# Patent

## Title
System and method for measuring particles in a sample stream of a flow cytometer using low-power laser source

## Abstract

A system and method for analyzing a particle in a sample stream of a flow cytometer or the like. The system has a light source, such as a laser pointer module, for generating a low powered light beam and a fluidics apparatus which is configured to transport particles in the sample stream at substantially low velocity through the light beam for interrogation. Detectors, such as photomultiplier tubes, are configured to detect optical signals generated in response to the light beam impinging the particles. Signal conditioning circuitry is connected to each of the detectors to condition each detector output into electronic signals for processing and is designed to have a limited frequency response to filter high frequency noise from the detector output signals.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser. No. 13/966,624 filed Aug. 14, 2013 (now allowed), which is a continuation of U.S. patent application Ser. No. 12/903,003, filed Oct. 12, 2010, and later issued as U.S. Pat. No. 8,564,776. U.S. patent application Ser. No. 12/903,003 is a continuation of U.S. patent application Ser. No. 11/593,312, filed on Nov. 3, 2006, and later issued as U.S. Pat. No. 7,835,000. The foregoing applications and patents are incorporated by reference herein in their entireties for any and all purposes.


STATEMENT OF GOVERNMENT RIGHTS
This invention was made with government support under Contract No. DE-AC52-06NA25396 awarded by the U.S. Department of Energy. The government has certain rights in the invention.


TECHNICAL FIELD
Embodiments are generally related to sensor methods and systems. Embodiments are also related to flow sensor analyzers, such as flow cytometers that move particles in a flowing fluid through a sensing region where multiple independent optical measurements are made on the particle. Embodiments are also related to portable systems for interrogating particles in sample streams of flow cytometers or the like.
BACKGROUND
Flow cytometry is a technology in which multiple physical and optical characteristics of single small or microscopic particles, such as cells or microspheres, are analyzed as they flow in a fluid stream through one or more beams of light. Flow cytometry is an integral technology in nearly every bio-medical discipline including diverse biological assays in clinical settings. Additionally, flow cytometry is an important analytical platform to perform biological point detection, bio-surveillance, and forensic analysis in support of homeland defense.
In the flow cytometer, particles are carried to the light beam intercept in a fluid stream. When particles pass through the light beam, they scatter the light and any fluorescent molecules present on or in the particle fluoresce. These resulting optical signals are directed by means of optics to appropriate detectors which generate electronic signals proportional to the optical impulses striking them. These electronic signals are processed to gather data on each particle or event and subsequently analyzed to provide information about the sample. Various particle properties, such as particle size, granularity and fluorescence intensity, can be determined by a flow cytometer recording how the particle under interrogation scatters the incident light beam and emits fluorescence.
Flow cytometers typically incorporate expensive lasers with highly stable outputs in order to obtain the high detection sensitivity and resolution necessary in many applications. Unfortunately, the size and expense of typical flow cytometers currently restricts their use to clinical and laboratory environments. Such flow cytometers cost more than 30,000 US dollars to purchase. For many potential users of flow cytometers, instrumentation size and cost are important considerations that may limit the acceptance of these systems in broader applications.
There is a continuing need to provide improved systems and methods for measuring particles in a sample stream of a flow cytometer or other flow based analyzers which can be implemented at low cost and with reduced infrastructure requirements, such as electrical power or other laboratory-based requirements. Reducing size and cost nearly always speeds acceptance and adoption of new technology.
SUMMARY
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
It is, therefore, one aspect of the present invention to provide for improved sensor methods and systems.
It is another aspect of the present invention to provide for improved methods and systems for measuring particles in a flow stream of a flow cytometer or like analyzer.
It is a further aspect of the invention to provide low cost methods and systems for measuring particles in a flow stream of a flow cytometer or other flow based analyzer.
The aforementioned aspects of the invention and other objectives and advantages can now be achieved as described herein.
According to one aspect, a system for interrogating a particle in a sample stream of a flow cytometer or the like has a laser light source for generating a low power light beam and a fluidics apparatus for transporting the particle in the sample stream at substantially low velocity resulting in extended transit times through the light beam for interrogation thereof. Also included in the system are one or more detectors for detecting optical signals resulting from the light beam impinging on the particle and signal conditioning circuitry, operably coupled to the detector(s), for conditioning output signals from the detector(s) into electronic signals for processing thereof. The signal conditioning circuitry can be a low pass filter circuitry for filtering high frequency noise from the detected optical signals.
Advantageously, the fluidics apparatus transports the particle in the sample stream at substantially low velocity through the focused laser beam resulting in extended transit times to increase sensitivity and the signal conditioning circuitry is configured to low pass filter the resulting detected optical signals, which permits high-sensitivity measurements to be made with simplified circuitry and low-cost components, such as a laser pointer and miniature detectors.
The light source can be for example a low power laser pointer module such as a diode pumped solid state (DPSS) green laser. The transit time of the particle through the light beam can be of the order of 100 microseconds or more. The low pass filter circuitry can have a maximum cut off frequency of about 10 KHz.
The detector can be for example a Photomultiplier tube (PMT), photodiode, APD or hybrid detector. The signal conditioning circuitry can comprise a pre-amplifier stage coupled to the output of a PMT detector with the low pass filter circuitry integrated in the pre-amplifier stage. The pre-amplifier can be for example a high input impedance voltage follower circuit coupled to a limited band width inverting amplifier.
The fluidics apparatus can include a hydrodynamically focused flow chamber, an acoustically focused flow chamber or an unfocused flow chamber. The flow chamber can be coupled to a slow flow delivery system for transporting the particle through the light beam with the substantially low velocity resulting in extended transit times (>100 microseconds).
According to another aspect, a system for interrogating a particle in a sample stream of a flow cytometer or the like has a low powered laser pointer for generating a light beam and a fluidics apparatus for transporting the particle in the sample stream at substantially low velocity through the light beam for interrogation thereof. Also, the system includes one or more detectors for detecting optical signals resulting from the light beam impinging on the particle and signal conditioning circuitry, operably coupled to the detector(s), for conditioning output signals from the detector(s) into electronic signals for processing thereof. The signal conditioning circuitry can include low pass filter circuitry for filtering high frequency noise from the detected signals.
According to another aspect, a method for analyzing a particle in a sample stream of a flow cytometer or the like comprises generating a low power light beam; transporting the particle at substantially low velocity in the sample stream through the light beam for interrogation thereof, detecting light signals generated in response to the light beam impinging on the particle, and signal conditioning the detected light signals into electronic signals for processing thereof, the step of signal conditioning comprising filtering high frequency noise from the detected light signals.
By transporting the particle at substantially low velocity through the light beam to increase sensitivity and low passing filtering the resulting detected optical signals, high detection sensitivity and resolution can be achieved using lower power and less stable light beams than those typically used in the flow cytometers of the prior art. The system permits the use of low cost and compact lasers whilst providing the detection sensitivity and resolution demanded by many applications.
The transit time of transporting the particle through the low power light beam can be about 100 microseconds or more. The high frequency filtering has a maximum cut off frequency of about 10 KHz. The low power light beam can be generated by a laser pointer module. The low power light beam can be detected by a PMT. The step of transporting the particle can include hydro-dynamically or acoustically focusing the sample stream through a flow chamber.



BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the present invention and, together with the detailed description of the invention, serve to explain the principles of the present invention.
FIG. 1 illustrates a block diagram of a system for interrogating a microscopic particle in a sample stream of a flow cytometer according to a preferred embodiment;
FIG. 2 illustrates the fluidic apparatus of FIG. 1 in more detail;
FIG. 3 illustrates a cross-sectional view of the flow cell taken along line A-A′ of FIG. 2;
FIG. 4 illustrates a schematic circuit diagram of a high input impedance limited bandwidth pre-amplifier of the system depicted in FIG. 1;
FIG. 5 illustrates a flow diagram outlining a method for interrogating a particle in a sample stream of a flow cytometer according to a preferred embodiment;
FIG. 6 illustrates a histogram of the Side Scatter (SSC) amplitude parameter on 10000 simulated events that demonstrates the stability of the laser pointer by virtue of the 1.24% coefficient of variation (CV) of the distribution;
FIG. 7 demonstrates the stability of the laser pointer by displaying SSC peak (amplitude) versus time during the entire 4 minutes required to collect 10000 simulated events;
FIG. 8 illustrates high-resolution and high-sensitivity performance on 6 populations of microspheres in the fluorescence area histogram, obtained by analyzing a mix of 1.87 and 2.8 μm blank (i.e. non-fluorescent) microspheres added to the RCP-20-5 2.1 μm diameter microsphere set using the system of FIG. 1;
FIG. 9 illustrates high-resolution SSC performance in a contour plot of SSC peak Vs fluorescence area, obtained by analyzing a mix of 1.87 and 2.8 μm blank microspheres added to the RCP-20-5 2.1 μm diameter microsphere set using the system of FIG. 1;
FIG. 10 displays a contour plot of SSC Peak Vs SSC Width obtained by analyzing the RCP-30-5A microsphere mixture with the system of FIG. 1, which illustrates the region of interest around the main peak used to gate the fluorescence data in FIGS. 11 and 12.
FIG. 11 illustrates excellent resolution of all 8 microsphere populations (in the 16-bit fluorescence peak data) obtained by analyzing the RCP-30-5A microspheres sample using the system of FIG. 1;
FIG. 12 illustrates excellent resolution of all 8 microsphere populations (in the 32-bit area data) obtained by analyzing the RCP-30-5A microspheres sample using the system of FIG. 1; and
FIG. 13 demonstrates the linearity of the system in a graph of fluorescence area Vs the calibrated intensity of the RCP-30-5A microspheres (in units of Mean Equivalent Soluble Fluorophore for phycoerythrin MESF-PE) obtained by measuring the RCP-30-5A microspheres using the system of FIG. 1.



DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
The illustrative embodiment provides an approach to interrogating microscopic particles in a sample stream of a flow cytometer, or other systems that use the flow cytometry paradigm, using a method and a system which enables a compact and inexpensive flow cytometer to be implemented, while having high detection sensitivity and resolution comparable to that of prior art flow cytometers.
Referring to FIGS. 1 and 2 of the accompanying drawings, which, respectively, illustrate block diagrams of the optical-electrical circuitry and fluidic circuitry of the system for measuring particles in a sample stream of a flow cytometer according to one embodiment. The system 1 has a light source 2 for generating a low powered light beam 3 and a fluidics apparatus 26, which is configured to transport particles in the sample stream 30 at a substantially low velocity through the light beam for interrogation. Detectors 4, 5 are configured to detect optical signals generated in response to the light beam 3 impinging the particles. Signal conditioning circuitry 6, 7 can be connected to each of the detectors 4, 5 to condition each detector output 81A, 81B into electronic signals 82A, 82B for processing and can be designed to generate a limited frequency response in order to filter high frequency noise from the detector output signals.
As will be explained in more detail below, configuring the system to transport particles in the sample stream at substantially low velocity through the light beam and filtering high frequency noise from the detector output using limited bandwidth signal conditioning circuitry, permits a compact and inexpensive diode pumped solid state laser to be used in a flow cytometer while maintaining high detection sensitivity and resolution using miniature detectors that require minimal support circuitry.
In the illustrative embodiment of the system of FIG. 1, the light source 2 for generating the low powered light beam is a low powered laser, typically having a 10 mW or less output. For example, the light source can be an inexpensive diode pumped solid state (DPSS) laser, such as a laser-pointer module. One example of a suitable laser pointer is an OEM version of a commercially available laser pointer (532 nm, 3.0 mW, model GMP-532-5F3-CP) from LaserMate Group, Inc. which emits a green laser beam. The laser pointer operates on 2.1-3.0 VDC and has a rated output of 3-5 mW. A power source 40, such as Tektronix power supply (model PS281), provides the DC voltage (typically 2.3 VDC @270 ma). Alternatively, a battery or very simple line-voltage operated power supply can be utilized to power the laser pointer with no observable degradation of the system performance. The laser head itself is a very compact device with reasonable manufacturer's specifications (TEM11, <1.4 mrad beam divergence, M2<2, and 5% output power stability). Advantageously, the laser pointer module is inexpensive, compact and results in the entire excitation source using less than 1 W of power, which greatly increases instrument portability.
Optics 9-11 are configured to direct the laser beam 3 to the analysis region 35 of the system, that is, the point where the stream 30, which is flowing from out of the page of FIG. 1, intercepts the light beam 3. A half-wave plate 9 and polarizing beam splitter 10 positioned between the light source 2 and a mirror 11, serve to attenuate the laser beam (to the desired power level). Mirror 11 is configured to reflect the beam 3 onto a focusing lens 12, which focuses the beam to a high light flux (10 μm diameter spot) at the analytical region 35 of the system.
Detectors 4, 5, which are configured as side-scatter light (SSC) and fluorescence (FL) detectors, respectively, are aimed at the analytical region 35 of the system. Collection lenses 13, 14 are arranged on either side of the flow cell 25 to collect and concentrate light onto the detectors 4, 5 through respective filters 15 (SSC), 16 (FL).
In the illustrative embodiment of the system of FIG. 1, the detectors 4, 5 are photomultiplier tubes which detect the optical impulse generated when the particle passes through the light beam 3 and produces a current signal proportional to the intensity and duration of the impulse. The photomultiplier tubes utilized in this example are miniature Hamamatsu 5783 or 6780 Photomultiplier tubes. These multi-alkali metal-package detectors typically have a gain of about 106 and radiant sensitivity of 60-80 mA/W in the 400-700 nm range, depending on the specific model used, and are operable with a high voltage of 250 to 1000V The exact voltage utilized depends on the individual PMT, the laser output power, and the intensity of the signals being measured (fluorescence or light scatter), which in turn is related to the particles being analyzed. Other low cost and compact light detectors can alternatively be employed. The detectors could be photodiodes, avalanche photodiodes or any small detector capable of detecting light emission. Utilizing low cost, compact PMTs further reduces the cost and size of the system.
