Abstract:
A system and method for use with a flow cytometer to improve event reading and data processing capabilities of the flow cytometer, while also providing efficient system configuration assessment capabilities. The system and method enables the flow cytometer to capture and sample an entire waveform representative of an event being read, and provides improved processing and analysis of the sampled data in a real time or near real-time basis. The system and method further enable the flow cytometer to assess its configuration and provide assessment results to an operator in an efficient and effective manner.

Description:
The present invention claims benefit under 35 U.S.C §119(e) of a U.S. Provisional Patent Application of Dwayne Yount et al. entitled “Hardware and Electronics Architecture for a Flow Cytometer”, Ser. No. 60/203,515, filed May 11, 2000, of a U.S. Provisional Patent Application of Michael Lock et al. entitled “Cluster Finder Algorithm for Flow Cytometer”, Ser. No. 60/203,590, filed May 11, 2000, of a U.S. Provisional Patent Application of Michael Goldberg et al. entitled “User Interface and Network Architecture for Flow Cytometer”, Ser. No. 60/203,585, filed May 11, 2000, and of a U.S. Provisional Patent Application of John Cardott et al. entitled “Digital Flow Cytometer”, Ser. No. 60/203,577, filed May 11, 2000, the entire contents of each of said provisional patent applications being incorporated herein by reference. 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     Related subject matter is disclosed in a copending U.S. Patent Application of Pierce O. Norton entitled “Apparatus and Method for Verifying Drop Delay in a Flow Cytometer”, Ser. No. 09/346,692, filed Jul. 2, 1999, in a copending U.S. Patent Application of Kenneth F. Uffenheimer et al. entitled “Apparatus and Method for Processing Sample Materials Contained in a Plurality of Sample Tubes”, Ser. No. 09/447,804, filed Nov. 23, 1999, and in a copending U.S. Patent Application of Michael D. Lock et al. entitled “System for Identifying Clusters in Scatter Plots Using Smoothed Polygons with Optimal Boundaries”, Ser. No. 09/853,037, filed even date herewith, the entire contents of each of these applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a system and method for providing improved event reading, data processing and system configuration capabilities in a flow cytometer. In particular, the present invention provides a system and method for use with a flow cytometer that enables the event reading components of the flow cytometer to capture and digitize substantially the entire optical waveform of each detected event, and provides improved, near real-time processing of the digitized waveform data and automated system configuration assessment capabilities. 
     2. Description of the Related Art 
     Flow cytometers known in the art are used for analyzing and sorting particles in a fluid sample, such as cells of a blood sample or particles of interest in any other type of biological or chemical sample. A flow cytometer typically includes a sample reservoir for receiving a fluid sample, such as a blood sample, and a sheath reservoir containing a sheath fluid. The flow cytometer transports the particles (hereinafter called “cells”) in the fluid sample as a cell stream to a flow cell, while also directing the sheath fluid to the flow cell. 
     Within the flow cell, a liquid sheath is formed around the cell stream to impart a substantially uniform velocity on the cell stream. The flow cell hydrodynamically focuses the cells within the stream to pass through the center of a laser beam. The point at which the cells intersect the laser beam, commonly known as the interrogation point, can be inside or outside the flow cell. As a cell moves through the interrogation point, it causes the laser light to scatter. The laser light also excites components in the cell stream that have fluorescent properties, such as fluorescent markers that have been added to the fluid sample and adhered to certain cells of interest, or fluorescent beads mixed into the stream. 
     The flow cytometer further includes an appropriate detection system consisting of photomultipliers tubes, photodiodes or other light detecting devices, which are focused at the intersection point. The flow cytometer analyzes the detected light to measure physical and fluorescent properties of the cell. The flow cytometer can further sort the cells based on these measured properties. 
     To sort cells by an electrostatic method, the desired cell must be contained within an electrically charged droplet. To produce droplets, the flow cell is rapidly vibrated by an acoustic device, such as a piezoelectric element. The droplets form after the cell stream exits the flow cell and at a distance downstream from the interrogation point. Hence, a time delay exists from when the cell is at the interrogation point until the cell reaches the actual break-off point of the droplet. The magnitude of the time delay is a function of the manner in which the flow cell is vibrated to produce the droplets, and generally can be manually adjusted, if necessary. 
     To charge the droplet, the flow cell includes a charging element whose electrical potential can be rapidly changed. Due to the time delay which occurs while the cell travels from the interrogation point to the droplet break-off point, the flow cytometer must invoke a delay period between when the cell is detected to when the electrical potential is applied to the charging element. This charging delay is commonly referred to as the “drop delay”, and should coincide with the travel time delay for the cell between the interrogation point and the droplet break-off point to insure that the cell of interest is in the droplet being charged. 
     At the instant the desired cell is in the droplet just breaking away from the cell stream, the charging element is brought up to the appropriate potential, thereby causing the droplet to isolate the charge once it is broken off from the stream. The electrostatic potential from the charging circuit cycles between different potentials to appropriately charge each droplet as it is broken off from the cell stream. 
