Improved single source multi-site photometric measurement system

Abstract: A multibeam photometric measuring device includes a light assembly (12) comprising a single modulated light source along with a filter assembly (19) for passing only a desired band of wavelengths and an optical system for suitably focusing light from the light source. A fiber rotor arrangement (24) is provided for sequentially connecting the collimated light source to each of the plurality of fibers (34) disposed in a distributor (32). Light passing through the wells of the sample plate (40) is captured by discrete photodetector cells (44) disposed on a one-to-one correspondence opposite the wells of the plate. A reference fiber links (52) light from the light source to a separate photodetector (45) to provide a reference light signal with which the sensed photodetector signals are compared. The sensed signals are suitably multiplexed and processed by circuitry which allows control over the accuracy with which the signals are processed on the basis of their instantaneous strengths as well as reference readings taken during system initialization. Enhancement of colour distribution resulting from chromogenic reactions is provided by automatic agitation by mechanism (54) of the reacting agents within the sample wells before a reading is taken at each of the sampling intervals of a kinetic measurement sequence.

Description: IMPROVED SINGLE SOURCE MULTI-SITE PHOTOMETRIC MEASUREMENT SYSTEM This invention generally relates to photometric devices for performing densitometric measurements on translucent sites. More particularly, this invention relates to an automatic scanning device using end-point and/or kinetic measurement techniques for measuring the optical density of a plurality of translucent samples in order to analyze their contents. A variety of techniques and devices are commercially available for the detection and measurement of substances present in fluid or other translucent samples by determining the light transmitivity of the sample. In particular, a number of photometric devices are capable of simultaneously performing individual assays on a plurality of liquid or other translucent samples. Such methods generally handle a multiplicity of samples by using "microplates" which contain a standard array (8 x 12) of wells and which are made of an optically transparent material. The optical density of the samples is measured by determining attenuation undergone by light as it passes through the translucent samples, contained in the microplate wells, to conventional photodetecting means. A widespread use for microplates is in the enzyme-linked immunosorbent assay (ELISA) technique which is used for detection and quantification of an extensive range of substances and biological cells in academic research and biotechnology as well as for clinical testing. In such assays, molecules of a marker enzyme (such as alkaline phosphatase) are deposited on the bottom and part of the way up the sides of each of the wells of a microplate; each well having been assigned to interact previously, directly or indirectly, with a sample containing an analyte of interest. The number of marker enzyme molecules bound to each well of the plate is a function of the concentration of analyte in the sample of interest. Determination of the activity of the bound enzyme, therefore, permits detection or quantification of the analyte. For determination of fluid-phase enzyme activity, current techniques for both research and clinical applications employ kinetic analysis which involves measurement of the initial rate of enzyme-catalyzed, chromogenic reactions in the presence of excess of the enzyme substrate; a procedure which has several well-known advantages over the alternative "end-point" analysis method of allowing the enzyme to react with a chromogenic substrate for a fixed period of time and then making a single optical density measurement after quenching the enzymes. In kinetic analysis, multiple readings are made within the initial (typically linear) reaction period and the intervals between readings are necessarily short (typically less than 30 seconds). By using kinetic analysis, the introduction of errors caused by (a) differences in initial optical density and/or (b) loss of independence from substrate concentration, is substantially avoided. Currently available automated optical density measurement instruments for microplates typically function by mechanically moving either the multi-well microplate or the optical components themselves in order to successively perform assays of samples located at the plurality of individual assay sites. This requirement places a severe restraint on the time required to actually measure the transmittance in all wells of the microplate, thereby making large scale kinetic analysis assay applications impractical due to the extended sampling times. In part because of this, "end-point" analysis is employed for ELISA protocols read by current instruments. A measurement system which is capable of reading a plurality of assay sites in sequence without relative physical movement of the microplate and the optical components iS disclosed in Wertz et al. US-A-4,448,534 (Figure 8) which discloses the use of fiber optic transmission means with a single light source sequentially coupled to a plurality of optical fibers which transmit light to the measurement sites. However, the apparatus described in the Wertz patent uses a highly inefficient system for coupling light from the light source into the optical fibers, which in turn leads to a variety of potential problems for kinetic measurements of enzyme activity. For example, the Wertz apparatus requires a high power light source and the increased light intensity can adversely affect the chemical reactions in the assay sites by increasing the operating temperature of the measurement system non-homogeneously and hence altering the rates of reaction in different wells to a different extent. In addition, such systems are unduly complex because of the wide fluctuation in signal levels generated as a result of the reception of light at the photodetectors after it has passed through the sample sites; this prevents efficient utilization of the overall dynamic range of amplification for the signal amplifiers of the measurement system. Another limitation of conventional microplate reading devices is their inability to make useful quantitative measurements for ELISA protocols performed in filter-bottom microplates. A principal problem is that the individual wells in such plates vary considerably in their initial optical density relative to air thereby introducing considerable error when end-point measurements are taken. Kinetic analysis, on the other hand, is not affected by this type of problem. Another major problem associated with conventional microplate reading devices, when used for assaying chromogenic reactions kinetically, is that they are subject to errors arising from erratic redistribution of the colored product as a result of phase separation and/or uncontrolled bulk movement of the aqueous phase of the sample during kinetic analysis. More specifically, in the case of ELISA protocols, where the enzyme is bound to the plastic surface of the microplate wells (on the bottom and/or part way up the sides), the bound enzyme interacts with an unstirred aqueous phase layer which typically causes localized phase separation of the colored product of the enzyme reaction due to its high local concentration. This separation introduces an unquantifiable error and a degree of non-linearity into such kinetic measurements. Even in cases where the colored product remains in true solution, erratic bulk movement of the aqueous phase leads to uneven redistribution of the concentrated product and hence to an unquantifiable error. US-A-4 482 251 shows a clinical analyser in which a shuttle of cuvettes is analysed one by one. While a cuvette is being tested, an agitator provides for agitation of that cuvette to stir the contents therein. A preferred form of agitator is a rotary solenoid controlled lever which strikes a particular cuvette whose contents are to be agitated. FR-A-2357881 shows an analysing apparatus in which clear microplates are dispensed on a conveyor belt and moved onto a station where reagents are added to the plates. Next the plates are moved to a station where sample liquids to be tested are added. Then the plates are moved to a station where movement is applied to the plates. Next, the samples are moved to an incubator chamber for a predetermined period of time. The plates are then moved to an image analysis system where the colour or distribution of particles is recorded and analysed by a data-analysis system. Lastly, the plates are ejected from the instrument. The "Titertek Twinreader", sold before the priority date of the present invention, is a programmable analysis device in which vibration of samples is available as an option. A single photodetector is moved as required to analyse each sample site. The present invention seeks to increase the overall efficiency of a kinetic analysis system and is characterised by repeatedly and simultaneously imparting mixing movement to the samples via the assay plate before each of a series of readings. This motion ensures uniform distribution of the color products resulting from chemical reactions and promotes homogeneity of the reacting agents, leading to more accurate transmittance readings and increasing the overall system efficiency. In addition, agitation increases the pH equilibration rate of an enzyme with its substrate solution. In an ELISA assay, for instance, substrate solution is added after the immobilized enzyme is washed with a washing solution (usually pH 7). If the pH of the substrate solution is significantly different from the washing solution (e.g., the commonly used substrate solution of alkaline phosphatase is pH 9 or 10), a lag phase of color development occurs due to the slow pH equilibration of the enzyme. Automatic agitation of reagent speeds up the pH equilibration and significantly reduces the lag phase. Thus, the present invention allows the construction of an automatic photometric device for multi-site kinetic and/or end-point densitometric assays, which is capable of accurately acquiring a large number of measurements in a relatively short period of time. It also allows the construction of a photometric measurement device which uses a single relatively low power light source leading to reduced ambient operating temperature and extended life. Such embodiments of the invention can give convenient control over the accuracy with which the sensed signals for this measurement system are processed. Embodiments of the present invention do not require relative mechanical movement of the microplate and the optical components of the system in order to perform successive multi-site assays. An important aim of this invention is to provide a photometric device which, by virtue of its ability to perform kinetic analysis, is able to make quantitative measurements of enzyme activity in filter-bottom microplates. Another important aim of this invention is to provide a photometric device with significantly enhanced color distribution in the assay of chromogenic reactions and which is not subject to measurement errors arising from nonuniform distribution of the colored product in such chemical reactions. It goes without saying that, through good design, an embodiment of the present invention will be efficient, economical and convenient to use. The present invention provides a device as set out in claim 1. The present invention also provides a method as set out in claim 19. Thus, the present invention provides positive direction of light to each of a plurality of sample cells, and does not require movement between the cells and the optical apparatus. The advantage of this in terms of accuracy and speed