Signal conditioning circuitry 6, 7 consists of pre-amplifiers connected to the anode of PMT detectors 4, 5 to provide high input impedance and limited bandwidth. The primary function of a pre-amplifier is to convert a current signal to a voltage signal for further use by data acquisition electronics. A power supply module 19 is assembled and electrically coupled to the detectors 4, 5 and signal conditioning circuitry 6, 7 to provide typically + and −5 VDC (200 ma) for the pre-amplifiers 6, 7, and +15 VDC (200 ma) to power and control the high-voltage 17, 18 for each PMT 4, 5. Further signal amplification, if needed, can be provided in a data acquisition system 80 which is coupled to the signal conditioning circuitry outputs for collection of the conditioned PMT output signals 82A, 82B.
Signal conditioning circuitry 6, 7 is designed to have limited bandwidth to filter any high-frequency noise in the PMT detector outputs. As shown in FIG. 4, which is a schematic circuit diagram of an example of the pre-amplifier circuitry, the pre-amplifier circuitry has a high input impedance voltage follower 60 coupled to a limited bandwidth inverting amplifier 61 in each of the pre-amplifiers 6, 7. The inverting amplifier 61 has an effective bandwidth of approximately 10 kHz and amplification factor of 3.3. Voltage follower 60 has a pair of resistors 62, 63 serially connected between the non-inverting input 65 of an operational amplifier 50 and ground 70 and provides high impedance to the output 71 of a respective detector 4, 5 which output is connected across resistor 63. The voltage follower output 72 is connected via input resistor 66 to the inverting input 69 of an operational amplifier 51 of the amplifier 61. A resistor 68 and capacitor 67 (RC) circuit is arranged in the feedback path of the operational amplifier between the operational amplifier output and the inverting input 69 and is selected to have an RC constant to provide a maximum cut off frequency of about 10 kHz.
Low pass filters other than the limited-bandwidth amplifier 61 can be utilized in system 1 to achieve the desired high-frequency filtering and need not necessarily be integrated in the pre-amplifier circuit. For example, the low pass filter could alternatively be implemented in signal conditioning circuitry after the pre-amplifier.
Incorporating a limited bandwidth pre-amplifier (effectively a low-pass filter) 61 in the signal conditioning circuitry is advantageous in that the high-frequency noise produced by low cost, relatively unstable light sources, such as laser pointer module, can be filtered out from the detected optical signals. By extending the transit times of the particles through the light beam to increase sensitivity and low passing filtering the resulting detected optical signals, high detection sensitivity and resolution can be achieved using lower power and less stable light beams than those typical utilized in the flow cytometers of the prior art. The system permits the use of low cost and compact lasers whilst providing the detection sensitivity and resolution demanded by many applications. Slow flow, that generates extended transit times of the particles through the focused light beam, permits high-sensitivity measurements to be made with simplified circuitry and low-cost components, such as the laser pointer and miniature detectors, to create a portable battery powered system with high performance. Both of these components have minimal power requirements which makes it possible to operate the system off of a battery or simple power supply such as a “wall wart” and 3-terminal voltage regulators with resulting performance is at least as good as the state-of-the-art systems currently available.
In typical flow cytometry data processing the pulse (or impulse) caused by a particle passing through the laser beam is characterized by 3 measurements: amplitude (or peak), duration, and area, and it is this correlated data collected by the data acquisition system that can be plotted as 1D histograms (as in FIG. 6). The correlated data can also be displayed in 2D as dot plots or contour plots (as in FIG. 9), or other displays depending on the software utilized. Specific regions on these plots can be sequentially separated by a series of subset extractions which are termed gates. Specific gating protocols exist for diagnostic and clinical purposes. The plots are often made on logarithmic scales.
A data acquisition system 80 is configured to collect conventional flow cytometric data files in which the event-based parameters of height, width, and area were collected from the electronic pulses derived from detectors 4, 5 and pre-amps 6, 7. Custom hardware boards are configured to convert the pre-amp output (2 V peak-to-peak) into a 14-bit digital data stream using a free running 14-bit 40 MS/sec ADC (Analog Devices—ADS5421Y). The output from each pre-amp 6, 7 is connected directly to separate ADC inputs. A field programmable gate array on the custom board captures the correlated digitized waveforms and sends them to a commercial digital signal processor board (OrSys—microline C6211CPU). The digital signal processor (Texas Instruments—TMS320C6211) extracts the pulse height, pulse area and pulse width parameters, sending the list-mode results to the host computer over FireWire (IEEE1394). The pulse parameters are recorded in FCS 3.0 data files with 24 bits for area, 16 bits for peak, 12 bits for width, and 28 bits for time (1 msec resolution). The pulse height, area and width were recorded in Flow Cytometry Standard (FCS) v.3.0 data files, which is an industry standard data file format for flow cytometry. Data acquisition techniques for flow cytometry are known in the art and will not be described in any more detail here.