     Because the cell stream exits the flow cell in a substantially downward vertical direction, the droplets also propagate in that direction after they are formed. To sort the charged droplet containing the desired cell, the flow cytometer includes two or more deflection plates held at a constant electrical potential difference. The deflection plates form an electrostatic field which deflects the trajectory of charged droplets from that of uncharged droplets as they pass through the electrostatic field. Positively charged droplets are attracted by the negative plate and repelled by the positive plate, while negatively charged droplets are attracted to the positive plate and repelled by the negative plate. The lengths of the deflection plates are small enough so that the droplets which are traveling at high velocity clear the electrostatic field before striking the plates. Accordingly, the droplets and the cells contained therein can be collected in appropriate collection vessels downstream of the plates. 
     Known flow cytometers similar to the type described above are described, for example, in U.S. Pat. Nos. 3,960,449, 4,347,935, 4,667,830, 5,464,581, 5,483,469, 5,602,039, 5,643,796 and 5,700,692, the entire contents of each patent being incorporated by reference herein. Other types of known flow cytometer, are the FACSVantage™, FACSort™, FACSCount™, FACScan™ and FACSCalibur ™ systems, each manufactured by Becton Dickinson and Company, the assignee of the present invention. 
     Although the flow cytometers described above can be suitable for reading events as intended, these existing systems do suffer from certain drawbacks. For example, in these types of instruments, the controller or central processing unit (CPU) does not ordinarily process the data obtained from reading the events in “real time”. However, it is desirable to process the data in real time or near real time to improve the efficiency of the flow cytometer and the ability to compare the readings of the events on a real-time or near real-time basis. 
     These existing systems also do not capture the entire image of the event. That is, when each event is read by detecting, for example, light fluorescing from the cell or particle of interest, these systems capture the “peak point” or peak intensity of the detected light. These systems also typically measure the duration during which the light is detected. By detecting these two parameters, the existing systems can use this data to determine characteristics of the event, such as the identity and size of a cell of interest. However, these techniques do not enable the existing systems to sample individual regions of the cell or particle of interest, nor are they capable of being performed on a real-time or near real-time basis. Furthermore, these systems are typically incapable of comparing data from multiple events effectively and in a real time or near real-time manner. 
     In addition, these types of existing systems do not provide a mechanism that indicates the configuration of the system to the operator effectively. For example, these types of systems are typically configured with multiple detector and filter arrangements that enable the different detectors to detect light having wavelengths within different wavelength regions. In such an arrangement, one detector can detect light with having a wavelength within the range of blue light, for example, while another detector can detect light having a wavelength within the range of green light. However, if an incorrect filter is placed in front of a particular detector, the detector will detect the incorrect light (e.g., green light instead of blue light). The system will therefore give erroneous results. However, the operator of the system will have difficulty determining which filters are arranged incorrectly, and in the worst case, the error may go unnoticed. 
     Accordingly, a need exists for an improved system and method for use with a flow cytometer to improve the event reading and data processing features of the flow cytometer to eliminate the above drawbacks. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a system and method for use with a flow cytometer to improve event reading and data processing capabilities of the flow cytometer, while also providing efficient system configuration assessment capabilities. 
     Another object of the present invention is to provide a system and method that enables a flow cytometer to capture and sample an entire waveform representative of an event being read, and which provides improved processing and analysis of the sampled data in a real-time or near real-time basis. 
     A further object of the present invention is to provide a system and method that is capable of indicating the configuration of a flow cytometer to an operator in an efficient and effective manner. 
     These and other objects are substantially achieved by providing a system and method for processing at least one signal representative of an event detected by at least one detector in a flow cytometer. The system and method employs a sampling device which is adapted to receive portions of the signal from the detector in time sequence and to generate a respective value representative of the respective magnitude of each respective portion of the signal as the respective portion of the signal is being received. The system and method further employ a storage device which is adapted to store the values generated by the sampling device. The sampling device can receive substantially all of the signal, and can generate the values which represent the portions of substantially all of the signal. The signal can be an analog signal representative of a light signal emitted from the event as detected by the detector. The system and method can further employ an arithmetic device which is adapted to, for example, subtract a designated value from each of the values generated by the sampling device. The designated value can be representative of an unwanted signal, such as crosstalk, detected by the detector, or can be representative of a characteristic of the detector. The sampling device can further be adapted to receive portions of a second signal from a second detector in time sequence and to generate a respective second value representative of the respective magnitude of each respective portion of the second signal as the respective portion of the second signal is being received, and the storage device can store the second values generated by the sampling device. The sampling device can receive the portions of the signal at a time different from that during which it receives at least some of the portions of the second signal, and the system and method can employ a comparator which is adapted to compare each of the second values with a respective one of the values to compare the signal to the second signal. 
     These and other objects are further substantially achieved by providing a system and for identifying a configuration of a detector unit of a flow cytometer. The system and method employ a port which is adapted to couple to a removable device that includes an optical clement, such as a mirror or filter, and a memory adapted to store information pertaining to the optical element. The system and method further employ a reader which is adapted to read the information stored in the memory when the removable device is coupled to the port. The system and method can also employ an indicator which adapted to provide an indication of the information read by the reader. 
