Optical spectroscopy sample cell

A sample cell for use in spectroscopy includes two cell portions that are mutually adjustable to enable a user to vary the cross sectional geometry of sample cell flow path thereof between at least two discrete configurations or positions. In a first position, the flow path may be provided with a relatively small cross-sectional area to provide a relatively small pathlength and generate high velocity fluid flow therethrough. The small pathlength enables use of relatively small sample volumes and facilitates optical measurement of samples, such as blood, which have relatively high optical density, absorbance or light scattering properties. In a second position, the flow path provides a larger cross-section adapted to permit larger agglomerations or clots to pass through the flow path to facilitate cleaning of the sample cell. The cell portions are maintained in slidable engagement with one another so that the adjustability of the flow path is accomplished by sliding movement of the cell portions relative to one another. In one embodiment, the cell portions are engaged along generally planar mating surfaces and the adjustability is accomplished by sliding the mating surfaces relative to one another. In another embodiment, the mating surfaces are superposed in spaced relation to one another, and the adjustability is accomplished by varying the distance therebetween.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to spectrochemical determination of analyte content
 of a sample, and more particularly to a spectroscopy sample cell for
 analyte analysis of turbid media such as blood.
 2. Background Information
 Spectrochemical analysis is a broad field in which the composition and
 properties of materials in any phase, (liquids, solids, gases or plasmas)
 are determined from the electromagnetic spectra arising from interaction
 (for example, absorption, luminescence or emission) with energy. One
 aspect of spectrochemical analysis, known as spectroscopy, involves
 interaction of radiant energy with material of interest. The particular
 methods used to study such matter-radiation interactions define many
 sub-fields of spectroscopy. One field in particular is known as absorption
 spectroscopy, in which the optical absorption spectra of liquid substances
 is measured. The absorption spectra is the distribution of light
 attenuation (due to absorbance) as a function of light wavelength. In a
 simple spectrophotometer, the sample substance which is to be studied,
 usually a gas or liquid, is placed into a transparent container, also
 known as a cuvette or sample cell. Collimated light of a known wavelength,
 .lambda., i.e. ultraviolet, infrared, visible, etc., and intensity
 I.sub.o, is incident on one side of the cuvette. A detector, which
 measures the intensity of the exiting light, I, is placed on the opposite
 side of the cuvette. The thickness of the sample is the distance d, that
 the light propagates through the sample. For a sample consisting of a
 single, homogenous substance with a concentration, c, the light
 transmitted through the sample will follow a well known relationship known
 as Beer's law which in part, states that the shorter the optical
 pathlength d, the less the absorption of light. If .lambda., I.sub.o, I,
 d, c, are known, a quantity known as the extinction coefficient can be
 determined.
EQU OD(.lambda.)=c*.epsilon.(.lambda.)*d
 Where:
 .epsilon.(.lambda.) is the extinction coefficient of the substance at
 wavelength .lambda.;
 OD(.lambda.) is the optical density of the sample at wavelength .lambda.
 (OD=-Log of the ratio: light transmitted (exiting)/light incident.
 (-Log.sub.10 I/I.sub.0)
 c is the concentration of the substance
 d is the pathlength or thickness of the sample through which the light
 propagates.
 Beer's law is useful when one considers samples which are mixtures of
 several different substances, j, each with known extinction coefficients,
 .epsilon..sub.j, and relative concentrations c.sub.j. In such a case, the
 optical density of the sample is given by:
EQU OD(.lambda.)=.SIGMA.c.sub.j *.epsilon..sub.j *(.lambda.)*d
 Inversely, if given a sample mixture's optical density spectra
 OD(.lambda.), and the extinction coefficients for each of the component
 substances, the unknown relative concentration of the component substances
 can be determined. Note, also, it is assumed that each of the component
 substances maintain the same extinction coefficients as when in a pure
 form, i.e. no chemical reactions occur that alter the extinction
 coefficients.
 Thus, if the absorption spectra for a given substance is known, its
 presence and concentration in a sample may be determined.