will be readily appreciated. Other aims, characteristics and advantages of the present invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified schematic representation of a photometric reading device embodying this invention; FIG. 2 is an illustration of the fiber distributor showing the various designated positions utilized to couple the light source to each of the plurality of optic fibers; FIG. 3 is an illustration of the disposition of the various optical fibers within a fiber manifold associated with the sample assay sites; FIG. 4 is a simplified block diagram of the processing circuitry; FIG. 5 is a flow chart representing the general sequence of operations involved in a typical sequential scan of the sample plate according to the illustrative measurement system; FIG. 6 is flow diagram of the operating sequence undergone by the system as part of the air calibration phase; FIG. 7 is a flow chart representing the sequence of operations involved in performing the read phase on a sample tray. FIG. 8 is a perspective view of a preferred embodiment of a multibeam photometer according to the present invention; FIG. 9 is a side elevation cutaway view of a microplate well inserted between the LED array and photodetector of the photometer of FIG. 8; FIG. 10 is a block diagram of the electronic control circuitry for the photometer of FIG. 8; FIGS. 11A and 11B are circuit diagrams of a portion of the control circuitry of FIG. 10; FIG. 12 is a timing diagram for portions of the circuit of FIGS. 11A and 11B; and FIG. 13 is a flow chart of a computer program for controlling the control circuit of FIG. 10. DESCRIPTION OF THE PREFERRED EMBODIMENT While the invention will be described in connection with certain preferred embodiments, it will be understood that it is not intended to limit the invention to these particular embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalent arrangements as may be included within the spirit and scope of the invention as defined by the appended claims. Referring now to the drawings, FIG. 1 is a simplified schematic representation of an exemplary kinetic measurement system 10 embodying the present invention. The system 10 includes a light assembly 12 capable of providing a finely focused reflected beam of light for coupling to the test or assay sites. The light assembly 12 comprises a single source of uniform, reflected and focused light 14 which, in the preferred embodiment, includes a tungsten-quartz bulb emitting light in a wavelength range of about 300 to 900 nanometers. The output of the light source 14 is passed through a chopper 16 which modulates the light at a fixed frequency which in the preferred embodiment is 800 hertz. The modulated light from the chopper 16 is collimated through a lens 18 onto a filter wheel 19 which extracts light of a selected wavelength and bandwidth which corresponds to the wavelength at which the test samples exhibit light absorbency. The filtered light is passed through a lens 20 which receives the collimated light and focuses it onto a precise point 22 from which the light can be coupled to selected fibers which conduct the light to the plurality of assay sites. The kinetic measurement system 10 utilizes a highly efficient and structurally simple light coupling and transmitting mechanism. As shown in FIG. 1, the collimated light exiting the light assembly 12 is directed to a light coupling arrangement which includes a cylindrical rotor 24 which is capable of being rotatably positioned accurately about its axis. The rotor 24 includes an optical fiber 26 having an input end 28 located at the center of the rotor 24 and coincident with the focal point 22 of the light output from the light assembly 12. The output end 30 of the fiber 26 is located near the periphery of the fiber rotor 24 so that as the rotor rotates the input end 28 of the fiber remains stationary with respect to the light assembly 12 while the output end 30 moves around a circular path. The rotor 24 thus permits efficient coupling of light from the light assembly 12 into selected ones of a plurality of light transmitting means which carry the light to corresponding assay sites. The light output from the fiber 26 in the rotor 24 is received by a fiber optic distributor 32 containing a multiplicity of optical fibers 34 having their input ends arranged in a circular array. This circular array of fiber ends has the same radius as the location of the output end of the fiber 26 in the rotor 24 so that as the rotor 24 is indexed about its axis, the output end of the fiber 26 is brought into alignment with successive fibers 34 in the distributor 32. On the output side of the distributor 32, the fibers 34 are brought together to form a fiber bundle 36 which leads to a fiber manifold 38 which aligns each of the fibers 34 with one of the assay sites (to be described below). The rotor 24 is indexed by means of a suitable stepper drive or other appropriate controllable displacement means so that it sequentially directs the collimated light from the focal point 22 into different fibers 34 of the bundle 36. The use of the optical fiber coupling arrangement to couple light from the light assembly 12 to individual optical fibers 34 within the distributor 32 constitutes a significant advantage because of the efficiency of transmission associated with the mechanism. More specifically, the optical fiber 26 ensures that substantially all the collimated light from the light source 14 is coupled into a selected optical fiber 34 without any significant diffusion at the point of coupling at either end of the coupling fiber 26. Thus, light from the source 14 is coupled through the rotor 24 only into a selected one of the optical fibers 34 with minimal undesired coupling of light into optical fibers adjacent to the selected fiber. This highly efficient coupling allows the use of a single light source with relatively low power capacity since a high percentage of light emanating from the source is transmitted to the site of measurement. The illustrated light source in FIG. 1 is a convenient and preferred form; however any light source or a plurality of light sources providing light of equal intensity and having the desired range of wavelength can be used. In this regard, an alternative form of photometric measurement, which is based upon a sequentially operated multibeam light source, will be described in detail below in connection with FIGS. 8-13. At the site of measurement, light emanating from any of the optical fibers 34 located within the fiber manifold 38 is directed to a corresponding assay site on a sample plate 40 through an array of lenses 42 disposed between the manifold 38 and the sample plate 40. The sample plate 40 may be a conventional microplate having a series of wells, usually an array of 96 wells arranged in 8 rows having 12 wells each. The sample plate is preferably mounted in an area where the ambient temperature is regulated, e.g., by means of a fan, so as to present isothermal conditions about the sample plate. This is important in minimizing inaccuracies from varying rates of reaction occurring at different sample sites as a result of a temperature gradient about the sample plate. An array of photodetectors 44 is provided on a detector board 46 in the form of a matrix conforming to the positions of the wells located on the sample plate 40. A second lens array 48 is positioned beneath the sample tray 40 and serves as means for focusing light from each well of the sample plate 40, after the light has passed through the well. The lens arrays 42 and 48 are of the conventional type and include apertures (not shown) which function to direct the light towards the individual lenses and minimize diffusion of light into adjoining lenses. The sample plate 40, or at least the bottom of each well in the sample plate, is translucent or transparent so that light coupled to a particular optical fiber within the fiber manifold 38 and then collimated onto a corresponding sample well penetrates the well and its contents, passes through the corresponding focusing lens disposed within the lens array 48, and reaches the corresponding photocell 44 located immediately below the sample well. The photodetector 44 senses the intensity of the light passing through the corresponding sample well and produces an electrical output signal proportional to the intensity of light incident on its surface. Each of the photodetectors 44 provided on the detector board 46 functions in a similar manner and provides a signal which is proportional to the varying intensity of the light impinging thereon. This varying intensity is caused by the varying transmittance offered by the sample as a chemical reaction progresses therein and alters its constituency. The resulting electrical signals from the photodetectors 44 are fed to an analysis and indication system 50 which processes the received signals and provides an external indication of the transmittance or optical density of each of the samples contained within the multiple wells of the sample plate 40. The measurement system of Fig. 1 also includes a single reference optical fiber 52 positioned adjacent to the focal point 22 so that the fiber 52 receives light continuously whenever the light source 14 is energized, i.e., whenever any of the sample sites is being tested. The light emanating from the output end of the reference fiber 52 is coupled through air or an empty sample well to a separate photodetector 45 on the detector board 46, and provides a reference photodetector signal whose significance will be described in detail below. The optic fiber distributor 32 also has an optical fiber designated as the "home" fiber which serves as a reference for determining the current position of the distributor 32 relative to the coupling rotor 24. The function of the home fiber will also be described in more detail below. In order to promote uniform distribution of the color resulting from chromogenic reactions in the samples being measured, the measurement system of FIG. 1 is provided with means for agitating the chemical solutions contained within the plurality of wells of the sample plate 40. More specifically, as shown in FIG. 1, the sample plate 40 is attached to an agitating mechanism 54 which oscillates the tray in a manner that brings about a thorough mixing of the chemical solutions contained within the sample wells. A common problem with conventional microplate readers, if used for kinetic analysis of ELISA, is that distribution of the color produced as a chromogenic reaction progresses may be uneven across the width of a sample well. Since kinetic optical density measurements generally involve the transmission of light through the central portion of a sample well, uneven distribution of color (e.g., due to color development on the walls or corners of a sample well) can severely distort the accuracy of the transmittance readings. Non-homogeneous distribution of color in the samples can also produce significant variations in the measured values from one cell to another, even if similar chemical reactions are occurring in the cells, thereby making the transmittance measurements unreliable. Although certain previous measuring techniques have suggested manual shaking of the microplate prior to reading, random diffusion of the color as the reaction progresses can introduce unpredictable, non-reproducible results into the otherwise generally linear variation of the kinetically measured optical density with respect to time; that is, the transmittance of the sample becomes dependent on the progressively non-homogeneous color distribution in the sample. This non-reproducibility cannot be overcome by shaking the microplate at the beginning of a kinetic reading cycle because, unlike conventional end-point measurement where the reaction is quenched prior to the measurement phase, kinetic reading involves measurement of optical densities while the reaction