     These and other objects are also substantially achieved by providing a removable device which is adapted for coupling with a port of a flow cytometer, and comprises an optical element, such as a filter or mirror, and a memory adapted to store information pertaining to the optical element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various objects, advantages and novel features of the present invention will now be more readily appreciated from the following detailed description when read in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a conceptual block diagram of the flow cytometer employing a system and method according to an embodiment of the present invention; 
     FIG. 2 is a cross-sectional view of the flow cytometer shown in FIG. 1; 
     FIG. 3 is a detailed view of an example of an emission block according to an embodiment of the present invention which is employed in the flow cytometer shown in FIGS. 1 and 2; 
     FIG. 4 is a top perspective view of an example of a support ring and flex circuits employed in the emission block shown in FIG. 3; 
     FIG. 5 is a bottom perspective view of the support ring and flex circuits shown in FIG. 4; 
     FIG. 6 is a side view of the support ring and flex circuits shown in FIGS. 4 and 5; 
     FIG. 7 is a conceptual top plan view of the emission block shown in FIG. 3; 
     FIG. 8 is a perspective view of an example of a removable mirror assembly for use with the emission block shown in FIG. 3 in accordance with an embodiment of the present invention; 
     FIG. 9 is a perspective view of an example of a removable mirror assembly for use with the emission block shown in FIG. 3 in accordance with an embodiment of the present invention; 
     FIG. 10 is a conceptual top view of the emission block shown in FIG. 3 illustrating exemplary paths in which light entering the emission block is reflected and propagates; 
     FIG. 11 is a block diagram illustrating an example of the electronic components employed in the flow cytometer shown in FIGS. 1 and 2 according to an embodiment of the present invention; 
     FIG. 12 is a block diagram illustrating anther example of the electronic components employed in the flow cytometer shown in FIGS. 1 and 2 according to another embodiment of the present invention; 
     FIGS. 13-16 are conceptual illustrations of an exemplary relationship between multiple lasers and multiple emission blocks in the flow cytometer shown in FIGS. 1 and 2 according to an embodiment of the present invention; 
     FIGS. 17-20 are conceptual block diagrams showing exemplary relationship between certain components shown in FIGS. 11 and 12; 
     FIG. 21 illustrates an example of a waveform as captured and sampled by the circuitry shown in FIGS. 11 and 12; 
     FIG. 22 is a conceptual block diagram of control circuitry for a PMT detector; and 
     FIGS. 23-27 illustrate exemplary waveforms and their processing by the circuitry shown in FIGS.  11  and  12 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A flow cytometer  100  employing an embodiment of the present invention is illustrated in FIGS. 1 and 2. As discussed in the background section above, the flow cytometer  100  includes a nozzle  102  having a flow cell  104  therein. The flow cytometer further includes a sample reservoir  106  for receiving a fluid sample, such as a blood sample, and a sheath reservoir  108  containing a sheath fluid. The flow cytometer transports the cells in the fluid sample in the cell stream to the flow cell  104 , while also directing the sheath fluid to the flow cell  104 . 
     Within the flow cell  104 , the sheath fluid surrounds the cell stream, and the combined sheath fluid and cell stream exits the flow cell  104  via an opening  110  as a sample stream. The opening  110  can have a diameter of, for example, 50 μm, 70 μm, 100 μm, or any other suitable diameter. As illustrated, due to characteristics of the sheath fluid, such as surface tension and the like, the sample stream remains intact until breaking off into droplets at the droplet break-off point  112 , which is at a certain distance from opening  110 . The distance from opening  110  at which the droplet break-off point  112  occurs, and the frequency or rate at which the droplets are formed, are governed by the fluid pressure, as well as the amplitude and frequency of oscillation of oscillating device  114  which can be, for example, a piezoelectric element. 
     As shown in FIG. 2, the oscillating device  114  is connected to an alternating voltage source  116  whose output voltage amplitude, frequency and phase is controlled by a controller  118  which can include, for example, a microprocessor or any other suitable controlling device. Further details of the controller  118  are described below. The amplitude of the alternating voltage signal output by alternating voltage source  116  can be increased or decreased by controller  118  to in turn increase or decrease the distance from opening  110  at which the droplet break-off  112  occurs. Likewise, the frequency of the alternating voltage signal output by alternating voltage source  116  can be increased or decreased by controller  118  to increase or decrease the rate at which droplets of sample fluid are formed at the droplet break-off point  112 . 
     To view the droplet break-off point  112 , a light source  119 , such an LED array, can be positioned in the region of the sample fluid stream containing the droplet break-off point  112 . The controller  118  can control the light source  119  to strobe at a described frequency, so that the detector  120 , such as a camera or other special viewing device, can be used to view the region of the sample fluid stream containing the droplet break-off point  112 . The flow cytometer  100  further includes at least one laser  122 , such as a diode laser, which is controlled by controller  118  to emit laser light. The emitted laser light intersects the sample stream at a point of interest  124  commonly referred to as a the interrogation point. 
     The laser  122  can be, for example, a red laser that emits light having a wavelength of at or about 633 nm, which is in the red light spectrum. Alternatively, laser  122  can be a blue laser that emits light having a wavelength of at or about 488 nm, which is in the blue light spectrum. Laser  122  also can be an ultraviolet laser that emits light having a wavelength of at or about 325 nm, or within the range of at or about 351 nm to at or about 364 nm, all of which are within the ultraviolet spectrum. As discussed in more detail below, flow cytometer  100  can include multiple lasers  122  that each emit their respective laser light to a respective interrogation point along the fluid flow stream. Also, if desired, a lens or filter  126  can be positioned between the laser  122  and the interrogation point  124  to filter out light of unwanted wavelengths from the laser light prior to its reaching the interrogation point  124 . 