 Optimum optical pathlength depends on a variety of factors. In general,
 relatively small pathlengths are preferred as they enable use of smaller
 sample volumes and facilitate optical measurement of samples, such as
 blood, which have relatively high optical density, absorbance or light
 scattering properties. However, problems tend to occur when pathlengths as
 small as a few thousandths of an inch are used with whole blood samples.
 Unfiltered human blood may contain various formed elements (cell
 aggregates, protein/fibrin chains, synthetic fibers, clots, etc.), which
 when introduced into a narrow sample cell, can become lodged in the sample
 cell. These "clots" can obstruct the flow of wash fluids or subsequent
 samples, which can adversely affect the optical measurements. When high
 velocity air or liquid is forced through the sample cell, these clots can
 sometimes be dislodged. The introduction of a cleaning solution containing
 bleach or pepsin may also be effective in removing protein and fibrin from
 the sample cell. These cleaning solutions however, can only be used in
 systems which can tolerate exposure to such agents. In some cases, the
 sample cell may have to be replaced or disassembled and manually cleaned
 to remove such clots.
 Optical measurements in whole blood can be further complicated by a number
 of issues. When red blood cells, the small "carriers" of hemoglobin (the
 light absorbing constituent to be measured), are suspending in serum, a
 light scattering (non isotropic) medium is produced. This is due to the
 differing indices of refraction of the red blood cell's intracellular
 fluid and the serum in which the cells are suspended. In such a case,
 Beer's law may not strictly apply and light scattering in the sample may
 result in significant errors in spectrophotometric measurements.
 The degree of light scattering in whole blood is influenced by a number of
 normally occurring conditions such as variations in serum protein and
 lipid content, red cell morphology (size and shape), red cell
 concentration (number of red cells per unit volume), cell rouleaux
 formation, and red cell orientation.
 Rouleaux formation occurs when a sample of whole blood is allowed to remain
 stationary, whereupon the red blood cells coalesce in an ordered fashion,
 forming "stacks" or "chains." This phenomenon, which is due, in part, to
 the bi-concave shape of the red cells and the colloids present in blood
 serum, significantly affects the optical properties of whole blood.
 Although some exceptions exist (e.g., certain animal bloods and rare
 variations in cell morphology/serum chemistry), this phenomenon occurs
 normally in most whole blood samples. The rate at which these rouleaux
 "chains" form is strongly dependent on red cell concentration.
 Rouleaux formation is a reversible phenomenon. If the blood sample is
 mixed, stirred or made to flow through a channel of some type, resulting
 shear forces within the fluid will cause the rouleaux chains to
 disassemble and break up. Stopping the blood flow will allow, once again,
 the chains to form. If allowed to sit for extended periods of time, these
 rouleaux chains will settle and eventually result in the separation
 (stratification) of the blood cells and the blood serum.
 Rouleaux formation alters the amount of light scatter observed in a given
 blood sample. It is important then, that a means of either accounting for
 or preventing the effect must be implemented. One way of preventing
 rouleaux chains from forming, is to force the blood to flow through a
 small channel. Shear forces, generated by differential velocities
 (velocity gradients) throughout the flow channel, if high enough, tend to
 prevent rouleaux chains from forming. A drawback of this approach however,
 is that use of a relatively small channel disadvantageously exacerbates
 the tendency for clogging or clotting, etc.
 Another factor to be considered is whether or not the red cells are
 "oriented" during analyte measurement. Red cells tend to become oriented
 in a particular manner while flowing through narrow channels at relatively
 high velocity. A "normal" red blood cell is bi-concave (i.e. shaped much
 like a donut; thinner in the middle than at the edges) and measures
 approximately 8 microns (8.times.10.sup.-6 meters) across by 2 microns
 thick. A single normal red blood cell, if suspended in a carrier fluid
 experiencing relatively high velocity laminar flow, will be acted upon by
 the shear forces generated by the differential velocities in the fluid.
 Because of the cell's asymmetric shape, these forces will tend to orient
 the cells in some non-random fashion. This orientation of red blood cells
 occurs in mass (whole blood) flow, and is affected by the size and shape
 of the flow cell, particularly by use of flow cells having relatively
 small or restricted openings.
