for the required mechanical properties and resistance to corrosion by the analyzed fluid. Preferably, the housing is in the form of a sleeve coaxial with the radiation transmitting bore. Preferably, each fiber optic probe is fitted with sealing means mounted, adjacent the inner end face thereof, between the ferrule of the probe and an end portion of the inner surface of the associated radiation transmitting bore section. The sealing means may be of any suitable kind such as, for example, an O-ring or where the ferrule and the sleeve are ferromagnetic, a ferrofluid material. The flow cell according to the present invention is especially advantageous when a small volume of an analyzed sample and consequently a small size fluid cavity are required. The required small dimension of the fluid cavity is readily achievable by mounting the fiber optic probes in such a manner that their end faces project into the fluid cavity to an appropriate extent and further by choosing a small diameter optical fiber cable. The combination of a small volume of the analyzed material and irradiation transversal to the direction of fluid flow, provides a high optical sensitivity of the cell. As a result substances present as solute in even a very low concentration can be measured. Also, radiation losses are effectively minimized and high efficiency of analysis is obtained. In view of the above, the fluid cell according to the present invention is specifically advantageous for use in IR analysis in general, and of liquid solutions in particular, which latter have very high absorption in the mid IR region and, therefore, put severe limitations on the length of the optical path. Finally, with the flow cell according to the present invention being small and comprising only a small number of components, its use is specifically advantageous in cases when the analysis process requires severe operating conditions such as, for example, heating of the cell. BRIEF DESCRIPTION OF THE DRAWINGS For better understanding the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a cross-sectional view of a fiber optic flow cell according to the present invention; Fig. 2 is a cross-sectional view of a section of the sleeve of the fiber optic flow cell shown in Fig. 1, drawn to a larger scale; Fig. 3 is a cross-sectional view of a fiber optic probe used in the fiber optic flow cell shown in Fig. 1; and Fig. 4 shows portion A of the fiber optic flow cell shown in Fig. 1, drawn to a larger scale. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT The fiber optic flow cell according to the present invention shown in the drawings is adapted for use in spectrometric analysis, in particular for IR absorption measurements of fluids (liquids and gases). As shown, the fiber optic flow cell 1 comprises a cylindrical sleeve or adaptor 2, which is formed with a relatively broad radiation throughgoing bore 3 serving for radiation transmission and transversal fluid throughgoing bore 4 serving for the passage of a fluid F to be analyzed, bores 3 and 4 defining at their intersection a fluid cavity 5. The flow cell 1 further comprises two fiber optic probes 6 and 7 inserted into the radiation transmission bore 3 of the sleeve 2 at opposite ends 8, 9 thereof. One of the fiber optic probes, e.g. probe 6 transmits radiation R from a radiation source (not shown) to the fluid cavity 5 and the other probe 7 collects the radiation exiting from the fluid cavity 5 and transmitting it to a spectrometer (not shown). The fiber optic probes 6 and 7 of the flow cell 1 are identical and the structure of such a probe is schematically illustrated in Fig. 3. As shown, the probe is in the form of a transparent optical fiber cable 10 surrounded by a tubular ferrule 11, both the fiber and the ferrule being made of materials inert to the fluid to be analyzed. The ferrule 11 may be in the form of any standard connector. The fiber optical cable used in the flow cell may be of a single mode type, in which case it may have a diameter as small as 3-5 microns, or of the multi-mode type. The ferrule 11 is formed with a circumferential abutment shoulder 12 spaced from an end face 13 of the probe to an extent that ensures that when the two probes 6 and 7 are mounted in the radiation transmission bore 3 with the shoulders 12 of the probes abutting on the end surfaces 8 and 9 of sleeve 2, the end faces 13 of the probes project into the fluid cavity 5 to such an extent as to obtain a desired end separation therebetween. The accuracy of the setting of the end separation between the end faces is ensured by the appropriate machining of the end faces 13 of the fiber optic probes 6, 7, of end surfaces 8 and 9 of sleeve 2 and of the abutment surfaces of the shoulder 12, and preferably all these surfaces are polished. The fact that the abutment shoulders 12 of the ferrule and the end surfaces 8 and 9 of the housing are disposed remote from the fluid cavity, ensures that practically no fluid or at least no particles, if any, comprised therein can reach the engaging surfaces 12, 8 and 9. Consequently, no specific cleaning of these surfaces is required, whereby their high surface quality is maintained. Transverse and angular alignment of the end faces of the probes is achieved by accurate diametric dimensions of the optical fiber cable and the ferrule, which are normally inherent in conventional fiber optic technology as known per se. As seen in Figs. 