     As further illustrated, the flow cytometer includes at least one fiberoptic cable  130  that receives laser light that has intersected the sample stream at the interrogation point  124  and has been scattered by the sample stream fluid and, in particular, by any cells or particles of interest present in the sample stream. The input port  132  of the fiberoptic cable  130  in this example is located in the same plane as the laser light being emitted from laser  122 , and at a 90° angle or about a 90° angle with respect to the direction of propagation of the laser light being emitted from laser  122 . The laser light scattered by the fluid stream and any cells or particles of interest at the interrogation point  124  is commonly referred to as side-scatter laser light. 
     As further illustrated, a detector  134  and filter  136  arrangement can be used to detect a portion of the laser light that has passed through the interrogation point  124  along the direction of propagation of the laser light being emitted by laser  122 , which is commonly referred to as the forward-scatter laser light. Also, if desired, an obscuration bar  138  can be position in the path of the forward-scatter laser light, in the path of the side-scatter laser light, or in both paths, to reduce the amount of side-scatter laser light entering fiber optic cable  130  or to reduce the amount of forward-scatter laser light entering detector  134 . The side-scatter laser light entering the fiberoptic cable  130  is input to an emission block  140  as described in more detail below. 
     As further shown in FIGS. 1 and 2, the flow cytometer  100  can include deflection plates  1142  and  1144  which can be controlled by controller  118  to allow droplets to pass to droplet collection container  1146 , or to deflect droplets that have been charged by charging unit  147  towards droplet collection containers  1148  and  1150 , as appropriate. In additional, a laser and filter arrangement  1152  and  1154 , detector and filter arrangement  1156  and  1158 , and detector and filter arrangement  1160  and  1162 , can be employed to monitor the manner in which the droplets are being deflected. Further details of the charging, deflection, and monitoring of the droplets are described in copending U.S. patent application Ser. No. 09/346,692, referenced above. 
     Further details of the emission block  140  will now be discussed with reference to FIGS. 3-10. As illustrated, emission block  140  includes a support ring  142  which can be made from stainless steel or any other suitable material. As shown, in particular, in FIGS. 4-6, support ring  142  has inner groves  144  in its inner surface and outer groves  146  in its outer surface. A first flex circuit  148  is mountable in support ring  142 . Specifically, the first flex circuit  148  includes projections  150  that are received into inner groves  144  of support ring  142  to thus mount the first flex circuit  148  inside support ring  142 . As can be appreciated by one skilled in the art, first flex circuit  148  is an integrated circuit board arrangement that includes a plurality of integrated circuits (not shown) and contact pads  152  that have contacts  154  which are adapted to provide connections to the circuitry in the first flex circuit  148 . 
     As further illustrated, a second flex circuit  156  is mountable to the support ring  142 . That is, the second flex circuit  156  includes projections  158  that can be received in the outer groves  146  of the support ring  142  to thus mount the second flex circuit  156  to the exterior of support ring  142 . An adhesive can be used to secure the first flex circuit  148  and the second flex circuit  156  to the support ring  142 . Like first flex circuit  148 , second flex circuit  156  is also an integrated circuit arrangement that includes integrated circuits  160  that are capable of carrying out certain data processing operation as discussed in more detail below. The second flex circuit  156  further includes contact pads  162  that include contacts  164  which provide connections to the circuitry in the second flex circuit  156 . 
     As further illustrated, the emission block  140 , first flex circuit  148  and second flex circuit  156  are housed within an outer housing  166  and inner housing  168 . As illustrated, the combination of the support ring  142 , first flex circuit  148 , second  156 , outer housing  166  and inner housing  168  form openings  170  and  172  as illustrated in FIG.  7 . Each of the openings  170  is configured to receive a mirror assembly  174  which includes a dichroic mirror  176 , the purpose of which is described in more detail below. Furthermore, each opening  172  is configured to receive a filter assembly  180 , the purpose of which is described in more detail below. In this example, emission block  140  is capable of receiving six mirror assemblies  174 - 1  through  174 - 6  and seven filter assemblies  180 - 1  through  180 - 7  (see FIGS.  7  and  10 ). However, the emission block  140  can be configured to include any suitable number of mirror assemblies  174  and filter assemblies  180 . 
     An example of a mirror assembly  174  is shown in FIG.  8 . As stated above, each mirror assembly  174  includes a dichroic mirror  176  that is capable of passing light having a particular wavelength (e.g., blue light) while reflecting light of all other wavelengths. The diachronic mirror assembly  174  includes a memory, such as an electrically, erasable read-only memory (EEPROM), in which is stored information pertaining to the type of dichroic mirror  176  in the mirror assembly  174 , along with other information such as the company of manufacture, the date and place of manufacture and so on, for purposes described in more detail below. The mirror assembly  174  further includes contacts  178  that provide electrical connection with the memory embedded in the mirror assembly  174 . Accordingly, when the mirror assembly  174  is inserted into an opening  170  as shown, for example, in FIG. 7, the contacts  178  of mirror assembly  174  engage with the contact  154  on the contact pads  152  of the first flex circuit  148 . Accordingly, the circuitry in the first flex circuit  148  can thus access the information stored in the memory of the mirror assembly  174  for the purposes described in more detail below. 