 The amount of light transmission through a sample of whole blood where the
 red cells are randomly organized differs significantly from one in which
 the cells are oriented (i.e. non-randomly). This aspect of the red cells
 thus tends to further complicate spectroscopic analyte measurement in
 whole blood samples.
 One attempt to overcome the aforementioned difficulties and provide
 accurate optical determination of analytes in blood samples has been
 embodied in instrumentation manufactured by Bayer Corporation of Medfield,
 Mass., known as the 800 Series analyzer. This instrumentation performs
 spectrophotometric measurements using lysed blood samples. The lysing
 process ruptures the red blood cell's membrane, releasing the
 intracellular fluid (primarily hemoglobin) into the surrounding serum.
 Lysing the red cells makes the sample relatively homogeneous and isotropic
 (i.e. essentially non-light scattering). Beer's law, as discussed above,
 then may be effectively applied to determine analyte concentration.
 Ultrasonic energy provided by ultrasonic cell disrupters is typically
 utilized to lyse the red cells. A drawback of this approach is that these
 cell disrupters add cost, take up space, require maintenance and may
 generate bubbles in the sample which generate unpredictable light scatter
 induced errors. Also, the lysing process alters the sample's serum
 chemistry. Because of this, integrated instruments which combine lysed
 blood spectrophotometry with other sensor technologies that require whole
 blood for their measurements require additional sample volume for
 analysis. In these systems, a sample is aspirated into the instrument,
 then "split" into two segments to effectively isolate a portion of the
 sample for lysing and an other portion for whole blood analysis.
 Thus, a need exists for an improved spectroscopy sample cell that avoids
 clogging, may be easily cleaned without special cleaning agents, is
 optimized for various operating modes including measurement, wash, fill,
 etc., is inexpensive and provides a stable mechanical environment for
 optical measurement repeatability.
 SUMMARY OF THE INVENTION
 According to an embodiment of this invention, a sample cell is provided for
 use in spectroscopic determination of an analyte in a fluid sample. The
 sample cell includes a sample path extending therethrough adapted for
 communicating fluid from an inlet, through a measurement zone to an
 outlet, the sample cell being selectively adjustable between a first
 position having a predetermined optical pathlength adapted for analyte
 measurement while the sample is in the measurement zone, and a second
 position having a predetermined other pathlength adapted for clearing the
 sample from the flow path.
 The above and other features and advantages of this invention will be more
 readily apparent from a reading of the following detailed description of
 various aspects of the invention taken in conjunction with the
 accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring to the figures set forth in the accompanying Drawings,
 illustrative embodiments of the present invention will be described in
 detail hereinbelow. For clarity of exposition, like features shown in the
 accompanying Drawings shall be indicated with like reference numerals and
 similar features as shown in an alternate embodiment or to represent
 movement in the Drawings shall be indicated with similar reference
 numerals.
 Briefly described, the present invention includes a sample cell 100 for use
 in spectroscopy, which includes two cell portions 48 and 52 that are
 mutually adjustable to enable a user to vary the cross-sectional geometry
 of sample cell flow path 16 thereof, between at least two discrete
 configurations or positions. In a first position, flow path 16 (FIG. 5A)
 may be provided with a relatively small cross-sectional area to provide a
 relatively small pathlength and to generate high velocity fluid flow
 therethrough. The small pathlength enables use of relatively small sample
 volumes and facilitates optical measurement of samples, such as blood,
 which have relatively high optical density, absorbance or light scattering
 properties. In a second position, flow path 16' (FIG. 5B) provides a
 larger cross-section to enable larger agglomerations or clots to pass
 through the flow path to facilitate cleaning of sample cell 100. Cell
 portions 48 and 52 are maintained in slidable, fluid tight engagement with
 one another so that the adjustability of flow path 16 is accomplished by
 sliding movement of cell portions 48 and 52 relative to one another. In
 one embodiment, the cell portions are engaged along generally planar
 mating surfaces 49 and 53, and the adjustability is accomplished by
 sliding the mating surfaces relative to one another. In another
 embodiment, mating surfaces 249 and 253 (FIG. 11) are superposed in spaced
 relation to one another, and the adjustability is accomplished by varying
 the distance therebetween.
