Abstract:
A cuvette cartridge for optical measurements is disclosed herein, the cartridge comprising a flat, adhesive sheet having a selected thickness that defines an optical path length perpendicular to the longitudinal axis of such sheet; wherein a portion of the adhesive sheet is cut out to define at least one optical chamber of a desired length; wherein the adhesive sheet is placed between a first flat sheet and a second flat sheet, the first flat sheet comprising at least one inlet hole and at least one outlet hole, the vent hole being located at a first end of the optical chamber and the inlet hole being located at a second end of the optical chamber. An integrated analytical system employing this cartridge and optionally including a vacuum chuck for holding and registering the cartridge are also disclosed.

Description:
RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 60/130,876, filed Apr. 23, 1999 entitled “Disposable Optical Cuvette Cartridge”; U.S. Provisional Application No. 60/130,918, filed Apr. 23, 1999, entitled “Spectrophotometric Analysis System Employing a Disposable Optical Cuvette Cartridge”; and U.S. Provisional Application No. 60/130,875, filed Apr. 23, 1999, entitled “Vacuum Chuck for Thin Film Optical Cuvette Cartridge.” 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a disposable optical cuvette cartridge for spectrophotometric measurements of liquid samples such as whole blood, to methods for constructing the cuvette cartridge and to methods of using the cuvette cartridge. 
     BACKGROUND INFORMATION 
     U.S. Pat. No. 5,430,542 disclosed a cuvette comprising two optically transparent liquid-impermeable sheets, wherein a third adhesive sheet is inserted between the two transparent plastic sheets and the three sheets are pressure sealed together. The adhesive sheet had cut outs defining the contour of an optical chamber, inlet port, and vent port. A liquid sample to be analyzed was placed in the optical chamber through the inlet port in the adhesive layer and the thickness of the adhesive sheet defined the optical path length of the liquid sample within the cuvette. 
     SUMMARY OF THE INVENTION 
     It has now been found that the disposable optical cuvettes of the art are improved if the inlet port and vent port are present in and pass through one of the liquid-impermeable sheets rather than through the adhesive layer. The invention provides an improved disposable optical cuvette based upon this discovery for use in the spectrophotometric analysis of liquid samples, methods for manufacturing the improved cuvette as well as analytical systems and methods which employ the improved cuvette. 
     Thus, in a first aspect, this invention provides a cuvette cartridge for optical measurements of analytes in liquid samples. The cartridge comprises a flat, adhesive sheet having a selected thickness that defines an optical path length perpendicular to the plane of the flat sheet. A portion of the adhesive sheet is cut out to define at least one optical sample chamber of a desired length. The sample chamber is completed when the adhesive sheet is placed between and sealed to a first flat sheet and a second flat sheet. The first flat sheet contains a inlet hole and a vent hole which connect to the optical chamber. The vent hole is located at one end of the optical chamber, and the inlet hole is located at a second end of the optical chamber so that liquid can fill the sample chamber. In preferred embodiments, the cuvette cartridge contains a plurality of optical chambers each with its own inlet hole and vent hole. 
     In another aspect, the invention provides a disposable optical cuvette cartridge for use in the spectrophotometric analysis of liquid samples which employs a pair of parallel sheets of material impermeable to the liquid samples. The two sheets are held in spaced relationship to one another by an intermediate layer which has at least one discontinuity defining a sample zone having a thickness set by the spaced relationship between the two sheets. At least one of these two sheets is optically transparent at the wavelengths of the spectrophotometric analysis and one of the two sheets carries an inlet port and a vent port for each of the sample zones. The thickness of the intermediate layer defines the optical path through the sample zone and the quantity of sample presented for analysis. The thickness of the intermediate layer is also selected taking into account the hydrophilicity of the materials of the various layers to permit capillary action to draw the liquid sample into the sample zone when the sample is placed at the inlet port. 