     A filter assembly  180  is shown in more detail in FIG.  9 . Filter assembly  180  includes a filter  182  that is capable of passing light of a certain wavelength (e.g., blue light) while blocking light of all other wave lengths. Furthermore, like mirror assembly  174 , filter assembly  180  includes a memory, such as ROM, in which is stored information pertaining to the type of filter  182  in the filter assembly  180 , the date, place, and company of manufacture, and so on. Filter assembly  180  also includes contacts  184  which provide electrical contact to the memory embedded in the filter assembly  180 . Accordingly, when the filter assembly  180  is inserted into an opening  172  as shown, for example, in FIG. 7, the contacts  184  of the filter assembly  180  engage with the contacts  164  on a contact pad  162  of the second flex circuit  156 . Hence, the circuitry in the second flex circuit  156  can then access the information stored in the memory of the filter assembly  180  for reasons discussed below. 
     As further shown in FIG. 3, for example, emission block  140  include a plurality of detectors  186  which, in this example, are photomultiplier tubes (PMTs). Each photomultiplier tube detector  186  has an opening therein (not shown) which is aligned with a dichroic mirror  176  in its respective mirror assembly  174 , and with a filter  182  in its respective filter assembly  180 , so that the detector  186  will receive light passing through its respective dichroic mirror  176  and filter  182 . Each detector  186  further includes a circuit board assembly  188  that include circuitry for processing the light received by its respective PMT detector  186 , as well as power and control circuitry for the PMT, as discussed in more detail below. 
     As shown in FIG. 3, for example, and in more detail in FIG. 10, the mirror assemblies  174  are angled so that the side-scatter laser light entering the emission block  140  from fiber optic cable  130  is reflected to all of the mirror assemblies  174  and to all of the filter assemblies  180 . Specifically, when the laser light enters the emission block  140  from fiber optic cable  130 , the laser light propagates to mirror assembly  174 - 1 . The dichroic mirror of mirror assembly  174 - 1  allows light having a certain wavelength to pass to filter assembly  180 - 1 , which also allows light of that wavelength to be detected by its respective detector  186 - 1 . Detector  186 - 1  outputs a signal representative of the detected light, which is processed as described in more detail below. 
     As further illustrated, the portion of the laser light reflected by mirror assembly  174 - 1  propagates to mirror assembly  174 - 2 , which functions in a manner similar to mirror assembly  174 . That is, the dichroic mirror of mirror assembly  174 - 2  allows light of a certain wavelength (e.g., green light) to pass to filter assembly  180 - 2  while reflecting light of all other wavelengths. Accordingly, the light passing to filter assembly  180 - 2  will pass through the filter of filter assembly  180 - 2  and be received by detector  186 - 2 , while the reflected light will propagate to mirror assembly  174 - 3 . As can be appreciated from the above description, mirror assemblies  174 - 3  through  174 - 6  will each allow light within a certain respective wavelength range to pass through to the corresponding filter assemblies  180 - 3  through  180 - 6 , respectively, while reflecting light of all remaining wavelengths. It is noted that the light reflected by mirror assembly  174 - 6  will propagate directly into filter assembly  180 - 7 , because no further reflection is necessary. Filter assembly  180 - 7  will therefore allow light within a respective wavelength to pass to its corresponding detector  186 - 7 . 
     As discussed above, each laser  122  (see FIG. 1) of the flow cytometer  100  is associated with a respective fiber optic cable  130  and emission block  140 . Accordingly, as discussed in more detail below, if flow cytometer  100  includes, for example, four different lasers  122 , then the flow cytometer will also include four emission blocks  140 , with each emission block  140  being associated with a respective laser  122  to receive side-scatter laser light in the manner described above. 
     An example of the electronics included in the flow cytometer  100  is shown in block diagram format in FIG.  11 . As discussed above, the flow cytometer  100  includes a controller  118  which, in this example, includes a data acquisition unit  190 , a status and control unit  192 , a droplet control module  222  and a fluidics control module  224 . As indicated, the data acquisition unit  190  includes a processor  194  which, in this example, is a real-time or near real-time CPU, such as a Pentium III processor or any other suitable processor. The processor  194  is coupled to the screen LCD  196  of the flow cytometer  100 , as well as a sample loader  198  and sample output device  200 . The processor  194  is further coupled to a hub  202  which provides data to and from work station  204  and processor  194  as described in more detail below. It is noted that the processor  194  provides the data pertaining to the event readings to the work station  204  in packet format in real-time or near real-time. The hub  202  further provides data to and from processor  194  and a prepper unit  206  which can be, for example, any type of sample preparation unit such as that described in U.S. patent application Ser. No. 09/447,804, referenced above. 
     The data acquisition unit  190  further include a plurality of data acquisition modules  208  that are each capable of acquiring data from respective circuit board assemblies  188  of the detectors  186  discussed above as described in more detail below. The data acquisition unit  190  further includes a master data acquisition module  210  that gathers the data from all of the other data acquisition modules  208  via a plurality of link-ports  211  and provides the data to processor  194  as discussed in more detail below. 
     As further illustrated, the processor  194  of data acquisition unit  190  communicates with the controller  212  of status and control unit  192  to control, for example, the fluid flow, drop delay, PMT driving voltage, and so on as described in more detail below. The status and control unit  192  include PMT modules  214  which, under the control of controller  212 , control the driving voltage of the PMT detectors  186  as discussed in more detail below. The status and control unit  192  further include a laser control module  216  which, under control of controller  212 , controls operation of laser  122 . The status and control unit  192  also includes a power and temperature control module  218  that controls, for example, the power to components of the flow cytometer  100 , as well as the temperature of the sheath and sample fluid. 