 Throughout this disclosure, the term "light" shall refer to electromagnetic
 radiation of a wavelength .lambda. within the visible or near visible
 range, from approximately 100-30,000 nanometers, including
 far-ultraviolet, ultraviolet, visible, infrared and far-infrared
 radiation. Similarly, throughout this disclosure, the term "optical" shall
 be defined as pertaining to or utilizing light as defined herein.
 Referring now to the drawings in detail, as shown in FIGS. 1A-1C, the
 primary purpose of sample cell or cuvette 10 is to confine the sample
 within a controlled geometry, while allowing for illumination and
 detection of the post interaction light intensity.
 Turning to FIG. 1A in particular, in a simple spectrophotometer 20, a
 sample 18 is placed into a cuvette or sample cell 10. Collimated light of
 a known wavelength, .lambda., i.e. ultraviolet, infrared, visible, etc.,
 and intensity I.sub.o, is generated by a light source 22 and is incident
 on one side of the cuvette. Exiting light of intensity I typically passes
 through a lens 24 and is measured by a detector 25.
 As shown in FIG. 1B, the thickness of sample 18 within sample cell 10 is
 the distance d that the light must propagate through. For a sample
 consisting of a single, homogeneous substance with a concentration, c, the
 light transmitted through the sample will tend to follow Beer's law as
 discussed hereinabove, to enable one to determine analyte presence and
 concentration within the sample.
 Typically, as shown in FIG. 1C, two opposing sides of the cell consist of
 flat "windows" or panels 26 which are transparent to the wavelengths of
 light being used for the measurements. These windows are transparent at
 least in a predetermined optical measurement zone or area 30. As shown,
 measurement zone 30 preferably includes a substantially circular area as
 indicated by the disc-shaped portion of sample 18 shown in FIG. 1C.
 Turning now to FIG. 2, the present invention enables a single, relatively
 simple, sample delivery system 28 to be utilized in integrated
 instrumentation (not shown) that combines whole blood spectrophotometry
 with other sensor technologies that utilize whole blood for their
 measurements. As used herein, the term "downstream" refers to the
 direction of fluid flow through delivery system 28, from a collection or
 storage device, to waste receptacle 32. The term "upstream" refers to a
 direction opposite the downstream direction. As shown, sample delivery
 system 28 includes a conventional sample aspirator probe 34 adapted for
 fluid communicating engagement at its upstream end with a conventional
 sample collection device or syringe 36 as shown. The aspirator probe
 communicates at its downstream end with a reagent valve 38, which in turn,
 communicates with a fluid path 40 defined, in part, by a conduit such as,
 for example, flexible tubing (not shown). Reagent valve 38 is also engaged
 with an array of calibration and wash fluid storage containers 42 as
 shown. The reagent valve is a device commonly known to those skilled in
 the art. Valve 38 operates in a conventional manner to selectively connect
 the calibration/wash fluids from container array 42, and a sample 18
 disposed within syringe 36, to fluid path 40. An optical sample cell 100
 of the present invention and various additional system sensors 44 are
 disposed within fluid path 40 downstream of reagent valve 38. System
 sensors 44 may include any number of individual sensors commonly known to
 those skilled in the art, such as for example, various electrochemical,
 optical, or electrical sensors including ion selective electrodes and
 luminescent sensors for such analytes as Calcium, Chloride, Oxygen, Carbon
 Dioxide, Glucose, pH, BUN, etc. A fluid pump 46 such as a conventional
 peristaltic pump is also disposed along the fluid path, preferably
 downstream of system sensors 44 as shown, and serves to draw the sample or
 calibration/wash fluids through delivery system 28. Fluid path 40
 terminates at waste receptacle 32. Delivery system 28 serves to
 alternately move a sample 18 and calibration/wash fluid, through sample
 cell 100 for respective analyte measurement and calibration/wash sequences
 in a manner to be discussed hereinbelow.
 Referring to FIGS. 3-5, the present invention includes a sample cell 100
 optimized for use in a spectrophotometer adapted to measure the
 concentration of various analytes within a moving or flowing sample of
 whole or lysed (human) blood. Sample cell 100 provides a means for
 optimizing flow to clear obstructions while also optimizing flow and
 optical pathlength for optical analyte measurement of blood (whole or
 lysed) or other samples of high optical density.