3 and 4, each fiber optic probe 6, 7 is provided with a sealing 14 mounted in a space between the radiation transmission bore 3 and the ferrule 11 of the probe 6, 7 adjacent the end face 12 thereof. The sealing 14 may be, for example, in the form of an O-ring (schematically shown in dotted lines in Fig. 3) or rather in the form of a ferrofluid material, in which case the ferrule and/or the sleeve or parts thereof should be made of magnetic material. As shown in Fig. 1, the fiber optic probes 6, 7 are secured in the sleeve 2 by means of connector nuts 15. In operation, fluid F of which absorbance is to be measured, is introduced into the fluid cavity 5 via one of the narrow fluid bores 4 and is discharged through the other bore 4 in a manner known per se and not designated in the drawings. The fluid is illuminated by a radiation beam delivered through one of the fiber optic probes, say probe 6. The radiation is transmitted across the fluid cavity 5, and is collected by fiber optic probe 7 and directed thereby to a spectrometer. Thus, with the above design, the fluid cavity of the flow cell can have very small dimensions and, consequently, can provide for an extremely small volume of sample to be analyzed, about 30 pl, whereby a high optical sensitivity of the cell is achieved so that low concentration e.g. lower than 100 ppm of solute in water or than 0.1 ppm of acetone in oil, can be measured. At the same time, the combination of a short optical path, small inlet and outlet optical apertures defined by the diameter of optical fiber cable, and the fact that the radiation proceeds transversely with respect to the fluid flow direction, ensures that virtually all the incident radiation interacts with the analyzed sample and that radiation losses are minimized. In this way the analysis is rendered highly efficient. It should be mentioned that the fiber optic cell according to the present invention may have design features different from those described above. Thus, the flow cell may be employed for spectrometric analysis using radiation other than infrared, in which case the fluid cavity may have a longer optical path, i.e. the distance between the end faces of the fiber optic probes may be equal to or even be greater than the width of the transverse fluid flow bores. The sleeve may be other than cylindrical and may be held in an appropriate casing. The abutment shoulders of the probes may be constitut- ed by annular rims around their end faces. The shoulders may be adapted to abut on spacings arranged inside the sleeve rather than abutting on outer, end surfaces thereof. The fixed position of the fiber optic probes in the sleeve may be achieved by any other suitable means.

Claims: CLAIMS: 1. An optical flow cell for use in spectral analysis of the kind that comprises a housing formed with intersecting first and second bores forming between them a fluid cavity, which first and second bores serve, respectively, for fluid throughflow and radiation transmission, said second bore holding in a fluid tight fit and releasable fashion two radiation transmitting optical probes having each an outer end portion and an inner end portion with an inner end face, the outer end portion of one probe being linked to a radiation source and the outer end portion of another probe being linked to a spectrometer, the inner end faces of the probes facing said fluid cavity and being spaced from each other; characterized in that each of said optical probes is in the form of an optical fiber cable having a ferrule mounted on the inner end portion of the cable and firmly held in an associated radiation transmitting bore section. 2. An optical flow cell according to Claim 1, wherein the spacing between the inner end faces of the probes is set by an engagement between abutment surfaces of said ferrule and said housing, which surfaces are disposed relatively remote from the fluid cavity. 3. An optical flow cell according to Claim 2, wherein said end faces of the optic probes and the mutually engaging surfaces of said ferrules and the housing are machined with high surface quality. 4. An optical probe according to Claim 3, wherein said end faces of the optic probes and said engaging surfaces of said ferrules and the housing are polished. 5. An optical flow cell according to Claim 1, wherein the inner end face of said fiber optic probes project into said fluid cavity. 6. An optical flow cell according to Claim 1, wherein the housing is in the form of a sleeve coaxial with the radiation transmitting bore. 7. An optical flow cell according to Claim 1, each optic probe is fitted with sealing means mounted, adjacent the inner end face thereof, between the ferrule of the probe and an end portion of the inner surface of the associated radiation transmitting bore section. 8. An optical flow cell according to Claim 7, wherein said sealing means are in the form of an O-ring. 9. An optical flow cell according to Claim 7, wherein the ferrule and the sleeve are ferromagnetic and said sealing means are in the form of a ferrofluid material. 10. An optical flow cell according to any one of the preceding Claims, adapted for use in IR spectrometric analysis.

Inventors: BORNSTEIN AHARON, BORNSTEIN, AHARON
Applicants: BORNSTEIN AHARON, ISRAEL ATOMIC ENERGY COMM, BORNSTEIN, AHARON, THE STATE OF ISRAEL, ATOMIC ENERGY COMMISSION, SOR
Application year: 1996
Main classification: G02B6
IPC codes: G01N21, G02B6