Referring now in more detail to the fluidic apparatus 26 of FIG. 2, the fluidic apparatus includes a flow chamber 25, which in this particular embodiment is an optical flow cell 25, and a slow-flow sample delivery system 37 configured to pass the sample through the flow cell 25 at substantially low velocity ( ̃5 cm/sec) such that the particles have extended transit times across the beam 3. Typically, these extended transit times are about 100 microseconds or more. Advantageously, by the fluidic apparatus extending the particles' transit times across the beam 3, the particle under interrogation has a longer residence time in the analytical region 35 thereby increasing the system detection sensitivity and resolution for a given light beam power so that relatively inexpensive miniature PMTs provide adequate performance.
The slow-flow sample delivery system 37 has gravity driven sheath flow from a sheath bottle 27, which is suspended above the flow cell 25 such that sheath fluid is fed to the flow cell via a sheath delivery line 28 connected between the lower end of the bottle and a side port of the flow cell assembly. A sample delivery tube or capillary 31 is connected to the bottom end of the flow cell 25 such that the sample to be interrogated can be delivered under pressure to the flow cell. In the center of the flow cell is a 250×250 μm square flow channel 30 and the sample delivery tube 31 occludes about 75% of the square flow channel, as best shown in FIG. 3, which illustrates a cross-sectional view of the flow cell taken along line A-A′ of FIG. 2. The capillary 31 has a 40 μm inner diameter ID 31A as indicated by the white circle in FIG. 3. The capillary, with an outer diameter of 245 μm, is inserted about 5 mm into the square flow channel 30, and this occlusion results in relatively low volumetric flow rates of sheath for focusing of particles. The low sheath rate results in extended transit times through the laser beam. A waste line 32 couples the upper end of the flow channel (of the flow cell) to a waste reservoir 33 that has a fluidic head to prevent oscillation in fluid flow rates. The waste line is connected to the flow cell upper end (cuvette) 39 by a short length of soft silastic rubber tubing 41 that is gently pressed up to the top of the cuvette and held in place by a metal arm. By adjusting the relative heights of the sheath supply bottle 27 and the waste reservoir 33, the transit time of single particles across the laser beam 3 can be varied from about 100 microseconds to milliseconds. The direction of flow in the flow cell is from bottom to top to help clear bubbles, as show in FIG. 2, and flowing out of the page in FIG. 1. The sample is introduced through the sample tube 31 and into flow cell 25 for focusing via the sheath stream. The sample is pushed in via pressure, while the sheath is gravity fed in the corners of the flow channel around the capillary. The sheath bottle can be raised above the flow cell (for gravity delivery) and pressurized to set the transit time.
In the illustrative embodiment of the system of FIG. 1, the flow cell 25 is a hydrodynamically focused flow cell. However, any particle focusing technique (e.g. acoustic, dielectrophoretic, etc. . . . ) that results in extended transit times (100 microseconds to milliseconds) can be employed. Furthermore, any flow chamber suitable for passing the sample stream through the light beam could be employed. All that is required is that the particle passes (centered in the flow channel) through the light beam 3 with sufficiently slow velocity so that the low powered light beam striking the particle results in optical signals which are detectable with sufficiently high sensitivity to enable the use of low powered laser beams.
A method 100 for interrogating a particle in a sample stream of a flow cytometer according to one embodiment will now be described with reference to FIG. 5 which is a flow diagram outlining the steps of the method 100. The method 100 can, for example, be implemented in the system of FIG. 1. As a general overview, initially a light beam is generated as indicated in step 101 of FIG. 5. In the system of FIG. 1, the light beam 3 is a low power light beam generated from a laser power module. The particle is transported in a sample stream through the generated light beam at substantially low velocity (step 102). For example, this step can be implemented in the system of FIG. 1 by hydro-dynamically or acoustically focusing the sample stream through the flow cell 25 and delivering the sample using the slow flow delivery system 37 to transport the particle through the laser beam 3 with a velocity which is sufficiently low to extend the transit time of particle through the light beam to about 100 microseconds or more.