progresses unquenched. Because the color development continues as the reaction progresses, initial shaking, even though it provides a certain degree of linearity, is insufficient to produce a uniform distribution of color throughout the reacting solution and causes low precision measurements. It has been found that agitation of the reaction agents immediately prior to each of the multiple readings taken within a kinetic reading cycle, in accordance with the present invention, significantly improves the measurement accuracy by ensuring a homogeneous distribution of the color product throughout the sample as the chemical reaction progresses. In the particular example illustrated in FIG. 1, such homogeneous color distribution is achieved by oscillating the entire microplate before each reading is taken. This oscillatory movement agitates the reacting agents within each of the sample wells so as to prevent any localized separation of the colored products of the enzyme reactions. In this manner variation in the transmittance of the samples within the sample wells is maintained substantially linear with respect to time over an entire kinetic reading cycle. In the particular embodiment shown in FIG. 1, the sample plate 40 is connected to an agitation mechanism 54 which comprises a motorized arrangement capable of imparting gentle oscillatory motion to the sample plate at a desired frequency of oscillation. According to a preferred embodiment, the agitation mechanism 54 oscillates the sample plate with the same reversible drive motor generally used to control the position of the sample plate, by energizing the motor repeatedly in opposite directions at the desired rate of oscillation. Satisfactory results are obtained by oscillating the sample plate over a distance of about 1/16th of an inch at a frequency of about 20 Hz. The measurement system is programmed to oscillate the sample plate before each reading. The oscillation is followed by a short time delay before the start of the reading phase to allow the reacting solutions within the sample wells to settle. The delay interval prevents erroneous readings due to reflection and/or refraction effects arising from ripples caused by the agitation process. Typical time periods are 3 seconds for the agitation phase and 1 second for the delay interval. The agitation need not be effected by mechanical means, as long as the objective of achieving homogenous color distribution effect is realized. Other suitable agitation means, such as "ultrasonic" vibration inducement, may be used, depending upon the desired efficiency and the constituency of the reacting agents. Further, the oscillatory movement of the sample plate need not be limited to lateral or rotational movements parallel to the plane of the sample plate; vertical oscillation (up-and-down movement) can also be used satisfactorily as long as spilling of test samples can be avoided. Referring next to FIG.2, the fiber manifold 38 is shown in more detail, including the disposition of the various optical fibers across the lower surface of the manifold. It will be noted that this particular embodiment of this invention is designed for the sequential assaying of samples contained in a conventional microplate which has 96 test sites or wells arranged in 12 x 8 matrix. Accordingly, as shown in FIG. 2, the fiber manifold 38 has 96 fibers arranged in 8 rows A, B, C, D, E, F, G and H, each row consisting of 12 fibers. For instance, the row A has 12 fiber ends A₁, A₂, A₃ . . . A₁₂. Also located in the manifold 38 are the output ends of the reference fiber 52 and the "home" fiber. The arrangement of the 12 rows A-H and the separation between adjacent fibers corresponds exactly to the arrangement of the sample test sites in the sample plate used for the assay. Thus, light transmitted through a particular fiber is collimated directly onto the corresponding sample well and passes through to the corresponding photodetector, thereby generating an electrical signal having a magnitude proportional to the intensity of the light impinging on the photodetector. FIG. 3 is a schematic representation of the fiber distributor arrangement showing the circular array of the input ends of the various fibers located in the fiber manifold 38. As shown, the distributor 32 carries the input ends of each of the 98 optical fibers disposed within the fiber manifold. These 98 fibers are arranged in a circular array around the distributor 32. The input end of the "home" fiber is located in a position preceding the first row A of optical fibers. Three opaque spots are located between the "home" fiber and the first fiber A₁ which corresponds to the first sample site to be assayed; these opaque spots serve to reference the start of a new assay sequence, as will be described below. Referring now to FIG. 4 there is shown a block diagram of the processing circuitry used for the analysis of the signals generated by the photodetectors 44. In the illustrative embodiment, the detector board 46 actually has 98 photodetectors 44, 96 of which correspond to the 96 sample sites of the sample plate 40 (FIG. 1), one of which receives light focused upon it from the reference fiber 52, and one of which receives light from the "home" reference fiber. It will be apparent that only a single reference photodetector is required if a single fiber is used to perform both the light and home reference functions. The signals generated by the 96 photodetectors for the sample sites are converted to a corresponding voltage form by a current-to-voltage converter 110. These signals are then passed through a band limiting low pass filter 111 and subsequently to a multiplexer 112 which functions to controllably select desired ones of the 96 signals for further analysis according to a pre-programmed sequence. The signals generated in response to light from the light reference fiber 52 and the "home" fiber are converted into their equivalent voltage form by current-to-voltage conversion units 113 and 114 and then passed through band limiting low pass filters 115 and 116, respectively. The outputs of the multiplexer 112 and the two low pass filters 115 and 116 are fed to a second multiplexer 118 which functions to controllably select one of the three signals at its input for further analysis. The "home" reference fiber provides a convenient means for locating the angular position of the fiber 26 in the rotor 24 relative to the fiber distributor 32. More specifically, the "home" reference fiber serves as a means for ensuring that the rotor 24 is positioned correctly so that the light output from the coupling fiber 26 is directed to the proper optical fiber in the distributor 32 to begin a sequential assaying process. Since any light entering the "home" reference fiber is transmitted directly to a dedicated "home" photodetector 44, the existence of a peak signal at the output of the "home" photodetector is an indication that the rotor 24 is positioned at a so-called "home" position where the coupling fiber 26 is aligned with the "home" reference fiber. Any time a new assay is to be started, the measurement system first positions the rotor 24 at its "home" position. In order to accomplish this, the rotor 24 is rotated while monitoring the output of the "home" photodetector, until a peak output signal from that photodetector is detected. Since the position of the "home" reference fiber relative to the first optical fiber A₁ is a known factor (in this case, separated by the three opaque spots), subsequent alignment of the coupling fiber 26 with the fiber A₁ can be conveniently accomplished by stepping the rotor 24 through three consecutive positions. The provision of the light reference fiber 52 serves as a calibration means by which only the differences in sensed light intensity from one reading to another may be utilized for analysis of sample transmittance, thereby eliminating the effects of variations in the light output of the light source, system drifts and the like. More specifically, each time an assay is performed, the signal produced by the light reference fiber is measured before the start of the reading cycle. Then all subsequent signals produced by the photodetectors for the test sites are scaled relative to the signal initially produced by the light reference fiber. In this way, only actual changes in the transmittance of the liquid samples as a result of the chemical reactions occurring at the sample site are measured. Any localized difference in intensity such as those resulting from fluctuations in light intensity from the light source over the course of its lifetime, or premature readings taken before the light source or other system parameters have had a chance to stabilize, are disregarded. Since the "home" reference fiber is coupled directly to the corresponding photodetector, it can also be conveniently used as a reference light reading source and can effectively provide all functions performed by the light reference fiber as described earlier, in addition to providing a reference for determining the position of the fiber rotor 24 relative to the fiber distributor 32. Returning now to FIG. 4, the signal selected by the multiplexer 118 is passed through a conventional bandpass filter 119 with a reasonably large bandwidth centered about the frequency at which the chopper 16 modulates the light passing through it (e.g., 800 hertz). The bandwidth of the bandpass filter 119 is selected to be sufficiently large to accommodate the fluctuation in signal settling time that occurs as a result of rapid readings being performed on the sample plate. The band-limited output of the filter 119 is then fed to a variable gain amplifier 120 whose function will be described below. The output of the amplifier 120 is fed to a two-way switch 122 which in its normally closed position provides a direct electrical connection from the amplifier 120 to the subsequent portion of the processing circuitry. When the two-way switch 122 is open, the preceding portion of the measurement system is cut off and the switch 122 serves as a virtual short circuit for measuring the signal generated only by the rest of the processing circuitry. Thus, the switch 122 serves as a means for determining the "dark current" flowing through the processing circuitry in the absence of any light passing through the fiber distributor 32. This dark current measurement is free from distortions and noise from sources preceding the two-way switch 122, because the section of the processing circuitry in which the current is measured is totally isolated from the rest of the measurement circuitry. The significance of taking such readings while the circuit is isolated from the light source will be described below. From the two-way switch 122, the output signal is passed through a high pass filter 124 which in combination with the low pass filters 111, 115 and 116 provides secondary bandpass characteristics to the processing circuitry. The output of the high pass filter 124 is fed through a phase detector 126 which in conjunction with a reference input from the light source chopper 16 and a low pass filter 128 functions to extract a direct current signal corresponding to the alternating current resulting from the modulating effect of the chopper 16. More specifically, the phase detector 126 effectively inverts the negative portions of the alternating output signals from the detector board 46 on the basis of the timing input provided from the chopper 16. The average value of the resulting signal represents the DC equivalent of the alternating signal. To extract this average value, the output of the phase detector 126 is fed through a low pass filter 128. The output signal from the low pass filter 128 is fed to a second variable gain amplifier 130 which is a high precision amplifier providing a series of well defined gain settings. The operation of the variable gain amplifier 130 in conjunction with the first variable gain amplifier 120 in providing controllable accuracy and increased dynamic range for the processing circuitry of this invention will be described in detail below. The output of the amplifier 130 is passed through a low pass filter 132 and then on to an analog-to-digital converter 134 which functions to translate the amplified analog signal into its corresponding digital form. The digital signals produced by the analog-to-digital converter 134 are then fed to a conventional digital microprocessor system which performs a series of mathematical calculations and comparisons required to determine the optical density of the samples on the basis of a predefined algorithm. The same digital microprocessor system is also used to regulate the sequential scanning of the plurality of test sites, the different multiplexing arrangements, and all related processing circuit functions. The measurement system can be made more efficient and economical by effectively augmenting the dynamic range of the analog-to-digital converter used to represent the analog signals defining the processed values of the various signals produced by the photodetectors. The variable gain amplifiers 120 and 130 function to adjust the extent to which the processed signals are amplified in such a way that even when the signals vary over a wide range of amplitudes, a major portion of the quantization levels of the analog-to-digital converter is util-ized without exceeding its original dynamic range. The actual operation of the amplifiers will be clarified by considering the case where the ADC 134 in FIG. 1 having a 12-bit capacity so that the total number of quantized output levels is 4096, ranging from 0-4095. As the wavelength of light extracted (by the filter wheel 18) from the light emanating from the light source changes, the intensity of the resulting light also varies. This, in combination with variations in light transmission characteristics from one optical fiber to another and the frequency response characteristics of the photodetectors themselves, as well as changes in transmittance due to chemical changes in the samples, can produce signals whose amplitudes vary over a significant range. In order to ensure that the signal with the largest possible amplitude does not result in an output value that exceeds the range of the ADC, it is conventional to use a low gain amplifier. However, the disadvantage associated with such an arrangement is that the low gain amplifier produces a low level digital output in cases where the intensity of light received at the photodetectors produces a relatively weak sensed signal, thereby utilizing the dynamic range of the ADC very inefficiently. Conventional recourse in such situations has been to upgrade the system by using an ADC with a higher bit-processing capacity, which significantly adds to the overall system cost. The variable gain amplifiers 120 and 130 initially process the incoming signal at very low gain settings. The resulting digital output is then compared with the maximum possible value of the ADC output in order to determine the maximum possible gain to which the incoming signal may be subjected without exceeding the maximum digital output value of the ADC. For instance, if an input signal produces an ADC output of about 50 counts at the initial gain setting (which are normally 1), the system compares this count value to the maximum count value possible with the ADC which, in the case of the 12-bit ADC, is 4095 and determines the extent to which the signal may be safely amplified so that the digital output falls within the maximum count value. A safety margin of about 10% is built into this dynamic ranging process by performing the above comparison not on the basis of the maximum output value of the ADC but instead by comparison with a value that is roughly 90% of the maximum output. For the 12-bit ADC, the actual count of 50 would hence be divided into a safety-adjusted maximum output of 3686 counts to give a desired amplification factor of about 74. Subsequent to this determination, the gain settings of the variable gain amplifiers are adjusted to closely approximate the desired gain factor. The above arrangement thus allows maximum utilization of the dynamic range of the measurement system, regardless of variations in the relative strengths of the signals generated by the photodetectors. The gain G1 of the first variable gain amplifier 120 is adjusted on the basis of the signal derived from light being transmitted through air, whereas the gain G2 of second variable gain amplifier 130 is adjusted on the basis of the signals derived from the sample site photodetectors. The actual sequence of operations involved in the adjustment of gain settings for the variable gain amplifiers as well as the overall method of operation of the measurement system will now be described below with reference to FIG. 5. FIG. 5 is a flow diagram of the general sequence of operations involved in a typical sequential scan. The sequence begins at step 150 where the various system variables, such as the number of readings to be made within the kinetic reading cycle, duration of agitation, the duration of the delay following the agitation phase, etc., are initialized. At step 152, the air calibration phase is carried out by performing a reading upon each of the sample wells through air. Measurements made during the air calibration phase are performed with a unity gain setting G2 for the second variable gain amplifier 130 and the gain G1 of the first variable gain amplifier 120 is optimized for the maximum dynamic range of the analog-to-digital converter. The various steps and measurements undergone as part of the air calibration phase 152 will be described in detail below. The air calibration phase 152 is followed by step 154 where a kinetic reading cycle is initiated upon the basis of the initialization data provided to the measurement system as part of step 150. The kinetic reading cycle includes the execution of the agitating, delay and reading steps at each of a series of pre-programmed discrete time intervals at which optical density readings are to be taken for the particular samples being measured. It will be apparent that, in the case of end-point analysis, the reading cycle will comprise the execution of the above steps only at a single time interval. Step 156 is the read phase, which includes a series of three steps beginning with the agitation phase 158 during which the sample plate is vibrated for a pre-defined time interval. Subsequently, at step 160, the settling phase takes place during a pre-defined delay interval in which the oscillation mechanism is dormant and the reaction agents within all the sample wells of the sample are allowed to settle down for a pre-defined time interval before obtaining the actual signal readings. At step 162 the measurement system obtains the transmittance reading on all the wells of the sample plate. This step includes optimization of the gain setting G2 for the second variable gain amplifier 130 while maintaining the gain setting G1 of the first variable gain amplifier 120 at the optimized value determined during air reference. The sequence of events involved in the read step 162 will be described in detail below. Following the read step 162, a check is made at step 164 to determine whether the system has completed the pre-defined kinetic reading cycle. If the answer at step 164 is no, step 166 continues the kinetic reading cycle. The read phase 156 is reiterated by the measurement system until the agitation phase and the accompanying delay and read sequences have been performed at each of the prescribed intervals of the kinetic reading cycle. If it is found at step 164 that readings have indeed been performed at all prescribed time intervals, the measurement system comes to a stop. This marks the end of the kinetic reading cycle. The reading cycle has been described above only with respect to the sequence of operations undergone by the illustrative photometric measurement divide in obtaining the various light readings required to calculate the optical density at the sample sites. It will be understood that the microprocessor system which forms part of the analysis and indication system (FIG. 1) processes the data resulting from the measurements as the reading cycle proceeds and initiates computation of optical density values (on the basis of a pre-defined algorithm as will be explained below) for those sample sites and time intervals for which required measurements have been completed. The following definitions and symbols will be used in the ensuing description of the various operations carried out by the measurement system of this invention during the air calibration and read phases. OD n : The calculated optical density of a given sample well (where n varies from 1 through 96 in order to designate each of the 12 sample wells positioned along each of the 8 rows A through H of the microplate). W n : The signal output of a photodetector corresponding to a given sample well containing the reacting sample. G1: The adjustable gain of the first stage variable gain amplifier 120 (controllable by a set of gain multiplication factors including 1, 2, 4, 8, 16, 32, 64 and 128). G2: The adjustable gain factor for the second stage variable gain amplifier 130 (adjustable by a set of gain multiplication factors including 1, 10 and 100). D n : The dark current reading taken with the two-way switch 122 of FIG. 4 in its open position, for a given sample well. This reading is taken with the first stage gain setting G1 set to 1 and at the same second stage gain setting G2 used to obtain the corresponding W n signal output. W.AIR n : The signal reading of a given sample well's air calibration taken with the secondary stage variable gain amplifier at a gain setting G2 = 1. D air : The dark current reading of an air calibration performed with the secondary stage gain setting G2 = 1. L.REF air : The light reference signal reading taken during an air calibration with the secondary stage gain setting G2 = 1. L.REF read : The light reference signal taken at the initiation of a READ cycle with the secondary stage variable gain setting G2 =1. D read : The dark current readings taken at the beginning of a READ cycle. Referring now to FIG. 6, there is shown a flow chart of the sequence of operations included within the air calibration phase of FIG. 5. At the first step 200, the measurement system positions the rotor 24 at the reference "home" position by sequentially displacing the rotor until the existence of a peak signal is detected at the output of the "home" reference photodetector. At this stage, the gain settings for both the first and second stage variable gain amplifiers 120 and 130 are set to unity. The next step in the air calibration sequence is step 202 where the measurement system switches to either the photodetectors for the light reference fiber or the photodetector for the home reference fiber in order to measure the light reference signal L.REF air . This reading is measured with the gain G2 of the secondary stage variable gain amplifier set to unity and the gain G1 of the first stage variable gain amplifier optimized to provide the largest count value at the output of the analog-to-digital converter 134 (according to the safety-adjusted dynamic ranging procedure described above). Also at step 202, the measured reference signal value L.REF air and the optimized first stage gain setting G1 L.air are stored in the memory of the microprocessor system for later use in the optical density calculations. At the succeeding step 204, the rotor is displaced through a designated number of positions relative to the "home" position so as to locate the rotor at a position corresponding to one of the three opaque spots X₁, X₂ and X₃ on the fiber distributor 32. According to the preferred embodiment, the rotor is actually displaced by two positions relative to its home position so that the coupler comes to rest at a position corresponding to the opaque spot X₂. In this position, the opaque spot effectively blocks the coupling of any light from the coupling fiber 26 into any of the fibers in within the fiber manifold 32 and hence isolates the light source from the photodetectors. At the succeeding step 206, the dual position dark current switch 122 activated and a dark current reading D air is taken with both the first and second stage gains G1 and G2 set to unity. The dark current reading D air represents the residual current flowing within the portion of the processing circuitry of FIG. 4 following the two-way switch 122. This value is subtracted from the signal reading of every sample well in order to provide a true representation of the transmittance value for the sample well at any designated time. Also at step 206, the measured dark current reading D air is stored in the system memory for later use in calculating the optical density. At this stage, the measurement system is ready to perform air calibration readings on each of the sample wells. Accordingly, at step 208, the rotor 24 is advanced to the position A₁ corresponding to the first sample well of the microliter plate, and the photodetector corresponding to the sample well A₁ is switched on. At the succeeding step 210, the air calibration signal reading W.AIR n for the sample well A₁ is taken with the gain G2 of the secondary stage variable gain amplifier set to unity and the gain G1 of the first stage variable gain amplifier optimized to a value G1 A1 ; the latter value represents the gain setting which allows the maximum safety-adjusted output from the analog-to-digital converter without exceeding its rated dynamic range. At the end of step 210, the measured signal reading W.AIR n (in this case n = A₁) and the optimized gain setting G1 A1 are stored in the memory. At step 212, the microprocessor system checks to determine whether air calibration has been performed at each of the 96 sample well sites on the sample microplate. If the answer at step 212 is negative, step 214 advances the rotor to a position corresponding to the succeeding sample well site before reverting to the air calibration step 210. If the answer at step 212 is positive, i.e., air calibration has indeed been performed on all 96 sample well sites, it marks the end of the air calibration sequence at step 216. It will be noted that the entire air calibration sequence is performed with the sample plate in its retracted position, i.e., away from the photodetector board so that light from the fiber manifold 38 is transmitted directly to the photodetectors. Referring now to FIG. 7, there is shown a flow chart of the sequence of operations undergone by the measurement system while performing the read phase. It should be noted that before actual reading is performed on a a sample plate, the measurement system proceeds through the air calibration phase with the sample plate in its retracted position. Prior to the start of a read phase, the sample plate is moved into its advanced position in preparation for the reading phase. At the start of a reading operation (step 300 in FIG. 7), the measurement system locates the rotor at the "home" position. For this purpose, the signal from the "home" reference fiber photodetector is tracked by the processing circuitry with both the first and second stage variable gain amplifiers having their gains G1 and G2 set to unity. Step 302 is then accessed, where a measurement of a light reference signal occurs. More specifically, the measurement system switches to the photodetector corresponding to the light reference fiber (or the photodetector corresponding to the home reference fiber if this fiber is being used to perform the functions of the light reference fiber), and a light reference signal L.REF read is taken with G2 set to unity and G1 optimized to its maximum value G1 L.air according to the dynamic ranging procedure described above. Also as part of step 302, the measured L.REF read value is stored in the system memory for later use during calculations of optical density. At the succeeding step 304, the rotor 24 is displaced through a designated number of positions to locate it at one of the three opaque spots X₁, X₂ and X₃ provided on the fiber distributor 32. More specifically, the rotor is stepped two positions relative to the home reference fiber so as to be located at the second opaque spot X₂. The following step 306 actuates the dual position switch 122 and measures a series of dark current readings D air.X with the first stage variable gain G1 set to unity. A single reading is taken at each of the possible gain settings G2 (in this case 1, 10 and 100) of the secondary stage variable gain amplifier. The measured values of D air.X are also stored within the system memory as part of step 306. At the succeeding step 308, the rotor is advanced to the first sample well position A₁. In addition, the system switches to the photodetector corresponding to the first sample well A₁ to begin the actual sequential reading cycle. The succeeding step 310 activates the agitation mechanism for a pre-designated time interval T₁ to promote homogeneous color distribution as described above. The agitation phase of step 310 is succeeded by a settling phase at step 312 during which the agitation mechanism is deactivated and the system dwells for a time interval T₂ to allow the agitated samples to settle down in preparation for performing transmittance readings upon them. The agitation phase at step 310 may entail displacement of the sample from its position between the fiber manifold and the photodetector board to permit oscillatory movement of the plate. Accordingly, the settling phase may actually take place during the time it takes to reposition the plate from its agitation position to its reading position. Immediately after agitation and the subsequent settling of the reacting samples within the sample plate, optical density readings are taken. At step 314, the signal W An for the first sample well, i.e., A₁, is measured with G1 set to the corresponding stored gain value G1 A1 determined as part of the air calibration (step 210 in FIG. 6). During this measurement, G2 is initially set to unity and then optimized to a value that produces the maximum safety-adjusted output value from the analog-to-digital converter of FIG. 2. Also, as part of step 314, the measured signal value W A1 is stored within the system memory for use in calculation of the optical density for that sample well. At the succeeding step 316, the microprocessor system performs a check to determine whether signal readings have been obtained for all 96 sample wells. If the answer at step 316 is negative, the microprocessor system advances the rotor to a position corresponding to the next sample well. At the same time, the processing circuitry switches to monitor the photodetector corresponding to the selected sample well. The system then reverts to step 310 and goes through the agitation, settling and read steps 310, 312 and 314 again. These three steps are reiterated until the check at step 316 produces a positive answer, indicating that signal readings have been obtained from all sample wells. This marks the end the reading cycle at step 322. It will be noted from the above description that for any given sample well designated by the letter n, the gain G1 n for the first stage variable gain amplifier 120 is individually determined for each sample well during the air calibration phase, and then is maintained constant for all subsequent readings in that kinetic reading cycle. The value of G1 is not adjusted again until the next air calibration phase. The gain G2 n for the second stage variable gain amplifier is set equal to unity for all air reference and light reference readings, so that the dynamic range of the second stage amplifier 130 is utilized only when actual transmittance readings are being taken, and not for air reference readings. Once the optical parameters defined and described above with respect to FIGS. 6 and 7 have been obtained by the measurement system for a given sample well, the calculation of the optical density OD n of the reacting sample contained within that sample well is calculated as: OD n =LOG 10 [(W.AIR n -D air )/(W n -D n ) * (L.REF read -D read )/(L.REF air -D air ) * (G2 n ) This equation represents the logarithmic value of the product of three separate quantities. The first quantity (W.AIR n -D air )/(W n -D n ) is the ratio of the adjusted signal readings for a given sample well (1) without any sample and (2) with a sample. All readings measured by the processing circuitry of the microprocessor system are adjusted for any offset voltages generated by the analog-to-digital converter or other system offsets and drifts by taking into account the corresponding dark current readings, as indicated in the above equation for OD n . For instance, the signal reading W.AIR n is normalized for dark current effect by subtracting from it the value D air of the corresponding dark current reading. Similarly, this signal reading W n is adjusted by subtracting from it the corresponding dark current reading D n . The second quantity (L.REF read -D read )/(L.REF air -D air ) is a measure of the ratio of the light reference readings obtained for a given sample well during the air calibration phase and the read phase. These two readings are also normalized on the basis of the corresponding dark current readings. Finally, the third quantity (G2 n ) in the above equation accounts for the effect of the dynamic ranging procedure described above, i.e., this quantity neutralizes the effects of amplification of the signal readings by the processing circuitry. The application of the above equation to the parameters measured as part of the kinetic reading cycle results in a highly accurate optical density measurement because the equation takes into account the effects of system offset voltages as well as those resulting from localized differences in light intensity and measurement conditions from one kinetic reading cycle to another, or from one sample well to another. The computation of the logarithms required to calculate the optical density readings is performed by storing all the required logarithmic values within the microprocessor system in the form of a look-up table and subsequently using the digitized output of the processing circuitry as an index to retrieve the appropriate logarithmic value. In previous systems, the output signals of the photodetectors of the detector board have been fed to a logarithmic amplifier to obtain the logarithmic values of the output signals. This technique is subject to a variety of problems and limitations because of the constant need to adjust the system for offset gains of the logarithmic amplifiers. In addition, any temperature drift in the computing hardware must be accurately tracked and appropriately compensated to retain the accuracy of computation. The computation of the logarithmic values is made substantially more accurate and independent of the system hardware parameters by storing within the microprocessor system memory all possible logarithmic values that would be required by the system in order to compute the optical density readings. More specifically, the look-up table contains logarithmic values corresponding to each of the possible output quantization levels for the analog-to-digital converter of the system (as shown in FIG. 4). Considering, for example, the case of a 12-bit analog-to-digital converter, the possible quantization levels range from 0 to 4095. This means that there are 4096 different values that a signal may take after it has been detected, processed and digitized. The logarithmic value corresponding to each of these 4096 possible values are stored within a logarithmic look-up table which is contained within the ROM portion of the microprocessor system. The logarithmic look-up table is defined in such a way that the digitized output from the analog-to-digital converter serves as an address or index that points to the corresponding logarithmic value stored within the look-up table. Because the look-up table is available within the microprocessor system memory, computation of the optical density becomes a simple matter of using the digitized output to extract the corresponding logarithmic value stored within the look-up table, and then performing simple mathematical subtractions. The process of selecting a particular photodetector from the multiplicity of detectors on the detector board is simplified by use of a tiered parallel addressing scheme for the photodetectors. More specifically, the photodetectors are divided into a selected number of blocks, each block consisting of a plurality of photodetectors. Corresponding photodetectors in the various blocks are connected in a parallel fashion in such a way that if, for instance, the first photodetector in the first block is addressed, the measurement system also automatically addresses the first photodetectors in the remaining blocks. Considering, for example, the case of a 96-well microplate, the detector board comprises 97 photodetectors (96 for the 96 sample wells and 1 for the "home" and light reference fiber). According to a preferred embodiment of this invention, the 96 sample-well photodetectors are divided into blocks consisting of 16 photodetectors each. The first block consists of the photodetectors A₁-A₄, B₁-B₄, C₁-C₄ and D₁-D₄, the second block consists of the photodetectors A₅-A₈, B₅-B₈, C₅-C₈ and D₅-D₈, the third block consists of the photodetectors A₉-A₁₂, B₉-B₁₂, C₉-C₁₂ and D₉-D₁₂, fourth block consists of the photodetectors E₁-E₄, F₁-F₄, G₁-G₄ and H₁-H₄, the fifth block consists of the photodetectors E₅-E₈, F₅-F₈, G₅-G₈ and H₅-H₈ and the sixth block consists of the photodetectors E₉-E₁₂, F₉-F₁₂, G₉-G₁₂ and H₉-H₁₂. With this block configuration, the first photodetectors in all of the blocks, that is A₁, A₅, A₉, E₁, E₅ and E₉, are connected in parallel, and the rest of the photodetectors are connected in a similar fashion. The addressing of individual photodetectors is considerably simplified since the same address is applicable to the selection of any one of the six photodetectors. Even though six photodetectors are turned on each time any one of them is to be actually monitored, the detectors within any given block are spaced sufficiently apart on the detector board so that the desired photodetector is not affected by light scattering from the other five photodetectors in that block. The following description with respect to FIGS. 8-13 relates to a preferred embodiment of a rapid solid state multibeam photometer for multi-site microanalysis which can perform a large number of readings in a short period of time without requiring mechanical movement of the optical components or the microplate or other multi-site assay sample. A number of light sources are arranged so that one light source shines on each of the assay wells or spots. At least one monolithic photodetector is employed such that light coming from a plurality of different assay sites can be detected at a plurality of different sites on the surface of the photodetector. The light sources are turned on in sequence so that only a single light source is shining at any one time. The signal detected by the photodetector will correspond to the particular well or spot in the sample beneath the light source which is turned on at that time. In the preferred embodiment, an array of 96 light-emitting diodes (LED's) is arranged above the 96 wells of a microplate or sample spots on microplate format filter. At least one silicon photodetector is arranged beneath the microplate or filter with an area corresponding to the area of a plurality of the wells on a microplate. Solder connecting lines to the top layer of the photodetector are arranged in a criss-cross fashion to correspond to the area between the sample sites so that light is not blocked. The use of the large photodetector results in a large noise problem. This noise is overcome by a number of features. By using bright LED's, the intensity of the signal relative to the noise is increased. The primary noise signal encountered is the 60 Hz line power signal. The effect of this noise signal is counteracted by taking 10 readings for each well, with each reading being synchronized to occur 36° after the previous reading in 60 Hz phase space. For 10 readings, the time between readings equals: ( 10n + 1 600 ) sec, where n is an integer. Thus, 60 Hz noise encountered on one reading should be cancelled out by noise encountered on another reading at an opposite phase when the 10 samples are averaged. Noise is also reduced by the use of high-frequency sequencing of the LED's rather than having the light source constantly on. A filter is then used to eliminate noise which occurs at frequencies above the LED sequencing frequencies. Measurements are made at this high frequency for both an on and an off state for each LED. The off state measurement will give a reference level corresponding to the noise in the system at frequencies below that of the sequencing frequency. Because the off state measurement is done immediately after the on state measurement, the noise level should be approximately the same in both states and cancel out. A disadvantage with using LED's and a large photodetector is that both have characteristics which drift with temperature. These characteristics can change in just a few minutes. Devices which utilize a tungsten light source and small photodiodes do not have drift problems of the same magnitude. This drift problem is overcome by the following steps. A reading is first taken without the sample being inserted to give a photosignal value corresponding to the light transmission through the airspace between th LED's and the photodetector. A second reading is then taken immediately thereafter of a microplate filled with water (blank plate), buffer or any other base solution in which the chemical reactions are to take place or of a blank filter. This will give a "blank" photosignal level reading corresponding to the light transmission through the blank plate. Immediately prior to each subsequent reading of chemical samples, a second reading through an airspace is taken prior to the reading through the chemical sample. The chemical sample reading is then compared to the "blank" reading and is adjusted for differences between the two airspace readings. The differences in the readings through air will be caused by drift of the detector and LED characteristics with temperature. The use of airspace readings eliminates the need to insert a blank prior to each sample. Turning now to FIG. 8, there is shown a perspective view of a presently preferred embodiment of a multibeam photometer of the present invention. A photometer 2 is depicted comprising circuit board 4, the details of which will be described below, monolithic photodetector 6, LED array 8, and drawer-type sample holder 10. A microplate or sample tray 14 is usually placed in opaque mask 16 adapted to isolate the sample tray well walls while allowing light to pass through each sample well from top to bottom or vice versa. This assembly is then placed in open receptacle 18 of sample holder 10. The reader is activated by returning sample holder 10 to the closed position which activates a microswitch, indicating that the sample may be read. FIG. 9 is a side elevation cutaway view of a microplate well inserted into mask 16 and placed between photodetector 6 and LED array 8. Although not depicted, microplate format filters may be read by placing the filter ina holder adapted to isolate each sample site one from another while permitting light to pass through each individual sample site. FIG. 10 shows a block diagram of the electronic control circuitry used. A central processing unit (CPU) 40 controls the operation of the circuit and for each LED to be illuminated sends a data signal on data bus 42 to decoders 44 and 46. The decoded signals are sent through appropriate lines of busses 48 and 52 to a current driver array 50 and a current sink array 54, respectively, and through drive bus 56 and sink bus 60 to activate a particular LED in LED array 8. Light then passes through sample 14 to photodetector 6. The signal from photodetector 6 passes through current-to-voltage converter 66, sample and hold circuit 68, analog-to-digital A/D converter 70, through data bus 72 and back to CPU 40 for analysis. CPU 40 can be a standard CPU board having a microprocessor, random access memory (RAM) and read only memory (ROM), such as the Microsystems 5323. In operation, when a particular LED is to be turned on, a digital code corresponding to that LED is transmitted along data bus 42 to decoders 44 and 46. Decoder 44 decodes a portion of the data designating the correct column of the LED to be illuminated, and activates the appropriate line of data bus 48 to current driver array 50. Decoder 46 decodes the portion of the data corresponding to the appropriate row of the LED to be illuminated and activates a data line of data bus 52 connected to current sink array 54. Current is then passed through the appropriate line of current driver bus 56, through the appropriate diode of 8 x 12 LED array 8 and then through an activated line of sink bus 60 to the corresponding current sink in current sink array 54. The light from a particular LED in LED array 8 will pass through a well in sample tray 14 and be detected by photodetector 6. The current generated in photodetector 6 will be converted into a voltage level by current-to-voltage converter 66. A sample and hold circuit 68 will sample this analog voltage level at a period in time controlled by CPU 40. This analog value will be converted into a digital value by A/D converter 70. The digital data is passed along data bus 72 to CPU 40. Data from CPU 40 can be passed along to a printer 74. CPU 40 is controlled by an operating program which decides when data is to be analyzed, printed or sent to decoders 44 or 46 to illuminate an LED. A software clock on a line 76 indicates when an LED is to be illuminated. The timing of this software clock relative to a 60 Hz line noise signal may vary because of the variation in the number of program steps needed to complete an operation being performed when the program is interrupted to take a reading. To avoid this variation, software clock 76 is gated with a hardware clock 78 in a gate 80. Accordingly, the absolute time of each LED illumination can be precisely controlled. The output of gate 80 is coupled to a clock input of decoders 44 and 46. Photodetector 6 is a rectangular silicon cell with N-doped material on the top and P-doped material on the bottom. The N-doped layer is thin enough to allow light to pass through. The natural reverse bias of the silicon cell due to the diffusion of holes and electrons is sufficient for purposes of the present invention and no reverse bias voltage is added. Due to the resistivity of the N-doped region, a grid of solder lines criss-crosses the top of the N-doped layer. The lines are located to be in between the wells of the microplate so that light transmitted from the LED's will not be blocked. Current driver array 50 is supplied with current by a constant current source 82. The amount of current can be set to correspond to the desired amount and intensity of light to be emitted by each LED. FIGS. 11A and 11B show the detailed circuitry of portions of the block diagram of FIG. 10. The circuitry of FIG. 11B is preferably located on the same circuit card as the 8 x 12 LED array. The circuitry of FIG. 11A can be located remotely from the LED array on a separate card. Referring to FIG. 11A, a current-to-voltage (I/V) converter 66 is shown which is adapted for coupling to the photodetector 6 of FIG. 