     In addition, status and control unit  192  further includes an emission identification (ID) module  220  that receives information from the first flex circuit  148  and second flex circuit  156  indicative of the locations of the mirror assemblies  174  and filter assemblies  180 , in the emission block  140 . That is, as discussed above, each mirror assembly  174  and filter assembly  180  includes a memory in which is stored information pertaining to its respective mirror or filter. The circuitry in the first flex circuit  148  is capable of accessing the memory in the filter assemblies  180 , and providing the content of this memory to the emission ID module  220 . Likewise, the circuitry in the second flex circuit  156  is capable of accessing the memories in the filter assemblies  180  and providing that information to the emission ID module  220 . The emission ID module  220  then can determine whether each of the mirror assemblies  174  and filter assemblies  180  are in the appropriate positions based on information pertaining to a desired configuration stored in a memory that was provided, for example, by work station  204 . If the emission ID module  220  determines that a mirror assembly  174  or filter assembly  180  is missing or in an incorrect location in the emission block  140 , or if an erroneous or faulty mirror assembly  174  or filter assembly  180  has been installed in the emission block  140 , emission ID module  220  will provide the appropriate data to, for example, the controller  212 , which can then provide the data to the processor  194 . The processor  194  can then provide this data to, for example, work station  204 , which can display an appropriate error message. This error message can indicate the location of the incorrect mirror or filter assembly in the emission block  140 , and the work station  204  can also display the filter and mirror configuration, which therefore greatly simplifies troubleshooting. 
     As further shown in FIG. 11, the master data acquisition module  210 , which is described in more detail below, receives from the data acquisition modules  208  event data that has been provided to the data acquisition modules  208  from the PMT detectors  186  of the emission blocks  140 . Prior to running the flow cytometer  100  to detect events, the work station  204  can download data via the hub  202  and processor  194  to the master data acquisition module  210 . This downloaded data is stored in a memory in the master data acquisition module  210  and indicates to the master data acquisition module  210  the channel configuration of the data acquisition modules  208 , so that the master data acquisition module  210  can recognize which channels of the data acquisition modules  208  are active, and the type of data (e.g., representative of side scatter blue light, side scatter red light and so on) that the data from each channel represents, as discussed in more detail below. 
     The master data acquisition module  210  further provides and receives data to and from the droplet control module  222  and the fluidics control module  224  to control the operation of the flow cytometer  100  in the manner described above. For example, the master data acquisition module  210  can receive high-speed clock data from the droplet control module  222  that gives the master data acquisition module  210  a time reference as to the rate of drop formation (e.g., 50 thousand drops per second). Master data acquisition module  210  can use this time base to synchronize a direction command signal which can be, for example, a four bit binary code, that the master data acquisition module  210  sends to the droplet control module  222  so that the droplet control module  222  can control the charging unit  147  (see FIG. 2) as appropriate to achieve the desired charging of the appropriate droplets containing a cell or particle of interest. By charging the droplet with the appropriate charge, the droplet control module  222  thus controls the amount and direction of deflection that the deflection plates  1142  and  1144  (see FIG. 2) deflect the charged droplet. The deflection plates  1142  and  1144  are included among the sorting hardware  235  shown in FIG.  11 . The droplet can be deflected, for example, to be received in one of any suitable number (e.g., sixteen) collection vessels  1142 ,  1146  and  1150 . 
     In addition, the master data acquisition module  210  can receive data from the processor  194  that has been acquired by, for example, detectors  120 ,  1156  and  1160  that provide information concerning the status of the break-off point  112  (see FIG. 1) as well as information pertaining to the droplet sorting. Based on this data, the master data acquisition module  210  can provide control signal to the droplet control module  222  to control, for example, drop delay, droplet formation and so on as discussed above with regard to FIGS. 1 and 2, processor  194  can further control the droplet control module  222  to control, for example, a cooling module  234  and an aerosol management module  236  to control the temperature of the sorted sample, for example, as well as to control sorting and aerosol containment management and safety devices in the flow cytometer  100 . It is also noted that the fluidics control module  224  can control the valve and pump drivers  226 , the agitation module  228 , the temperature control module  230  and the multiport valve HPLC  232  to control the temperature of the fluid sample and sheath fluids, to agitate the sample in the sample reservoir  106  (see FIG.  1 ), and to control the flow of fluids in the flow cytometer  100 . 
     It is further noted that the flow cytometer  100  need not include all of the electronics shown in FIG.  11 . For example, if the flow cytometer  100  is not equipped to perform droplet sorting, certain components shown in FIG. 11 can be omitted. As shown in FIG. 12, the hardware of the data acquisition unit  190  and status and control unit  192  can consolidated into a data acquisition unit  190 - 1 . The components of the data acquisition unit  190 - 1 , such as the processor  194 , data acquisition modules  204  and master data acquisition module  210  operate in a manner similar to those described above with regard to FIG.  11 . However, the data acquisition module  190 - 1  includes an SCI controller  238  which performs the operations performed by status and control I/F unit  192  shown in FIG. 11, such as controlling the driving voltages of the lasers  122  and power and temperature sensor module  218  which operates as described above. The SCI controller  238  further controls operation of the driving voltage of detectors  186  in a manner described below, and receives and processes the mirror and filter assembly position information received from the emission block  140  in a manner similar to the emission ID module  220  described above. 