Thereafter, optical signals, generated in response to the light beam impinging on the particle, are detected (step 103 of FIG. 5). This latter step can be performed by the PMT detectors 4, 5 of the system of FIG. 1. High frequency noise is then filtered from the detected light signals (step 104) for processing (step 105). In the system of FIG. 1, step 104 is implemented by means of the limited bandwidth inverting amplifier 61 of the signal conditioning pre-amp and step 105 is performed by the data acquisition system of the system of FIG. 1.
Experimental Examples and Results
Specific results that have been obtained using the system and method of interrogating a particle in a sample stream of a flow cytometer according to the illustrative embodiments will now be described in which the interrogated samples were fluorescent calibration microsphere sets. The microsphere samples were concentrated 5-10× via centrifugation prior to use to compensate for the volumetric flow rate in this example ( ̃0.3-0.6 μl/minute) due to the narrow bore (40 μm ID) quartz capillary tubing utilized to deliver the sample to the flow cell. All microsphere samples were purchased from Spherotech Inc. (Libertyville, Ill.), and included: Rainbow Calibration Particles RCP-30-5A (8 peaks), RCP-20-5 (4 peaks), CP-15-10 (blank 1.87 μm dia.), and CP-25-10 (blank 2.8 μm dia.). The specific type used is given in the text for each example.
In these examples, the laser 2 was the aforementioned green laser pointer module (532 nm, 3.0 mW, model GMP-532-5F3-CP) from LaserMate Group. The miniature Hamamatsu PMTs described above were employed as detectors 4, 5, which were mounted in optical tubes that held bandpass filters 14, 15 and the light collection lenses 12, 13 in a light-tight assembly surrounding the flow cell 25. The high NA aspheric lenses 12, 13 collected light from the flow cell, which was passed through band pass filters to the detectors 4, 5. A 515-545 nm band pass filter was used in the side-scatter (SSC) light channel while a 565-605 nm band pass filter was used in the fluorescence channel.
In these examples, high purity water served as the sheath fluid which was gravity fed from a 2-L sheath bottle 27 suspended about 20 cm above the flow cell 25, which was a 2 cm long fused-silica cuvette 2.5×2.5 mm cross-section with a 250×250 μm flow channel. Polyethylene tubing (0.75 mm ID, about 1.5-m long) was utilized as the waste line 32. A 3-m long continuous column of water, from the sheath bottle 27 to the waste reservoir 33, stabilized the slow flow through the gravity-driven system. Sample solutions were driven from a 0.5-ml Eppendorf tube by nitrogen gas pressure through the sample delivery tube 31 which was a Polymicro Technologies (Phoenix, Ariz.;) fused silica capillary tube (40 μm ID, 245 μm outer diameter (OD),  ̃30-cm long) inserted 4-5 mm into the 250 μm square channel of the flow cell. Sheath fluid flowed around the capillary, in the corners of the channel, vertically (from bottom to top in FIG. 2; out of the page, in FIG. 1), which facilitated dislodging bubbles from the flow cell. The inserted tip of the capillary, ground to a 14° taper by New Objective, Inc. (Cambridge, Mass.), is positioned 300 μm below the interrogation region 35. The small-bore capillary served two purposes. First, it predisposed the fluidic system to slow-flow because it occluded about 75% of the flow channel. Second, its flow resistance permitted control of the sample delivery rate with a sensitive electronic pressure regulator (MiniPR-NC-1500-5-NR; Redwood Microsystems, Menlo Park, Calif.). The elevated sheath bottle 27 was pressurized to provide  ̃2 psig in order to reach a transit time of about 250 microseconds.
The combination of scattered and fluorescent light was collected by the collection lenses 12, 13, filtered by filters 14, 15 and picked up by the detectors 4 (SSC) 5 (Fluorescence), and fluctuations in brightness at each detector were converted to electrical signals which were conditioned by the signal conditioning circuitry, analogue to digital converted and then processed into data. FCS files were analyzed using FCS Express from DeNovo Software.
Laser stability was first measured by slightly misaligning the flow cell so that some of the laser light was refracted into the SSC detector pathway (without any particles in the flow channel). The data acquisition system 80 was electronically triggered to collect 10000 simulated events, recording the pulse height, width and area measurements from the SSC channel. These results, depicted in FIG. 6, indicate that the laser output was stable during the data collection interval and the precision of the measurements is indicated by the coefficient of variation (CV) of the histogram peak, which was 1.24% for both SSC Peak and SSC Area. The laser stability over time is clearly shown by the stability of the scattered light over 4 minutes of collection (FIG. 7).