     In yet another aspect, the invention provides a method of manufacturing a thin film disposable cuvette cartridge for use in the spectrophotometric analysis of liquid samples. This method involves sealably bonding three flat layers to one another in parallel alignment. The first and third layers are flexible sheets of thin film material impermeable to the liquid samples. The intermediate second layer is also made from a material which is impermeable to the liquid samples. This layer, which has a thickness which is preselected to provide a desired spacing between the first and third films following bonding, has at least one discontinuity. These discontinuities in the intermediate layer define at least one sample zone having a thickness set by the spaced relationship between the two sheets. At least one of the first and third sheets is optically transparent at the wavelengths employed in the spectrophotometric analysis and one of these two sheets carries an inlet port and a vent port for each of the sample zones. The thickness of the intermediate layer defines the optical path through the sample zones and is sized to cause capillary action to draw the liquid sample into the sample zone when the sample is placed at the inlet port. 
     In yet an additional aspect, the invention provides an integrated system for the spectrophotometric analysis of liquid samples. This system includes a cuvette cartridge into which liquid samples are placed for optical measurements of analytes, an excitation laser, capable of exciting fluorescence in the analytes, and a fluorescence detector capable of detecting the fluorescence excited in the analyte and providing information concerning the presence of the analyte based on the detected fluorescence. The cartridge comprises a flat, adhesive sheet having a selected thickness that defines an optical path length perpendicular to the plane of the flat sheet. A portion of the adhesive sheet is cut out to define at least one optical sample chamber of a desired length. The sample chamber is completed when the adhesive sheet is placed between and sealed to a first flat sheet and a second flat sheet. The first flat sheet contains a inlet hole and a vent hole which connect to the optical chamber. The vent hole is located at one end of the optical chamber, and the inlet hole is located at a second end of the optical chamber so that liquid can fill the sample chamber. The excitation laser is focused on the sample in the optical sample chamber and the fluorescence is detected from the chamber. 
     The invention also provides an apparatus for firmly holding the disposable thin film optical cuvette cartridge for use in the spectrophotometric analysis of liquid samples in a flattened and registered position as is generally preferred for spectrophotometric analysis of samples contained within the cuvette cartridge. This apparatus is a vacuum chuck which is specifically adapted to work with a cassette cartridge that itself includes a pair of parallel sheets of material impermeable to the liquid samples. The two sheets are held in spaced relationship to one another by an intermediate layer which has at least one discontinuity defining a sample zone having a thickness set by the spaced relationship between the two sheets. At least one of these two sheets is optically transparent at the wavelengths of the spectrophotometric analysis. At least two locating pin holes pass through the two sheets and the intermediate layer. 
     The vacuum chuck has a continuous perimeter which is shaped to surround the sample zone when a cuvette cartridge is placed upon it and has additional areas within the perimeter. The perimeter and the additional areas are coplanar and define a flat planar surface upon which the cuvette cartridge may be placed. The flat planar surface has indentations extending below the surface defined by the perimeter and the additional areas. These indentations are located beneath the sample zones when the cuvette cartridge is placed on the chuck. The chuck can also present features for accurately registering the cuvette cassette. These can include pins, ridges or other barriers which engage the cassette or markers that may be read by automated registration equipment, or the like. In the embodiment shown in the drawings, the chuck has at least two pins extending upwards from the planar surface and located in the planar surface so as to engage the at least two locating pin holes present in the cuvette cassette. The chuck includes a vacuum source connected to the indentations to apply a vacuum thereto to pull the cuvette cartridge into a locked and registered flat position on the planar surface when the locating pin holes of the cassette are mated with the pins of the chuck. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention will be further described with reference being made to the accompanying drawings in which: 
     FIG. 1 is a generally schematic exploded perspective view of a cassette cartridge of this invention; 
     FIG. 2 is a perspective not-to-scale view of an assembled cassette cartridge of this invention; 
     FIG. 3 is a cross-sectional view at  3 — 3  of the assembled cassette cartridge of FIG. 2; and 
     FIG. 4 is a perspective view of a vacuum chuck which can be used to hold the cassette cartridge in position in use in the analysis of liquid samples. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1,  2  and  3 , an optical cuvette cartridge is shown generally at  10 . Optical cuvette cartridge  10  comprises two substantially flat sheets  12  and  14 . These sheets are “liquid impermeable” which term denotes that they contain and are not affected by the liquid samples which are placed in the cuvette cassette for analysis. One, and more typically both, of these sheets are “transparent” which term denotes that the sheets are substantially optically transmissive and transmit a substantial amount, such as at least about 25% and preferably at least about 50%, of light shone upon them at the wavelengths which are relevant to their use as sample cuvettes in spectrophotometric analysis settings. 
     Sheets  12  and  14  are preferably flexible, nonrigid plastic films. Since many of the spectrophotometric analysis techniques which are carried out using the cuvette cassettes of this invention involve laser stimulation of fluorescence in the samples, preferably, the material used to make these plastic films has a low fluorescence in the ranges which would interfere with the signals of interest to the user. 
     Poly(ester)s including poly(ethylene terephthalate) and other terephthalate ester polymers, poly(urethane)s, cellulose polymers, acrylic polymers, poly(carbonate)s, poly(styrene) and poly(vinyl fluoride)s are representative examples of the types of materials which can be used for sheets  12  and  14 . Sheets  12  and  14  are “thin films” which denotes that they are from about 0.25 mils (6 microns) to about 25 or even 100 mils or more in thickness. Preferred embodiments of the present invention use sheets of polyesters, and especially poly(ethylene terephthalate) glycol from 0.5 mils to about 25 mils, and particularly 10 mils or so in thickness. These materials are available commercially from ICI, duPont, and the like under trademarks and tradenames such as Mylar®, Melinex®, Melinex® 725 (duPont) and the like. 
     The surface tension of sheets  12  and  14  is generally important. The surface tension of plastic sheets is reflected in the degree of wettability (“hydrophilicity”) that the sheets process. Many of the plastics having optical properties which are useful in the cuvette cassettes may present high surface tensions and hence low levels of hydrophilicity which render them unacceptable for use, because they resist wetting by the aqueous liquid samples and thus do not permit the aqueous liquid samples to be drawn into the sample zone. As is well understood in the art, the surface tension of a plastic film can be altered by chemically treating the surface of the film with a surface tension lowering agent, by physically modifying the surface or by plasma treating the surface. Any of these treatments may be employed to provide surfaces with levels of hydrophilicity adequate to permit the aqueous liquid samples to be drawn into the sample zone. 
     Although the sheets are shown to be in substantially rectangular form in the embodiment shown in the drawings, the actual shape may be varied to adapt the resulting cuvette cassette to fit the spectrophotometric analysis apparatus. 
     Sheets  12  and  14  are spaced apart from one another but adhesively secured together by an intermediate layer  16 . Layer  16  has one or more discontinuities  16   a  which are areas where the layer  16  has been cut away or is otherwise not present. These areas provide sample zones having an optical path length corresponding to the thickness of layer  16 , which for the purposes of the present explanation is preferably about 50 to about 200 microns, preferably 75 to about 175 microns and more preferably about 125 microns. 
     Layer  16  can be provided by an accurately laid down layer of a dimensionally-stable adhesive such as the pressure-sensitive acrylic adhesives and pressure-sensitive butylene polymer adhesives, silicone adhesives, hot melt thermoplastic adhesives and the like which is printed upon one of the sheets  12  or  14  in a pattern which provides the desired discontinuities. Such printing can be carried out using conventional techniques including silk screening, lithography, printing from an “x-y” adhesive dispenser and the like. In this case the layer  16  is printed upon one of sheets  12  and  14  and thereafter the second of sheets  12  and  14  is laminated on top of layer  16 . The adhesive can be laid down as a liquid or as a semisolid. It can be partially cured in place before the additional layers are laminated to it. It could also be cured in place after the top and bottom layers are in place as long as care is taken to maintain the proper spacing between the layers (such as by the placement of spacer elements) and to assure that the integrity of the discontinuities which define the sample zones is maintained. Curing can be effected by light, heat, or ultraviolet radiation or the like depending upon the particular adhesive employed. 
     More typically at this time, layer  16  is a preformed sheet of adhesive or a preformed sheet of adhesive-coated polymer which is adhered to sheets  12  and  14 . Alternatively, layer  16  could be a sheet of heat-sealable polymer film which is heat sealed or otherwise laminated between sheets  12  and  14 . Any adhesive which will bond to the polymer films, which will not be adversely affected by or which will not itself affect the typically aqueous liquid samples can be used. These include acrylic adhesives, cyanoacrylates, silicones, epoxies, esters, poly(olefin)-based adhesives and the like. In a particularly preferred embodiment, layer  16  is a double-sided adhesive sheet, preferably a material such as Arclad® 8482 (Adhesives Research, Inc.) or other similar acrylic-based film. 
     Although not a requirement, it is often advantageous if the material of the intermediate layer  16  has spectrophotometric properties which are distinguishable from those of layers  12  and  14 . By noting the difference in properties, one can determine accurately when the spectrophotometer is reading the spectrophotometric profile of the sample and when it is reading information for areas outside the sample zone. Similarly, sophisticated automated spectrophotometric readers can locate the edges of the sample zone by noting the properties of layer  16  and make their readings on the contents of the sample zone safely away from the edges. 
     The sample zones present in the discontinuities in layer  16  are each accessed by a pair of openings present in sheet  12 . One of these, inlet hole  20 , is at one end or side of the sample zone and the other, vent hole  22 , is spaced away from it at the other end or side of the sample zone. 
     The method of construction of cuvette cartridge  10  when layer  16  is itself a polymer film, involves the cutting, such as but not limited to laser, rotary or matched metal pattern die cutting, of the double-backed adhesive sheet  16  as indicated at  16   a , to provide a readily removable piece  16   b , having the desired contour of an optical chamber  18 . Inlet hole  20  and vent hole  22  are cut into flat sheet  12 , for example by punching. 
     The adhesive  16  sheet is then applied to one of the flat sheets  12  and  14  and the removable cut piece  16   b  is then peeled off. The other flat sheet is then applied over the adhesive sheet  16 . The remainder of the adhesive sheet  16  will hold the two transparent flat sheets together providing a liquid tight seal and will form an optical chamber of defined volume, shape, and optical path length. Inlet hole  20  is generated by the placement of the flat sheet  12  over the adhesive sheet  14  such that the inlet hole  20  located at one end of the optical chamber  18  and outlet or vent hole  20  is located at the other end of the optical chamber. 
     In use, a drop, which will typically range from about 5 to 50 microliters, more preferably about 15 microliters, of a liquid sample is placed on the inlet hole  20 . Surface tension holds the drop on the hole and capillary action brought about by the cross-section of the sample zone  18  draws the sample into the zone  18  and fills it completely, with excess sample remaining at the inlet hole  20 . This obviates the need for a well of a defined volume or for accurate measurement of the volume of the drop applied to the hole. 
     One advantage of the cuvette cartridge of this invention is its ability to incorporate a plurality of sample zones in a single unit. Thus, while the cassette can include a single sample zone  18 , preferably two to about 500 optical chambers  18  are provided in a single cuvette cartridge assembly. Certainly, the sample cassette can include many more than 500 optical chambers, if desired. As various types of automated sample handling equipment are perfected, the density of sample zones can increase dramatically. The present invention will find excellent application in such settings. 
     In many present applications there is a preference for the cassette to have from about 8 to about 100 and especially, from about 30 to 50 optical chambers. Referring generally to FIGS. 1-3, this multiplicity of optical chambers is provided by locating the desired number of removable-pieces  16   b  on the adhesive sheet and a corresponding number of inlet and vent holes on flat sheet  12 . The inlet and vent holes may be staggered or arranged in an antiparallel orientation as indicated in FIG. 1 to provide, for example, closer spacing of optical chambers on the flat sheet. In an embodiment, the center to center spacing of inlet holes is about nine millimeters for ease in filling with a multichannel pipette. 
     In some embodiments, at least one additional optical chamber  24  may be provided in the cartridge as a reference or calibration aide as described below. This reference optical chamber may optionally lack inlet hole  20  and/or outlet hole  22  and is provided to remain empty and serve as a calibration blank or to contain a calibration solution or other standard the measurement of which can be used in the accurate determination of the analyte in the samples. 
     Bar code  25  or other labels may be added to the assembly, preferably on flat sheet  12 , in a location adjacent to the optical chambers so as not to obscure measurement of the samples in the optical chambers. 
     In some embodiments, pinholes  26  and  28  may be provided to assist in the registration of the cartridge on a platform during an optical measurement. Such pinholes are sized to engage locating pins present in the optical measurement system and will typically range from about {fraction (1/32)} to about ¼ inch, preferably about 0.125 inch, in size and may optionally be located in all three layers of the cartridge so as to provide a hole through the entire thickness of the three layers. 
     In typical use, the inlet holes are filled with a liquid sample, such as but not limited to blood, serum, urine, saliva, homogenate of cells or tissue, suspension or cells or tissue or constituents thereof, or analytes in a reaction mixture, or solution of analytes, or combination thereof. One or more optical chambers may be filled with a control sample, such as a solution of a dye of desired optical properties. A cover  30  may be placed over the flat sheet  12  after filling of the inlet ports to reduce evaporation of the samples or other loss. Cover  30  is preferably fabricated out of the same materials as flat sheets  12  or  14 . The inlet ports may also be covered with a tape or other material to prevent sample evaporation. The tape or other material should be applied so as not to obscure measurement of the sample in the optical chamber. 
     It will be appreciated that the disposable thin-film cuvette cartridges with their extremely small sample volumes and their flexible construction could require very precise spectrophotometric measurement methodologies to give accurate information concerning the samples. To this end it can be important to accurately and positively position the thin-film cassette in the spectrophotometric measurement apparatus. 
     In one embodiment, the thin-film cassette cartridge is placed on a support such as a vacuum chuck, which can hold the cassette in accurate registration and in a very well-defined profile, such as a very flat profile. Such a vacuum chuck is shown generally in FIG.  2 . Chuck  40  presents a generally flat top surface  41  which includes a continuous perimeter  42 , a series of raised areas  43 , which together with perimeter  42 , make up a flat surface upon which the thin film cassette is placed. Top surface  41  also includes a series of depressed areas  44  which are below the plane defined by perimeter  42  and raised areas  43 . When cassette  10  is paced onto this chuck, pinholes  26  and  28  in the cassette are aligned with pins  45  and  46  to accurately register the cassette. A vacuum is applied to the volume defined by depressed area  44 . This pulls the cassette down onto the chuck and seals it firmly onto the plane defined by raised areas  43  and perimeter  42 . 
     This vacuum chuck  40  is fabricated out of a material such as metal, plastic, glass, ceramic, and so on which is stable and nonreactive with the sample cassette and preferably with the aqueous samples. In one embodiment the vacuum chuck may be mounted on an optical measurement device such as a laser fluorescence scanner. Use of such a vacuum chuck is advantageous in that the cartridge is held flat against the chuck, thereby facilitating focusing of the optical instrument and reducing a need to refocus during measurements of multiple optical chambers on the cartridge. Furthermore, the use of a vacuum chuck to hold the cartridge flat obviates any requirement that the cartridge itself be rigid and thus allows the use of flexible materials for the cartridge. It will be appreciated by those of skill in the art that the vacuum chuck needs to be constructed and employed in ways which cause the sample cassette to reproducibly present a consistent optical path through the various sample (and optional calibration) zones. To this end, it is generally desirable to check the degree of vacuum drawn in the vacuum chuck, the spacing of the vacuum zones in the chuck and the like to assure that when the particular sample cassette is placed on the vacuum chuck and there held in place, there is not an unacceptable degree of distortion. 
     In the embodiment wherein the vacuum chuck and the cassette are used in an optical instrument such as a spectrophotometer, the samples in the cassette may be scanned with a laser and the contents of the sample determined by reference to the fluorescence emanating from the sample. In this case, it is advantageous to minimize background fluorescence, such as fluorescence generated by the materials from which the chuck is fabricated. In this case, it may be advantageous to fabricate the chuck with depressed areas  44  in the form of slots aligned with and directly beneath the optical chambers on the cartridge when the cartridge is placed on the vacuum chuck. Such slots can reduce fluorescence by distancing the material of the chuck from the focal plane of the optical instrument used. In a typical use of the cuvette cartridge, a liquid sample is applied to an inlet hole, thus filling the optical chamber by capillary action. The cartridge is placed in a position for an optical measurement, such as on a platform of a laser scanner. A confocal laser scanner is used to scan the optical chamber. Typically the scanner will utilize the natural fluorescence of the adhesive sheet to define the edges of the optical chamber. In an embodiment, a reference optical chamber can be used by the scanner to detect the edges of the optical chamber. These measurements can then be applied to the optical chambers on the cartridge.