     The operation of the above components in relation to the operation of flow cytometer  100  will now be described. As discussed above, flow cytometer  100  will typically employ more than one laser  122  to sample more than one type of cell or particle of interest, or more than one characteristic of a cell or particle of interest. The following discussion will assume that the flow cytometer  100  includes four lasers  122 , each emitting light having a different wavelength. 
     As discussed above and as shown conceptually in FIGS. 13-16, if the flow cytometer  100  includes four lasers  122 - 1  through  122 - 4 , then the flow cytometer  100  will include four corresponding fiber optic cables  130 - 1  through  130 - 4  that feed the respective side-scatter laser lights to the respective emission blocks  1401  through  140 - 4 . As further shown, the laser light emitted from these respective lasers  122 - 1  through  122 - 4  strike respective interrogation points  124 - 1  through  124 - 4  on the fluid stream. In this example, the interrogation points are displaced by about  133  micrometers along the direction of flow of the fluid stream, which translates into a spacing of about 22 microseconds for a fluid stream flowing at a rate of 6 meters per second. As shown in FIG. 16, this spacing also permits inter-laser mixing to occur. For example, the side scatter laser light from interrogation point  124 - 3  can enter the fiber optic cable  130 - 4  dedicated to receive side scatter laser light from interrogation point  124 - 4 . The mirror assemblies  174  and filter assemblies  180  in the emission blocks  140 - 1  through  140 - 4  can be configured to eliminate any light of undesired wavelengths as discussed above, in the event that unwanted inter-laser mixing occurs. 
     Further details of the relationship between the detectors  186 , a data acquisition module  208 , master data acquisition module  210 , processor  194  (real time CPU) and the work station will now be described with regard to FIGS. 17-21. In this arrangement, each data acquisition module  208  can receive data from four detectors  186  from any of the emission blocks  140 . For purposes of this discussion, data acquisition module  208  is configured to receive side scatter laser light that has been generated by the four different wavelength lasers  122 - 1  through  122 - 4 . 
     As illustrated, the analog data signals from the detectors  186  are input to their respective data acquisition module  208  as 2 MHz bandwidth (BW) analog signals. Further details of the data acquisition module are shown in FIGS. 18 and 19. That is, the signal from each detector  186  is input to a respective analog-to-digital (A/D) converter  240  where the analog data is converted into digital data. As illustrated, each A/D converter  240  have differential inputs to maximize common mode rejection of the received analog signals. The frequency (e.g., 10 MHz) at which the A/D converters  240  are operating enable the A/D converters  240  to take multiple samples (e.g., 10 or 20, or more) of the waveform as shown in FIG. 21 in real-time or near real-time. As indicated, the intensity of the signal will typically increase to a maximum when the particle or cell of interest is at the center of the interrogation point, and then drop-off to a minimum as the cell passes out of the interrogation point  124 . Accordingly, each individual sample of the waveform will have a value representing the characteristic (e.g., height) of that sampled portion of the waveform. This sampling of the entire or substantially the entire waveform improves the details at which the waveforms can be analyzed and compared, for example, to other waveforms representative of other events. Accordingly, this sampling allows for a more detailed sampling of the characteristics of each event. 
     The digital data output by each A/D converter  240  is provided to a respective delay circuit  244  which imposes a respective delay on the digital data as described in more detail below. As shown, for example, in FIG. 19, the delay imposed by each delay circuit  244  is set to compensate for the delays between the interrogation points  124 - 1  and  124 - 4  as shown in FIG. 15 or, in other words, to compensate for the time delay that occurs between when the side scatter light representative of a particle or cell of interest at interrogation point  124 - 1  is received by a detector  186  in emission block  140 - 1  and when the side scatter light representative of that particle or cell of interest reaching interrogation points  124 - 2  through  124 - 4  are subsequently received by detectors  186  in their respective emission blocks  140 - 2  through  140 - 4 . 
     The digital data from each delay circuit  244  is provided to a respective channel field programmable gate array (FPGA) circuit  246 , which provide the data to a Super Harvard Architecture Computer (SHARC) unit  248 . It is noted that each channel FPGA circuit  246  can process the characteristics of the data samples to produce data representing a single characteristic of the analog waveform, such as the width or height of the waveform, if desired, instead of passing all of the samples (e.g., 20 samples per waveform as discussed above) to the SHARC unit  248 . Also, the channel FPGA circuits  246  will add a time stamp to their respective data prior to passing the data to the SHARC  248 . Under the control of a programmable logic device, versa-module Eurocard interface (PLD VME I/F) unit  250  and a trigger FPGA unit  252 , the SHARC unit  248  provides the digital data via a link port  211  to the master data acquisition module  210  as indicated. 
     Specifically, prior to running the flow cytometer  100  to detect the events, the workstation  204  can download channel data to the trigger FPGA unit  252  of each data acquisition module  208  via the hub  202  and processor  194 . This channel data indicates to the channel FPGA circuits  246  whether they should collect the data from their respective delay circuits  244 , that is, whether they are receiving data on an active channel. The channel data further indicates to the trigger FPGA unit  252  when the trigger FPGA unit  252  should trigger the SHARC  248  to transfer the event data received in parallel from the channel FPGAs  246  to the master data acquisition module  210  via the link port  211 . 