 In one embodiment, as shown in FIG. 3, the sample cell includes three
 components, namely, a first or base portion 48, a fluid seal or gasket 50
 and a second or cover portion 52. Base portion 48 includes concave entry
 and exit recesses 54 and 56, which terminate at entry and exit apertures
 55 and 57, respectively. Gasket 50 includes an elongated cutout or slot 58
 sized and shaped for superposed alignment with peripheries of both
 apertures 54 and 56 to comprise a sample cell flow path 16 (FIG. 5A) that
 extends from entry recess 54, through optical measurement zone 30, and to
 exit recess 56 when sample cell 100 is assembled as shown in FIG. 4. As
 also shown, mating surface 53 includes a hollowed out or substantially
 concave recess 59 that will be discussed in greater detail hereinbelow.
 As shown in FIG. 4, components 48, 50 and 52 are sealably and slidably
 superimposed or stacked atop one another. In other words, base portion 48
 and cover portion 52 are slidable relative to one another while a
 fluid-tight (i.e. gas and liquid tight) seal is maintained therebetween.
 As shown, gasket 50 may be compressed between mating surfaces 49 and 53 of
 portions 48 and 52, respectively, to facilitate this fluid-tight
 engagement. However, mating surfaces 49 and 53 may be provided with
 sufficient smoothness and uniformity to provide sufficient slidability and
 sealability without using gasket 50. Both portions 48 and 52 may be
 fabricated from any number of suitable materials, using a wide range of
 fabrication methods. For example, portions 48 and 52 may be fabricated
 from glass, crystal, quartz, sapphire or from a polymeric plastic
 material. In a preferred embodiment, portions 48 and 52 may be fabricated
 by injection molding a suitable thermoplastic material such as, for
 example, acrylic, polycarbonate, ultra high molecular weight or low
 density polyethylene, polyacrylonitrile, etc. The seal material may be an
 elastomer such as a hydrofluorocarbon elastomer sold under the trademark
 VITON available from DuPont Dow Elastomers Corporation. The choice of
 particular materials may be made based upon their mechanical, chemical and
 optical properties.
 Base and cover portions 48, 50 are maintained in mating engagement as
 shown, by suitable support means (not shown). The distance between mating
 surfaces 49 and 53 at optical measurement zone 30, taken along optical
 path p orthogonal to the surfaces, defines optical pathlength d (FIG. 5A).
 Optical pathlength d may be predetermined by various means, such as by
 interposing a precision spacer or shim between base and cover portions 48
 and 52. Alternatively, one or both mating surfaces 49 and 53 may be
 recessed within optical measurement zone 30 to establish the desired
 pathlength d. For example, surface 49 may be recessed in the optical
 measurement zone, as shown in FIG. 5A. One skilled in the art will
 recognize that such a recessed portion must be substantially planar and
 parallel to the opposite mating surface within measurement zone 30 to
 provide a relatively constant optical pathlength d.
 Turning now to FIG. 5A, a portion of fluid flow path 40 (FIG. 2) as it
 extends through sample cell 100 is referred to as cell flow path 16. This
 path 16 includes entry aperture 55, recess aperture 54, measuring zone 30,
 exit recess 56, recess 59 and exit aperture 57. As shown, sample cell 100
 is disposed in a first measurement or closed position, in which optimal
 pathlength d is maintained within optical measurement zone 30. Optimal
 pathlength d is determined by considering many factors. Several
 significant factors to be considered include the spectrophotometer's light
 source intensity, detection system characteristics such as available
 dynamic range, signal to noise ratio, resolution and sensitivity, as well
 as the sample substance's concentration and absorption properties
 (extinction coefficients) in the wavelength region of interest.
 Moreover, red blood cell orientation is a function of cell concentration
 and morphology, sample flow velocity and flow channel geometry. It has
 been demonstrated experimentally that cell orientation may be controlled
 by adjusting these variables to generate relatively high shear force
 conditions. Advantageously, such high shear forces tend to promote red
 cell orientation. Optical pathlength d of the present invention is thus
 preferably predetermined to be sufficiently small so that when other flow
 conditions are controlled, the cells become oriented. This orientation
 serves to generate predictable light scatter properties during analyte
 measurement to facilitate accurate analyte measurement using samples 18 of
 whole blood.
 Balancing these factors, sample cell 100 of the present invention may be
 provided with an optical pathlength d within a range of approximately
 0.003 to 0.004 inches, preferably about 0.0035 inches. This relatively
 small optical pathlength d facilitates measurement of samples having high
 optical density, such as whole or lysed blood. Moreover, when sample cell
 100 is disposed in this measurement position as shown, the transverse
 cross sectional area of flow path 16 throughout measurement zone 30
 (defined herein as a cross-section taken parallel to optical path p and
 transverse to the downstream direction) is relatively small, preferably
 about 0.000175 square inches or less. This cross-sectional area tends to
 provide sufficient flow velocity to generate desired shear forces at a
 flow volume as low as approximately 7 microliters per second (.mu.l/sec)
 to facilitate optical measurement of whole blood.
 Referring to FIG. 5B, cover portion 52 has been slidably moved relative to
 base portion 48 and is thus indicated as 52'. As shown, sample cell 100 is
 disposed in its second or open position, in which recess 59 of cover
 portion 52 has been moved into optical measuring zone 30 of base portion
 48. This movement is effected by any suitable means, such as an air
 cylinder or other pneumatic device, linear stepper/DC motor, solenoid,
 etc., or by manual manipulation. When in this open position, flow path
 16', in the vicinity of measurement zone 30, is provided with a relatively
 large pathlength d' parallel to light path p, and a relatively large
 transverse cross-section.
 Operation of sample cell 100 includes moving cover 52 into the measurement
 position as shown in FIG. 5A for analyte measurement. This provides a
 relatively small predetermined cross sectional area and optical pathlength
 d within the optical measurement zone 30 so that a sample 18 of blood
 passes through at relatively high red cell velocities and shear forces. As
 discussed hereinabove, these aspects tend to advantageously reduce
 rouleaux formation and promote red cell orientation in whole blood
 samples, while providing an optical pathlength d that facilitates optical
 measurement of high optical density samples such as whole or lysed blood.
 After the sample has been analyzed, cover 52 is moved relative to base 48
 into the second or wash position wherein wash fluid may be fed into the
 flow path to clean sample cell 100 for a subsequent sample 18. Many wash
 sequence options are facilitated by cell 100. For example, a wash sequence
 may begin with the sample cell in the closed measurement position. This
 would provide relatively high wash fluid velocity for cleaning the sample
 out of the optical measuring zone. The sample cell may then be switched
 into the open position during washing. Washing in the open position
 permits relatively large obstructions such as fibrin, clots, synthetic
 fibers, or other agglomerations, etc., to be freed. Moreover, sample cell
 100 may be alternated back and forth between open and closed positions
 during the wash sequence if necessary to help dislodge obstructions.
 Turning now to FIG. 6, an alternate embodiment of the subject invention is
 shown as sample cell 200, which is in many respects similar or analogous
 to sample cell 100. Sample cell 200 includes a base or body portion 248. A
 tubular sample flow path 216 extends through base 248 and includes entry
 and exit portions 82 and 84 having sample entry and exit apertures 255 and
 257, respectively. As shown, this sample cell is preferably a pneumatic
 device and includes a fluid inlet 62 which facilitates use of air or
 similar fluid pressure to actuate cell 200 between its open and closed
 positions, as will be discussed hereinafter.