10. A resistor 84 and capacitor 86 can be adjusted to provide high frequency filtering of the signal from photodetector 6. The ouput voltage signal of I/V converter 66 is applied to a sample and hold circuit 68. The sampling and holding functions of sample and hold circuit 68 are activated by a trigger input 86. This input originates from one of the data lines in bus 88 to CPU 40 of FIG. 10. Output 90 of sample and hold circuit 68 is provided as an input to A/D converter 70. The maximum value which A/D converter 70 can accept is 10 volts. Resistor 84 back in I/V converter 66 can be adjusted so that the largest signals expected to be detected from the photodetector 6 will produce a voltage of just under 10 volts. The digital output of A/D converter 70 appears on data bus 92. Data bus 92 serves to provide the digital values from A/D converter 70 to CPU 40, as well as allowing the passage of data from CPU 40 to decoders 44 and 46. When the CPU is transmitting data to decoders 44 and 46, the tristate outputs of A/D converter 70 will be directed to assume a high impedance state. The outputs of A/D converter 70 are controlled by a chip enable signal 94. The digital output values from A/D converter 70 consist of two 8-bit bytes. A digital signal on the byte address/short cycle input A₀ activates first the first byte and then the second byte on the outputs of A/D converter 70. Thus, more than one output of A/D converter 70 is shown coupled to each data line. A decoder 96 provides an enable signal 98 to decoders 44 and 46 of FIG. 11B. This signal also serves as the software clock input to synchronizing circuit 80. The ouput of synchronizing circuit 80 is an inhibit signal 100. Referring to FIG. 11B, data signals D0-D6 are shown input to decoders 44 and 46. A constant current source 82 provides current for current drivers 50 to LED array 58. The rows of LED array 8 are coupled to current sinks 54, which are inverters having an open collector output. When a particular LED is selected, a 7-bit digital code will appear on data lines D0-D6. When an enable signal is provided and the inhibit signal is removed, decoder 44 will activate one output line and decoder 46 will activate one output line. The line activated by decoder 44 will switch one of current drive transistors 50 which will provide current from current source 82 to all of the LED's in one of columns A-H. The output line activated by decoder 46 will turn on one of inverters 54 to a low state output, thereby coupling the selected one of rows 1-12 to ground. This will allow current to flow through the selected one of the LED's in the selected one of rows A-H. A front panel display 12 in FIG. 11A has "ready," "load," and power on indicator lights as well as "set blank," "read tray," and "optical density (OD)/range" switches. The switches and lights are controlled by signals from the CPU as discussed below. The operation of the circuit of FIGS. 10 and 11 can be understood with reference to the timing diagram of FIG. 12. The sample and hold trigger signal is signal 86 of Fig. 11A, the decoder inhibit is signal 100 of FIG. 11A and FIG. 11B, and the A/D chip enable signal is signal 94 of FIG. 11A. Two separate readings are taken for each sample, one with the LED on and one with the LED off. The transmittance level is the difference between the on and off readings. The sequences of the on and off readings are shown separately. All 96 LED's are activated in sequence, and the process is repeated 10 times over equally spaced phase positions of a 60 Hz line noise signal. The sample and hold trigger signal on line 86 produces a pulse every 250 microseconds (ms). This pulse pulse is generated in the CPU with the hardware clock from a crystal oscillator. The decoder inhibit signal is produced by a combination of the hardware clock and a software clock. The software clock signal is produced as soon after an interrupt signal in the CPU program as the operation of the CPU program can be interrupted . This amount of time can vary from 4 to 20 clock cycles, depending upon the status of program operation. When such a software clock signal is produced on line 98, it is combined with the hardware clock in synchronizing circuit 80 to produce a down going pulse on the decoder inhibit signal at time 104. This causes a particular LED to be turned on as explained before. Sample and hold circuit 68 is enabled for sampling a value at time 106. This sample is locked in at a hold time 108. This hold time is delayed past the LED turn-on time 104 to allow the signal to be processed through current-to-voltage converter 66 and filtered through resistor 84 and capacitor 86. A/D converter 70 has its tristate outputs enabled at a point of time 110 and data is read out in two bytes under the control of two chip enable pulses 112. Similarly, a sequence for recording a sample when the LED is turned off begins at a point of time 114 when the inhibit signal again appears, thereby disabling the decoders and removing current from the current drivers to turn off the LED's. Sample, hold, start of A/D and read of A/D signals follow accordingly. Ten on and off readings of this type are taken for each LED with all 96 LED's being sequenced for the first on and off reading, then all 96 being sequenced for the second on and off reading, and so forth. Because the most predominant noise signal is the 60 Hz line voltage, these 10 readings are equally spaced along different positions of the phase of the 60 Hz cycle. Thus, a first reading may be taken along the rising edge of the first 60 Hz cycle, and after the remaining 95 LED's are sequenced, a second reading is taken at a higher point on a corresponding later 60 Hz cycle. Because of the time difference between an LED on and LED off reading the ground or reference voltage, which is actually changed by the 60 Hz noise signal, will move slightly, thereby causing an error. This movement will be slightly upward for a reading taken on one part of a 60 Hz cycle, and slightly downward for a reading taken during another portion of a 60 Hz cycle. Thus, by equally spacing the readings across the 60 Hz cycle, the errors in the 10 readings should cancel out when an average value is taken. Prior to insertion of a microplate, which is to be read or used as a blank plate, drawer 10 is pulled out and the plate reader will do a reading of the airspace in between the LED's and the photodetector and after completion will light up the "load" indicator in front panel 12. The reading of microplates containing experimental samples is preceded by the insertion of a microplate containing a reference fluid such as water (or a set of reactants at time equals zero (t = 0) to give a "blank," or t = 0 reading. The "ready" indicator will light up when the tray is all the way in and the instrument is capable of responding to the "set blank" or "read" switch. The readout on printer 74 can be either several digits of resolution or can be rounded to a single digit as controlled by the "optical density (OD) range" switch. After the blank value has been determined and drawer 10 pulled out fully, an experimental sample tray can be placed in the drawer. The plate reader will again measure the airspace between the LED's and the photodetector immediately prior to reinsertion of drawer 10. The photosignal values as modified by the various wells of the sample tray are compared to values for the blank tray and are adjusted for variations in the airspace reading. Thus, a blank tray can be inserted initially and will not have to be reinserted for sample readings taken over the course of several hours or days. The adjustments for drift in the LED's and the photodetector with temperature over this time period is made by the comparison of the two airspace readings immediately before the blank tray insertion and immediately before the particular sample tray insertion. The on-board software program which controls the operation of the microplate reader is shown in the flow chart of FIG. 13. The plate reader is first turned on and housekeeping functions are performed by the software (step A). The operator then pulls drawer 10 out. The "drawer out" state is then detected (step B) and the LED's are scanned (step C) to produce a series of photosignal values (step D) corresponding to light transmission through the airspace. These values are put in a Table 3 in memory. Once drawer 10 has been reinserted (step E), the front panel switches are checked to see whether the drawer contains a blank tray (step F). For a blank tray, the LED's are scanned (step G) and the photosignal values are put in a Table 1 in memory (step H). The airspace values of Table 3 are then transferred to another Table 2 (step I) to provide a set of airspace readings close to the time of the blank plate reading producing the values of Table 1. The airspace readings are done constantly when drawer 10 is out, thus constantly updating Table 3 prior to drawer 10 containing a tray being inserted. If drawer 10 is inserted and the "set blank" switch is not thrown, the front panel switches are examined to determine whether the samples are to be red (step J). If the "read" switch has been thrown, the LED's are scanned (step K) and the transmittance values are put in Table 4 in memory (step L). The optical densities for the samples are then calculated (step M) and printed (step N). The calculation of optical density (OD) is done in accordance with the following equation: OD n = log (n 1 ) (n 3 )/(n 4 ) (n 2 ). where the photosignal value of the nth site in Table X equals n X ; and the optical density of the sample at site n equals OD n . Each LED is scanned starting with the first and proceeding through the 96th as discussed earlier. Ten repetitions are done equally spaced over the phases of a 60 Hz cycle. The final photosignal values used in the calculation described above are given by the sum of the 10 or minus off signals. As can be seen by the above equation, the optical density is the log of the blank value divided by the sample value adjusted for differences in the airspace readings. A photosignal reading in accordance with the flow chart of FIG. 13 is initiated by an interrupt signal to a microprocessor in CPU 40. In between readings initiated by the interrupt signal, the program in the CPU can cause data to be calculated or transferred to one of the tables or printed out on a printer. The actual reading itself is sandwiched in between these other functions as dictated by the interrupt signal. Because the interrupt signal will be interrupting a program operation, a certain amount of steps after the interrupt signal are necessary to basically hold the place in the program by moving data into memory or completing a step or otherwise. Thus, the amount of time between the initiation of an interrupt signal and the reading of an LED could vary (usually by between 4 and 20 clock cycles). To prevent this from causing jitter in the readings, the interrupt is provided at least 20 clock cycles prior to when it is anticipated that a reading should be taken and is, in effect, ANDed with a real time clock in synchronizing gate 80 as shown in FIGS. 3 and 4. This ensures that the location of each reading relative to the phase of the 60 Hz noise signal will be precisely controlled. Rather than manually flicking the switches of control panel 12, the operation of the plate reader can be controlled by another computer, such as a microcomputer coupled to the plate reader. The attachment of such a host computer overrides the operation of the switches. The rapid solid state multibeam photometer as described above is characterized by its ability to reliably read transmittances of multi-site solid support assay samples in less than about 2 seconds without movement of the sample or holder. This is achieved by employing at least one monolithic photodetector of an area at least equal to the area of a plurality of multi-site solid support assay sites.

Claims: A device (10) for measuring an optical characteristic of samples contained in a plurality of sample sites disposed on a multi-site assay plate (40), the device including: a light source (8;12,24,32) for providing light to more than one of said sample sites (A₁-H₁₂) for transmission to photodetector means (44) and means (50) for analysing the output of said photodetector means (44) to determine and indicate the optical density of said sites; photodetector means (6;44) positioned in relation to said assay plate in such a way that light is vertically transmitted through each of said sites and is received by a corresponding photodetector wherein a signal is provided in response to said received light; means (54) for simultaneously imparting mixing movement to said samples by imparting repeated oscillatory movement to said assay plate (40) for a predetermined time before a series of at least two readings is performed on a given sample site; and a means (50) for kinetic analysis of the signal of the photodetector means relating to the given site, characterised in that: said photodetector means (4;44) are disposed on a detector board (46), said mixing means impart said mixing movement before each reading of said series, and said light source (8;12,24,32) comprises at least first and second light transmitting paths (8;32) for directing light respectively to at least a first and a second of said sample sites (A₁-H₁₂), and said detector board (46) comprises at least first and second photodetector means (8;44) for receiving light transmitted along said first and second light transmitting paths respectively through said first and second sample sites. A device according to claim 1, wherein said light source comprises a plurality of light emitting elements (8) providing light to a respective plurality of the light transmitting paths and said readings are performed on selected ones of said plurality of sample sites. A device according to claim 1 or claim 2 wherein the light source (12) comprises a single source (14) adapted through a light transmitting means (24,32) to be selectively applied in a predetermined sequence to selected ones of the light transmitting paths. A device according to any of the preceding claims, wherein said light source provides light of a wavelength of about 300nm to 900nm. A device according to claim 3 or 4 wherein said light transmitting means comprise optical transmission fibers, said coupling means having: an optical rotor (24) receiving substantially all of said light at an input end and transmitting it to an output end; and a fiber optic distributor (32) including said plurality of transmission optical fibers disposed in a manner allowing each of said transmission optical fibers to be selectively coupled to the light coming out of said output end of said optical rotor; said optical rotor capable of being displaced in such a way that said light is coupled from said source to successive ones of said transmission optical fibers and hence to successive ones of said sample sites. A device according to any one of claims 3 to 5 which further includes: means (16) for modulating the light source at a predetermined rate; and means (19) for filtering light of a desired wavelength from said light source for application to said sample sites. A device according to any one of claims 3 to 6 wherein said means for analysing the output of said photodetector means includes: means (112,118) for sequentially selecting the analog output signals of desired photodetectors for analysis; means (134) for converting said analog signals to their corresponding digital values; means (120,130) for amplifying said analog signals in such a way as to utilize a substantial portion of the dynamic range of said analog to digital conversion means; means for storing precalculated values corresponding to said digitized values, accepting said digitized values and retrieving the corresponding stored precalculated value in response to each of said digital values; and means for processing said retrieved values in-order to calculate the optical density of said samples. A device according to claim 7, wherein said means for analysing the output of said photodetector means further includes means (111,115,116,119,124,126, 128) for analysing that portion of said analog signal which is modulated at said predetermined rate. A device according to claim 7 or claim 8 wherein said means for amplifying further includes: a variable gain amplifier means (120,130) for receiving said analog signals and providing an amplified output signal; means for comparing the digital value provided by said analog to digital converter means n response to said amplified output signal with the maximum possible value of said analog to digital converter means; and means for adjusting the gain of said amplifier means to the maximum gain possible without exceeding the maximum possible value of said analog to digital converter means. A device according to claim 9 wherein said amplifier means includes: a first amplifier (120) having a gain set in response to a light signal in the absence of a sample; and a second amplifier (130) having a gain set in response to a light signal passing through a sample. A device according to claim 5 wherein said distributor also contains a first reference optical fiber (HOME) directly coupling said light to a home photodetector as a means for referencing the relative positioning of said rotor (24) with respect to said distributor (32). A device according to any one of the preceding claims wherein a calibration means (52) is coupled to a reference photodetector (45) to eliminate effects of variations in output of the light source. A device according to claims 5 or 11 wherein the distributor further includes means (X₁-X₃) for preventing light signals from being coupled into said light transmitting means, and said means for analysing the output of said photodetector means includes means for measuring any output from said photodetector means in the absence of light being coupled to the transmitting means. A device according to any one of the preceding claims which further includes a drive motor (54) for advancing and retracting said assay plate relative to said detector board. A device according to claim 14 wherein said drive motor further serves for imparting said mixing movement to said assay plate. A device according to any of the preceding claims wherein said multi-site assay plate (40) is a 96-well microplate disposed in an 8 x 12 matrix. A device according to any of the preceding claims wherein said plurality of sample sites comprises 96 sample sites. A device according to any of the preceding claims where the device reads transmittances of said plurality of sample sites on said multi-site assay plate in less than 2 seconds. A photometric kinetic method for analysing samples contained in a plurality of sample sites disposed on a multi-site assay plate (40), including: providing light from a light source (12) to selected ones of said plurality of sample sites (A₁-H₁₂) on said assay plate (40) and means (50) for analysing light signals received by transmission through said selected sites; transmitting light through each of said selected ones of said plurality of sample sites (A₁-H₁₂) to photodetector means (44) and generating an output in response to said light received wherein the light passes through the sample; analysing the output to determine and indicate the optical density of the selected sites by passing the light vertically through the selected samples site(s); imparting oscillatory mixing movement to said multi-site assay plate (40) for a predetermined time before a series of at least two readings is performed on said selected sites in a manner sufficient to bring about thorough mixing of solutions contained within the sample sites; kinetically analysing the output of the photodetector means, characterised in that: said mixing movement is imparted before each reading of said series, and light is directed to at least a first and a second of said sample sites (A₁-H₁₂) along respective first and second light transmitting paths and is received by respective first and second photodetector means (44).

Inventors: CHOW CALVIN Y H, HUMPHRIES GILLIAN M, KUNG VIOLA T, LACY MICHAEL M, PARCE JOHN WALLACE, CHOW, CALVIN Y.H., HUMPHRIES, GILLIAN M., KUNG, VIOLA T., LACY, MICHAEL M., PARCE, JOHN WALLACE
Applicants: MOLECULAR DEVICES CORP, MOLECULAR DEVICES CORPORATION
Application year: 1986
Main classification: G01N21
IPC codes: G01N21, G01N35, G01N1