The pulse data for a series of microspheres were collected using the data acquisition system 80. FIG. 8 demonstrates excellent resolution and sensitivity obtained from analyzing a mixture of the 4 population set of RCP-20-5 microspheres with non-fluorescent 1.9 μm (CP-15-10) and 2.8 μm (CP-25-10) polystyrene microspheres added. The three dimmest microspheres that are not well resolved in the Fluorescence Area histogram (below 104) of FIG. 8, are completely resolved in the contour plot of FIG. 9, a 2D display of SSC versus Fluorescence Area.
The data depicted in FIGS. 10-13 resulted from analyzing the RCP-30-5A microsphere mix with the system depicted in FIG. 1. SSC width versus SSC area are displayed in a contour plot to indicate the gating region used to identify the individual microspheres (see FIG. 10), which are then presented in 1D histograms in FIGS. 11 and 12. FIG. 11 shows baseline resolution of all 8 microsphere populations using the amplitude parameter. FIG. 12 displays exemplary data on the area parameter with all 8 populations baseline resolved over a span of 5 orders of magnitude. As shown in FIG. 13, plotting the means of the fluorescence area peaks (the means for the 8 populations) against the estimated mean equivalent soluble fluorophore (MESF) values measured in units Phycoerythrin (MESF-PE) molecules, provided by the manufacturer, demonstrates the excellent linearity and sensitivity of the system. Extrapolating from this linear fit of fluorescence vs. MESF PE suggests that the system can detect as few as 50 fluorphores of PE per particle.
The components of the system in this example (laser pointer, PMTs and aspheric lenses for focusing and collection optics) cost approximately $1000. Using 1 mW of laser power through the system to analyze RCP-30-5A microspheres resulted in baseline resolution of all 8 peaks and demonstrated detection of  ̃50 fluorophores per particle. With the exception of particle analysis rate and number of parameters, this experimental example of the system has demonstrated comparable performance to that of flow cytometers that use expensive lasers and detectors that cost >$10,000 using components that cost less than $1000.
The aforementioned experimentation and results are for illustration purposes only and are not intended in any way to limit embodiments of the system or method to such an example. The system and method of the illustrative embodiments could be implemented in a range of different applications, such as biomedical diagnostics, homeland defense and point of care devices, to measure other particles and particle parameters. Examples of such particles and measuring parameters are volume and morphological complexity of cells cell pigments, DNA (cell cycle analysis, cell kinetics, proliferation etc.), RNA, chromosome analysis and sorting (library construction, chromosome paint), proteins, cell surface antigens (CD markers), intracellular antigens (various cytokines, secondary mediators etc.), nuclear antigens, enzymatic activity, pH, intracellular ionized calcium, magnesium, membrane potential, membrane fluidity, apoptosis (quantification, measurement of DNA degradation, mitochondrial membrane potential, permeability changes), cell viability, monitoring electropermeabilization of cells, oxidative burst, characterizing multi-drug resistance (MDR) in cancer cells, glutathione, various combinations (DNA/surface antigens etc.). Other examples of such particles and measuring parameters are pollen, spores, paint pigment particles, plankton, and other small or microscopic organisms.
The embodiments and examples set forth herein are presented to best explain the present invention and its practical application and to thereby enable those skilled in the art to make and utilize the invention. Those skilled in the art, however, will recognize that the foregoing description and examples have been presented for the purpose of illustration and example only.
Other variations and modifications of the present invention will be apparent to those of skill in the art, and it is the intent of the appended claims that such variations and modifications be covered. For example, in the illustrative embodiment of the system 1 depicted in FIG. 1, a single laser is employed to provide a light beam 3 and a pair of detectors 4, 5 are arranged to detect side scatter and fluorescence. However, the system can have a single detector or more than two detectors and/or more than one laser as required. Furthermore, whilst the system of the illustrative embodiment is arranged to measure a plurality of particles passing through the light beam in succession, as is known in flow cytometer technology, the system could alternatively be configured to simply measure a single particle. Those skilled in the art would also understand that the system and method for analyzing a particle in a sample stream can be implemented in flow based analyzers other than flow cytometers.
The description as set forth is not intended to be exhaustive or to limit the scope of the invention. Many modifications and variations are possible in light of the above teaching without departing from the scope of the following claims. It is contemplated that the use of the present invention can involve components having different characteristics. It is intended that the scope of the present invention be defined by the claims appended hereto, giving full cognizance to equivalents in all respects.

## Claims

1. A particle interrogation system, comprising:
a flow chamber having an interior and defining an interior cross-sectional area;
a particle delivery tube operably coupled to the flow chamber,
the particle delivery tube being at least partially inserted into the flow chamber so as to partially occlude the interior cross-sectional area of the flow chamber and define an unoccluded region of the interior cross-sectional area of the flow chamber,
the particle delivery tube being configured to deliver a sample fluid containing one or more particles into the flow chamber;
a sheath fluid container in fluid communication with the interior of the flow chamber, the sheath fluid container configured to deliver a flow of a sheath fluid to the unoccluded region of the interior cross-sectional area of the flow chamber;
wherein flow through the flow chamber opposes the force of gravity;
wherein the sheath fluid flow is assisted by gravity;
the particle interrogation system being configured to hydrodynamically focus the one or more particles within the flow chamber;
a laser source configured to impinge a laser beam on the one or more particles disposed within the flow chamber after the one or more particles are hydrodynamically focused, the laser source having an output power of less than 10 mW;
at least one detector configured to receive one or more optical signals resulting from fluorescence or light scattered from the one or more particles; and
signal conditioning circuitry operably coupled to the at least one detector.

2. The system of claim 1, wherein the laser source comprises a non-stabilized compact laser.

3. The system of claim 1, further comprising the sheath fluid container in fluid communication with the flow chamber, the fluid communication being via a gravity-fed delivery line.

4. The system of claim 1, wherein the signal conditioning circuitry comprises a pre-amplifier stage coupled to the output of the detector, and wherein the low pass filter circuitry is integrated in the pre-amplifier stage.

5. The system of claim 1, wherein the particle interrogation system is configured to hydrodynamically focus the one or more particles such that the particles are centered in the flow channel.

6. The system of claim 1, wherein the system is configured to provide a transit time of between about 100 microseconds and 1 millisecond for a particle transported through the laser beam.

7. A particle interrogation system, comprising:
a flow chamber having an interior and defining an interior cross-sectional area;
a particle delivery tube operably coupled to the flow chamber,
the particle delivery tube being at least partially inserted into the flow chamber so as to partially occlude the interior cross-sectional area of the flow chamber and define an unoccluded region of the interior cross-sectional area of the flow chamber,
the particle delivery tube being configured to deliver a sample fluid containing one or more particles into the flow chamber;
a sheath fluid container in fluid communication with the interior of the flow chamber, the sheath fluid container configured to deliver a flow of a sheath fluid to the unoccluded region of the interior cross-sectional area of the flow chamber;
the particle interrogation system being configured to spatially focus the one or more particles;
a laser source configured to impinge a laser beam on one or more spatially focused particles disposed within the flow chamber, the laser source having an output power of less than 10 mW;
at least one detector configured to receive one or more optical signals resulting from fluorescence or light scattered from the one or more particles;
signal conditioning circuitry operably coupled to the at least one detector; and
wherein the flow chamber is rectangular and wherein the particle delivery tube comprises a cylinder.

8. A method for interrogating one or more particles in a flow chamber, the method comprising:
delivering a sample fluid including one or more particles to a flow chamber having an interior and defining an interior cross-sectional area that is partially occluded by a particle delivery tube that delivers the sample fluid to the interior of the flow chamber;
delivering a sheath fluid from a sheath fluid container that is in fluid communication with the interior of the flow chamber and is configured to deliver the a sheath fluid to the unoccluded region of the interior cross-sectional area of the flow chamber, wherein the sheath fluid flow is assisted by gravity;
hydrodynamically focusing the one or more particles within the flow chamber;
flowing, at a velocity, one or more particles through a laser beam from a laser source having an output power of less than 10 mW, the laser beam impinging on the one or more particles flowing in the flow chamber after the one or more particles are hydrodynamically focused, wherein flow through the flow chamber opposes the force of gravity;
detecting scattered light or fluorescence resulting from the laser beam impinging on the one or more particles; and
signal conditioning the scattered light or fluorescence into electronic signals for processing thereof.

9. The method of claim of claim 8, wherein the signal conditioning comprises filtering high frequency noise from the detected scattered light or fluorescence.

10. The method of claim 8, wherein the sheath fluid is gravity-fed.

11. The method of claim 8, wherein the scattered light or fluorescence is detected by at least one of a PMT photodiode, an avalanche photodiode, or a hybrid detector.

12. The method of claim 8, wherein the laser beam is generated by a non-stabilized compact laser.

13. The method of claim 8, wherein the hydrodynamic focusing comprises hydrodynamically focusing the one or more particles such that the particles are centered in the flow channel.

14. The method of claim 8, further comprising adjusting the velocity of the one or more particles based at least in part on the power output of the laser source.

15. The method of claim 8, further comprising providing a transit time of between about 100 microseconds and 1 millisecond for a particle transported through the laser beam.