 Referring to FIG. 7, sample cell 200 includes body 248 having a generally
 planar surface 63 and a cavity or trough 60 disposed therein. Cavity 60 is
 generally "U" or horseshoe shaped in the plane of surface 63 to
 substantially surround three sides of a central position or locus 65 of
 base 248. The cavity extends to a predetermined depth within base 248,
 terminating at a substantially planar bottom wall portion 67 disposed
 generally in parallel to mating surface 249 (FIG. 8) which will be
 discussed hereinbelow. Portion 67 includes a series of through holes 66
 spaced generally equidistantly about locus 65. Each through hole 66 has a
 central axis a (FIG. 10) which preferably extends substantially
 orthogonally to bottom wall portion 67. A lifting shoe 61 is sized and
 shaped for slidable registration within cavity 60. A series of guidepins
 64 are spaced along shoe 61 at predetermined locations to slidably and
 coaxially extend through corresponding through holes 66. Shoe 61 and
 cavity 60, including guidepins 64 and holes 66, are provided with
 predetermined dimensions sufficient to provide a predetermined range of
 motion of shoe 61 within cavity 60, in an axial direction defined by axis
 a, as will be discussed hereinbelow.
 Sample cell 60 also includes a diaphragm 70 preferably fabricated from an
 elastomeric or rubber-like material, such as a polyvinyl elastomer.
 Diaphragm 70 is sealed in a fluid or airtight manner about its periphery
 to the periphery of cavity 60. In a preferred embodiment, the diaphragm is
 sealed to a peripheral ledge or lip 72 extending inwardly, generally
 orthogonally relative to the axial direction, from the periphery of cavity
 60. A pneumatic chamber plug 74, which includes air inlet 62, is sized and
 shaped for receipt within cavity 60 in superposed relation with shoe 61
 and diaphragm 70. Plug 74 is immovably fastened, in a gas tight manner to
 cavity 60, such as by ultrasonic welding or use of an adhesive or epoxy
 bond 77 (FIG. 10), so that plug 74 and diaphragm 70 form a gas tight
 pneumatic chamber 75 (FIG. 10) in fluid communication with air inlet 62.
 As also shown, a tube support 76, provided to support air inlet 62, is
 adapted for receipt within a corresponding receptacle 78 of base 248. A
 flexible elliptical gasket or elongated O-ring 80 is also provided, as
 best shown in FIG. 8.
 Turning now to FIG. 8, guidepins 64 are adapted to extend completely
 through holes 66 (FIG. 7) to protrude from substantially planar mating
 surface 249 of base 248 as shown, for connection to second portion or
 window 252 (FIGS. 9-11). A series of support pads 86 are disposed in
 spaced locations along surface 249, preferably proximate each guidepin 64.
 Entry and exit portions 82 and 84 of sample flow path 216 extend through
 base 248 and terminate at entry and exit recesses or openings 254 and 256
 disposed in mating surface 249. As shown, openings 254 and 256 are
 disposed substantially at locus 65 between guidepins 64 and pads 86.
 Openings 254 and 256 are spaced a predetermined distance from one another,
 so that the portion of surface 249 disposed therebetween and between an
 elliptical channel 82 forms a window 226 in the optical measurement zone
 30 of sample cell 200. As shown, elliptical channel 82 is recessed within
 surface 249 and encircles or surrounds openings 254 and 256. Gasket 80 is
 configured to be received within channel 82.
 Referring now to FIG. 9, window 252 is shown engaged with one of the
 guidepins 64, while being supported on its opposite side by a portion of
 spectrophotometer 20. A predetermined compressive force f, provided by any
 convenient spring means (not shown) is applied to base 248 in a direction
 substantially orthogonal to mating surface 249 so that gasket 80 is
 sealingly compressed against window 252. The portions of window 252 and
 surface 249 enclosed by gasket 80 effectively form the sample chamber
 portion of flow path 216, including optical measurement zone 30, of sample
 cell 200.
 Turning to FIG. 10, sample cell 200 is shown in its closed position, in
 which window 252 is substantially fully compressed against gasket 80 and
 engaged with support pads 86. Pneumatic chamber 75 is nominally at ambient
 pressure. Optical pathlength d is nominally the same as the thickness of
 pads 86. As discussed hereinabove with respect to sample cell 100, optical
 pathlength d may be approximately 0.003 to 0.004 inches, preferably
 approximately 0.0035 inches.