     Details of the master data acquisition data module  210  are shown FIG.  20 . That is, the master data acquisition module  210  includes a multi-SHARC unit  256  that includes a SHARC event classification unit  258 , a SHARC drop classification unit  260  and a SHARC event assembly unit  262 , the details of which are described below. The master data acquisition module  210  further include a gate FPGA  264 , a logarithmic look-up table  266 , and a data FIFO unit  268 . Furthermore, the master data acquisition data module  210  includes an FPGA module  270  that includes a drop control FPGA  272  and a trigger FPGA  274 . The master data acquisition module further include a PLD VME I/F  276 . The details of these components are described below. 
     Specifically, prior to running the flow cytometer  100  to detect the events, the workstation  204  can download channel data to the trigger FPGA unit  274  of master data acquisition module  210  via the hub  202  and processor  194 . This channel data indicates to the trigger FPGA units  252  of each data acquisition module  208  whether they should trigger their respective SHARC  248  to transfer the event data received in parallel from the channel FPGAs  246  to the master data acquisition module  210  via their respective link port  211 . That is, when the trigger FPGA units  252  provide their respective indications to the trigger FPGA unit  274  indicating that event data has been received on their appropriate respective channels, the trigger FPGA unit  274  will signal the trigger FPGA units  252  to trigger their respective SHARCs  248  to transfer the event data received in parallel from the channel FPGAs  246  to the master data acquisition module  210  via the link port  211 . 
     When the master data acquisition module  210  receives the event data via the linkports  211 , the event data is input to the SHARC event assembly  262 . The SHARC event assembly  262  assembles the data into lists, tables or buffers based on their time-stamp that has been added by the channel FPGAs  246 . That is, the SHARC event assembly  262  uses the time stamps to determine which data is associated with which event. 
     If no sorting of cells is to be performed, the SHARC event assembly  262  passes the lists, tables or buffers of data to the data FIFO unit  268 . The data FIFO unit  268  sends the lists, tables or buffers of the data via the VME bus  254  to the processor  194 . The processor  194  can then provide the data to the work station  204  for further display in, for example, a scatter plot diagram, a graphical representation, and so on. 
     However, if cell sorting is to be performed, data received by the SHARC event assembly unit  262  is processed by the SHARC event classification unit  258  and SHARC drop classification unit  260 . For example, the flow cytometer  100  can be run to sample a portion of the cell sample to therefore provide initial sample data to the work station  204  as discussed above. The work station  204  can display the detected events on, for example, a scatter plot which can be reviewed by the operator. The operator can select certain cells of interest to be sorted by selecting, for example, a region on an interactive display screen of the work station  204 . The work station  204  can then pass the desired cell sorting data to the master data acquisition module  210  via hub  202  and processor  194 . The master data acquisition module  210  stores this cell sorting data in, for example, the logarithmic lookup SRAM  266 . 
     When the operator reactivates the flow cytometer  100  to continue processing the sample, the SHARC event classification unit  258  and SHARC drop classification unit  260  can access the data in the logarithmic lookup SRAM  266  in real-time or near real-time to determine which data received by the SHARC event assembly unit  262  represents cells to be sorted. The SHARC event classification unit  258  and SHARC drop classification unit  260  can then provide signals to the Drop Control FPGA  272  which can provide the appropriate direction command signal to the droplet control module  222  so that the droplet control module  222  can control sorting as discussed above. The SHARC event assembly  262  can then pass the lists, tables or buffers of data to the data FIFO unit  268 , which sends the lists, tables or buffers of the data via the VME bus  254  to the processor  194  as discussed above. The processor  194  can then provide the data to the work station  204  in real-time or near real-time for further display in, for example, a scatter plot diagram, a graphical representation, and so on. 
     Additionally, the event data can be used to process the sample waveforms in various ways. For example, the above system, in particular, the controller  212  (FIG. 11) or SCI controller  238  (FIG. 12) can adjust system can adjust the voltages applied to the detector  186  (PMTs) to adjust the relative zero point of the PMT detector  186 . For example, as shown in FIG. 22, the PMT and circuit board  188  includes a DC high voltage power supply  280  that provide the driving voltage to the PMT socket  282  that drives the PMT. The current from the PMT generated upon, for example, detection of side scatter light as described above is converted by a current voltage converter  284  so that the voltage signal is provided to the respective channel data acquisition module  208  as described above. Voltage control and serial control signal are provided from the PMT controllers  214  in, for example, the respective channel data acquisition module  208  to adjust the base voltage of the PMT, to therefore adjust the relative zero point of the PMT. Accordingly, this adjustment can be used to perform the gain adjustment as shown, for example, in FIG. 23 to increase the height of the smaller waveform to be consistent with the heights of the red and blue waveforms. 
     In addition, as shown in FIGS. 24-27, the SHARC event assembly  262  the master data acquisition module  210  can compare the entire sample wave form of data obtained from different detectors  186  and can perform different types of processing functions on this data in a real time or near-real time basis. For example, the event data representative of the red side scatter light signal received at time T can be delayed so that it can be compared with the event data representative of the blue side scatter light signal received at time T+1 as shown in FIG. 24, so that the signals can be time correlated as shown in FIG.  25 . Furthermore, as shown in FIGS. 26 and 27, the data signals can be processed to remove crosstalk that can occur as discussed above. In this event, the blue data represented as the “blue+red crosstalk” can be processed to remove a percentage of the red signal that is affecting the magnitude of the blue data, so that the magnitudes of the blue and red data can be made similar for comparison as shown in FIG.  27 . 
     Although only a few exemplary embodiments of the present invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims.