Abstract:
An infra-red liquid sampling cell has a sapphire base plate with a part-spherical concave recess. A sapphire upper plate is hinged relative to the base plate larger radius of curvature than the recess. When the upper plate is lowered into contact with the base plate its convex surface contacts the liquid sample thereby excluding air bubbles and, when in contact with the edge of the recess, produces a well-defined sample thickness. An infra-red source directs radiation down through both plates and the liquid sample to a detector.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to optical sampling arrangements and to spectroscopic apparatus including such arrangements. 
   Where optical measurements, such as for spectroscopic purposes, need to be made on liquid samples, the sample is usually placed in a transmission liquid cell  1  of the kind shown in  FIG. 1 . The cell  1  consists of two windows  2  and  3 , which are optically transparent to the wavelengths of interest, and which are separated by a spacer  4 . The cell  1  is placed in the optical path between a source of optical radiation  5  and a radiation detector  6 . The sample  7  to be measured is contained in the cavity  8  between the windows  2  and  3 . The pathlength of the cell  1  is determined by the thickness of the spacer  4  and this is arranged to be sufficient so that the sample  7  absorbs a measurable amount of the optical radiation at the wavelengths of interest. The pathlength can vary from several millimetres in the UV and visible regions to just microns in the mid-infrared region. 
   These liquid transmission cells are widely used but suffer from a number of disadvantages. They can be difficult to fill and are prone to trapping air bubbles, which can prevent accurate measurements being made. Shorter pathlengths, as needed for near and mid-infrared radiation, can be particularly difficult to provide when used with viscous samples. The cells can be very difficult to clean, especially with viscous or sticky samples. This often requires the entire cell to be disassembled and significant quantities of solvent may be needed, which are often flammable or hazardous. The cells usually have a large number of parts, which have to be correctly aligned and assembled to ensure they do not leak. It can also be difficult accurately to reproduce the pathlength when the cell has to be taken apart and rebuilt, such as after cleaning. This is a particular problem when making quantative measurements. Furthermore, the relatively large size of the cell and the large number of components can make it difficult to control or stabilize the temperature of the cell. 
   BRIEF SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an alternative form of optical sampling arrangement and spectroscopic apparatus. 
   According to one aspect of the present invention there is provided an optical sampling arrangement including a first element having an upper surface with a smoothly curved concave cavity in which a liquid sample can be placed and a second element having a lower surface with a smoothly curved convex formation adapted to locate with the cavity and contact an upper surface of the liquid sample in the cavity, the curvature of the convex formation being shallower than that of the concave cavity, and at least one of the first and second elements being transparent to optical radiation such that optical radiation can be directed through the thickness of the liquid sample. 
   Preferably both the first and second elements are optically transparent. The cavity is preferably circular in the plane of the upper surface of the first element. The first and second elements may contact one another along a line of contact at the intersection of the cavity with the upper surface. Alternatively, the upper surface of the first element may be provided with a contact land around the cavity having a curvature parallel with that of the convex formation such that the convex formation makes contact with the land. The first element may have a lower surface that is flat and parallel with the upper surface. Alternatively, the first element could have a lower surface provided with angled faces. The arrangement may include a source of radiation arranged to direct radiation into one of the angled faces at an angle to the normal to the face. The arrangement may include a radiation detector arranged to receive radiation transmitted through an angled face at an angle to the normal to the face. Alternatively, the first element may have a lower surface with a curved profile. The upper surface of the second element may be flat. Alternatively, it may have a curved profile and its upper surface may be concave with the same centre of curvature as the convex surface on the lower surface of the second element. The first element is preferably mounted below the second element in a fixed position, the second element being movable up and down relative to the first element. The second element may be hinged relative to the first element. The surfaces of both the first and second elements are preferably transparent. Alternatively, one of the elements could have a reflective layer on an outer surface arranged to reflect optical radiation back through the element. The second element may have a reflective surface arranged to reflect optical radiation passing through the first element from below and through the sample back down through the sample and the first element. The optical radiation is preferably in the near infra-red region. One or both elements may be of sapphire. The concave cavity and the convex surface formation are preferably part spherical. 
   According to another aspect of the present invention there is provided spectroscopic apparatus including a source of optical radiation, a radiation detector and an optical sampling arrangement according to the above one aspect of the present invention located in the optical path between the source and the detector. 
   The source of optical radiation is preferably in the near infra-red and the detector is responsive to radiation in the near infra-red. The source and detector may be located on opposite sides of the optical sampling arrangement or on the same side. The apparatus may include a housing containing the source and detector, the first element being sealed in an upper surface of the housing. 
   Apparatus including a spectroscopic liquid sampling cell according to the present invention will now be described, by way of example, with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
       FIG. 1  is a schematic, side elevation view of prior art spectroscopic apparatus including a liquid sampling cell; 
       FIG. 2  is a schematic, side elevation view of a first embodiment of spectroscopic apparatus according to the present invention; 
       FIG. 3  is a plan view of the lower part of the sampling cell shown in  FIG. 2 ; 
       FIG. 4  is a schematic, side elevation view of a second embodiment of apparatus according to the present invention; 
       FIG. 5  is a plan view of the lower part of the sampling cell shown in  FIG. 4 ; 
       FIG. 6  is a side elevation view of modified apparatus; 
       FIG. 7  is a side elevation view of another modified apparatus; 
       FIG. 8  is a perspective view of the upper surface of a further modified lower element; and 
       FIG. 9  is a perspective view of the upper surface of a fifth modified lower element. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference first to  FIGS. 2 and 3 , the spectroscopic apparatus includes a housing  10  supporting a liquid sampling cell  11  on its upper surface and containing an optical detector  12  within it, mounted directly below the cell. The detector  12  provides an output to a processor  13 , which, in turn provides an output representation of the nature of the liquid  14  in the cell  11  to utilisation means, such as a display  15 . The cell  11  comprises a lower element or base plate  16 , fixed with the upper surface of the housing  10 . The cell  11  also includes an upper element or plate  17  mounted on an arm  18 , which is hinged on the housing  10  so that it can be raised to lift the upper plate away from the lower plate  16  and can be lowered to press the upper plate into contact with the lower plate. The arm  18  also supports a source  19  of optical radiation in the near infrared region. The radiation produced by the source  19  is directed downwardly through the upper plate  17 , through the thickness of the liquid sample  14  and the lower plate  16  to the detector  12 . As shown, radiation from the source  19  is focussed, such as by a converging lens (not shown), but the radiation could, instead, be collimated. Only radiation passing through the central region of the cell  11  is detected to ensure that variation in path length across the cell does not introduce a significant measurement error. The upper and lower plates  17  and  16  are both circular and are made of a material that is both transparent to the radiation and is not damaged by the range of substances with which the apparatus is to be used. A preferred material is sapphire but other materials may be possible. 
   The liquid sample  14  is contained within a cavity  20  located centrally in the upper surface  21  of the lower plate  16 . The cavity  20  is concave and is smoothly curved over its entire surface. As shown in  FIG. 3 , the cavity  20  is circular in the plane of the upper surface  21  and typically has a diameter “d” of about 8 mm. The profile of the cavity  20  is part spherical with a radius of curvature of about 9.3 mm, the centre of curvature being located perpendicularly above the plane of the flat part of the upper surface  21  by a distance of about 8.4mm, giving a depth of cavity at its centre, when open, of about 0.9 mm. The edge of the cavity  20  meets the upper surface  21  of the plate  16  at a sharp edge  22 . The lower surface  23  of the lower plate  16  is flat and parallel to the flat, outer part of the upper surface  21 . 
   The upper plate  17  has a flat upper surface  30  but its lower surface  31  is formed with a convex, smoothly-curved profile extending across the entire lower surface. The diameter of the upper plate  17  is slightly greater than the diameter d of the cavity  20 . The convex profile is also of a part-spherical shape and its radius of curvature is typically about 20 mm, that is, it is greater than that of the cavity  20  so that the curve is shallower than that of the cavity. The centre of curvature of the surface  31  is located directly above that of the cavity  20 . It can be seen, therefore, when the upper plate  17  is lowered into contact with the lower plate  16 , that the lower surface  31  of the upper plate contacts the lower plate at a circular line of contact, around the edge  22 . Because the curvature of the cavity  20  is greater than that of the lower surface  31  of the upper plate  17 , the cavity, when closed, resembles the shape of a positive meniscus lens, being deepest at the centre and becoming thinner towards the edges. Typically, the depth of the cavity  20  at the centre, when closed is about 0.5 mm. The outer surfaces  23  and  30  of the cell  11  are flat but they could be formed with spherical lens surfaces so that they can provide a part of the optical system, such as to collimate or focus the beam of radiation. For example, the upper surface  30  of the upper plate  17  could be concave with the same centre of curvature as its lower surface, as shown by the broken line  30 ′ in  FIG. 2 . This would make the system insensitive to small changes in orientation of the upper element  17  when it is removed and replaced. 
   The separation, in the cavity  20 , between the upper surface  21  of the lower plate  16  and the lower surface  31  of the upper plate  17  is accurately reproducible. The liquid sample  14  is readily placed in the cavity  20  with the upper plate  17  in a raised position and it is then lowered, contacting the liquid first in the centre so that no gas bubbles are formed. Any excess liquid is displaced to the side. When the upper plate  17  is pressed into contact with the lower plate  16 , the cavity  20  is closed and the liquid sample  15  is in optical contact across the entire cavity with both the upper and lower plates, thereby forming an efficient optical transmission cell  11 . 
   It can be seen that the smooth concave shape of the cavity  20  enables it to be filled and to be cleaned easily after use since there are no crevices in which the sample  14  can be trapped. 
   Different pathlengths can be produced readily simply by providing interchangeable upper elements with different radii of curvature. 
   The upper element need not contact the lower element at a sharp edge, as in the arrangement of  FIGS. 2 and 3 , instead an arrangement of the kind shown in  FIGS. 4 and 5  could be used. In this, the upper plate  117  has the same shape as before but the upper surface  121  of the lower plate  116  is modified by the addition of a narrow annular band or contact land  140  extending around the outside of the cavity  120 . The contact land  140  is curved with the same profile as that of the lower surface  131  of the upper plate  117  so that, when the upper plate is pressed into contact with the lower plate  116 , the land seals with the upper plate around the edge of the cavity. This helps retain the sample  114  in the cavity  120 , which can be particularly useful with samples containing volatile components. 
   In the arrangement shown in  FIG. 6 , the upper surface  230  of the upper element  217  is coated with a layer  250  of a material that is reflective at the wavelength of the radiation of interest. The radiation source  212  is located below the sample cell  211 , within the housing  210  and directs a beam of radiation upwardly at an angle away from the axis of the cell through the lower plate  216 , through the thickness of the liquid sample  214  and into the upper plate  217  where it is reflected back by the layer  250  through the thickness of the sample and the lower plate to the detector  213 . It can be seen that, in this arrangement, the radiation makes two passes through the thickness of the sample  214 . The beam diameters are kept as small as possible to avoid problems from the variation in pathlength across the beam. The variation in pathlength can, however, be an advantage by helping to avoid interference fringes that can arise from parallel surfaces. This arrangement has an advantage that the optical source  212 , the detector  213  and the entire optical path through air can be contained within the housing  210 . The housing  210  can, therefore, be sealed with a gas-tight seal and the interior of the housing can be purged with a dry gas. This avoids problems that can arise in the infrared region caused by variations in the water vapour content of ambient air. It can also be useful where the sample needs to be heated or cooled, such as to melt more viscous materials or to stabilize the temperature for quantative accuracy. By using a material of high thermal conductivity, such as sapphire, for the lower element  216  and maintaining a low thermal mass for the upper element  217 , the sample  214  can be thermally stabilized quickly. The heating or cooling system can be confined to the lower element  216  in the apparatus housing  210  to simplify the design and make it suitable for use in hazardous areas. 
   It is not essential for the lower surface of the lower element to be flat. Instead, the lower surface could have a curved profile or it could be angled in the manner shown in  FIG. 7  where the lower element  316  is in the form of a right-angle prism with two lower faces  360  and  361  inclined at 90° to one another. The radiation source  312  is located to direct a beam of radiation into the left-hand lower face  360  at an angle away from the normal so that it is refracted into the element  316 . Similarly, the detector  313  is located to receive the beam emerging from the right-hand lower face  361  after refraction. This arrangement eliminates any back surface reflections from the output path, which is important in eliminating interference fringing from the measured spectrum. The upper surface  330  of the upper element  317  is coated with a reflective layer  350  and is formed with a central concave recess  330 ′ . This further helps eliminate unwanted beams and reduces the sensitivity of the system to small changes in orientation of the upper element  317 . 
   The arrangements of the present invention can have various advantages. The cell is easy to fill and, when closed, helps eliminate air bubbles. The direct, mechanical contact of the upper and lower elements ensures an accurate and reproducible pathlength. The system can be used with viscous materials, even when using short pathlengths. In addition to liquids, the apparatus can accept some types of semi-solids, such as slurries, waxes, gels, pastes, putties and the like. The apparatus can easily be heated, cooled or thermally stabilized. By heating the cell, solid samples with a low melting point can be tested. The cell is very easy to clean, even with viscous and sticky samples. A disposable alcohol-impregnated wipe may be all that is needed to clean the cell before the next sample. Only small quantities of sample are needed and many can be wiped off with a tissue after measurement. 
   In the arrangements described above, the cavity in which the sample is contained is of circular shape when viewed in plan. Alternative shapes are, however, possible, such as of a part cylindrical shape, as shown in  FIG. 8 , or an annular shape, as shown in  FIG. 9 . In such arrangements, it will be appreciated that the upper element would have a matching shape but with a shallower curve. 
   Although, in the arrangements described above, both the upper and lower elements are of optically-transparent material, it is only essential for one of these to be transparent. For example, in a reflective system, the lower element could be transparent and the upper element could be opaque with a reflective lower surface. Radiation from a source below the lower element would pass through the lower element into the sample and be reflected by the lower surface of the upper element back through the sample and the lower element to a detector below the cell. The material from which the upper element is formed would, of course, have to be non-reactive with the samples to be tested. 
   It is not essential for the apparatus to include a radiation source and detector, instead, an external source and detector could be used, the apparatus having mirrors or the like to direct the radiation into and out of the cell. Fibre-optics could be used to bring radiation into or out of the cell. 
   The apparatus could be provided as a flow cell, having an inlet and outlet by which liquid enters and leaves the cell cavity. The cylindrical shape cavity shown in  FIG. 8  may be particularly useful in flow cell applications since the inlet and outlet could be located at opposite ends of an elongate cavity. The detector could be of the imaging kind, such as including an array of detector elements.