 Referring now to FIG. 11, a p redetermined fluid (i.e. air) pressure has
 been applied to pneumatic chamber 75 through air inlet 62 which serves to
 expand the chamber and bias lifting shoe 61 and window 252 away from base
 248 against the bias of force f. Lifting shoe 61 is moved by this
 expansion until the shoe engages or "bottoms out" against bottom wall
 portion 67. This engaged or bottomed out position defines predetermined
 pathlength d', which is optimally as large as approximately 0.015 to 0.025
 inches. Sample cell is thus disposed in its open position. In this
 position, gasket 80, though decompressed relative to the closed position
 shown in FIG. 10, is still disposed in fluid sealing engagement with
 window 252 as shown.
 Although preferred embodiments have been shown and described, several
 alternate embodiments of the sample cell of the present invention are
 envisioned. For example, sample cell 100 may be provided with multiple
 discrete optical pathlengths d such as by providing one or both surfaces
 49 and 53 with a stepped profile. In this manner, a single sample cell of
 the present invention may be used to analyze samples having a relatively
 wide range of discrete absorption properties. Moreover, a further
 variation may include provision of a position in which a cell portion of
 known pathlength containing a "calibrator" may be disposed within the
 measurement zone 30. This calibrator may be a dye or an optical
 interference filter permanently disposed within a portion of one or both
 of the base 48 and cover 52. Such a feature may be useful when running
 diagnostics on instrument componentry such as light source 22 or other
 spectrophotometer components.
 The sample cell of the present invention thus provides numerous benefits
 relative to the prior art. In particular, the cell is optimized for
 various operating modes, to enable optimized optical measurement of whole
 blood, while it also facilitates optimized washing, etc., to reduce
 difficulties associated with clogging. Moreover, the present invention
 substantially eliminates the need for a "hemolyzer" and the associated
 hydraulic components required to segment the sample into lysed and whole
 portions. This advantageously reduces system cost, size and tends to
 increase overall system hardware reliability. In addition, elimination of
 the lysing step advantageously maintains sample integrity. This permits
 measurements to be taken by other sensors, for which whole blood is
 required, on the same sample volume to effectively reduce the overall
 sample volume required for analysis. Still further, while the sample cell
 of the present invention facilitates optical measurement of whole blood
 samples, it also advantageously enables measurement of other samples
 having high optical density or light scattering properties such as lysed
 blood samples. The present invention thus provides the dual functionality
 of optical measurement of both lysed blood and whole blood samples.
 The present invention may also substantially reduce the need for special
 cleaning agents to remove large protein formations which tend to occur
 when running many blood samples. In addition, clearing of such formations
 may be effected automatically, without manual user intervention.
 Moreover, the present invention may be conveniently incorporated into an
 instrument which incorporates all measurement sensors, fluid handling
 components (valves, tubing, etc.) and calibration liquids, into a limited
 use, disposable "cartridge." Advantageously, such a configuration may
 provide the user with a system in which sensor and fluidic maintenance is
 not required. All system components that come in contact with the sample
 or reagent fluids may be disposed of when the cartridge expires.
 Moreover, the predetermined dimensions of the first and second portions of
 the sample cells 100 and 200, provided by use of rigid components movable
 between predetermined positions to define optical pathlength d provides a
 stable mechanical environment for optical measurement repeatability. The
 sample cell of the present invention is also relatively inexpensive to
 manufacture.
 Although the present invention has been described for use in absorption
 spectroscopy, one skilled in the art should recognize that the adjustable
 sample cell of the present invention may be utilized in conjunction with
 various other spectrochemical analysis techniques, including for example,
 luminescence or electromagnetic spectra emission measurement, without
 departing from the spirit and scope of the present invention.
 Moreover, although sample cell 200 has been described as a pneumatic device
 that utilizes air or other fluid pressure to move first portion 248
 relative to second portion 252, any suitable device or technique may be
 utilized to effect such movement, without departing from the spirit and
 scope of the present invention. For example, any suitable means, such as
 an air cylinder, linear actuator or stepper/DC motor, solenoid, etc., or
 even manual manipulation, may be utilized.
 The foregoing description is intended primarily for purposes of
 illustration. Although the invention has been shown and described with
 respect to an exemplary embodiment thereof, it should be understood by
 those skilled in the art that the foregoing and various other changes,
 omissions, and additions in the form and detail thereof may be made
 therein without departing from the spirit and scope of the invention.