Flow cytometer apparatus and method

1. A flow cytometry apparatus comprising:

a flow channel for directing a fluid stream containing sample particles through a particle interrogation location;

a laser operable to emit a plurality of electromagnetic radiation pulses, said pulses being directed along a beam path from the laser to the particle interrogation location, and said pulses being emitted at a frequency so as to impinge on particles passing through the particle interrogation location multiple times;

a timing circuit operable to produce a timing signal indicative of the arrival of pulses at the interrogation location;

a detector which detects electromagnetic radiation from the interrogation location and operable to output a time-varying analog signal indicative of the intensity of the detected electromagnetic radiation;

an analog to digital converter which receives the time-varying analog signal as input and to sample the analog signal to produce a digitized output; and

a processor operable to analyze the digitized output from the analog to digital converter as a function of the timing signal, wherein said processor is operable to process the digitized output as multiple digital samples in the form of a pulse waveform.

2. The apparatus of claim 1 , wherein the laser is operable to emit electromagnetic radiation pulses having a width of about 1-100 picoseconds at a pulse frequency of about 50-150 MHz at a power of about 100-500 milliwatts.

3. The apparatus of claim 2 , wherein the laser is operable to emit electromagnetic radiation pulses having a width of about 5-20 picoseconds at a frequency of about 70-100 MHz.

4. The apparatus of claim 1 , wherein the timing circuit comprises a sensor which senses light corresponding to the electromagnetic radiation pulses including scattered light generated by interaction of each pulse with the fluid stream or including light of the electromagnetic radiation pulse.

5. The apparatus of claim 1 , wherein the timing circuit comprises a clock that is operable to trigger the laser to emit a pulse.

6. The apparatus of claim 1 , wherein the detector detects fluorescent emissions stimulated by an excitation energy of the pulses.

7. The apparatus of claim 1 , wherein the laser is selected from a list consisting of: a mode-locked solid state laser, a Q-switched laser, and a cavity dumping laser.

8. The apparatus of claim 1 , wherein the detector is a photomultiplier tube.

9. The apparatus of claim 8 , wherein the photomultiplier tube has a response time of less than about 2 nanoseconds.

10. The apparatus of claim 1 , wherein the processor is operable to extract at least one of the following features from the digitized output: critical slope difference; pulse rise time; pulse peak; and pulse area.

11. A method of analyzing particles contained in a fluid stream as they flow through an interrogation location, said method comprising:

emitting a plurality of electromagnetic radiation pulses from a laser at a frequency so as to impinge on particles passing through the interrogation location multiple times;

intermittently illuminating the fluid stream and the particles contained therein by directing said pulses along a beam path from the laser to the interrogation location;

detecting electromagnetic radiation from the interrogation location;

generating a time-varying analog signal indicative of the intensity of the detected electromagnetic radiation;

generating a timing signal indicative of the arrival of a pulse at the interrogation location;

converting the time-varying analog signal into a digital signal; and

analyzing the digital signal as a function of the timing signal to determine characteristics of the particles in the fluid stream, wherein the step of analyzing the digital signal comprises analyzing the digital signal as multiple digital samples in the form of a pulse waveform.

12. The method of claim 11 , wherein the step of converting the time-varying analog signal to a digital signal comprises sampling the analog signal at a time that is synchronized to coincide with the illumination of the fluid stream by a pulse.

13. The method of claim 11 , wherein the step of illuminating the fluid stream results in excitation of a fluorophore associated with said particles and the step of converting the time-varying analog signal to a digital signal comprises sampling the analog signal at a predetermined time after illumination of the interrogation location and within a fluorescent lifetime decay of said fluorophore.

14. The method of claim 13 , wherein the step of sampling the analog signal at a predetermined time after illumination of the interrogation location and within a fluorescent lifetime decay of said fluorophore further comprises sampling the analog signal at a fluorescence emission peak following each laser pulse excitation.

15. The method of claim 11 , wherein each pulse contains sufficient power to saturate a fluorophore associated with said particles.

16. The method of claim 11 , wherein the step of detecting electromagnetic radiation from the interrogation location comprises using a photomultiplier tube to detect fluorescent emissions from a fluorophore associated with said particles.

17. The method of claim 11 , wherein the step of emitting pulses of electromagnetic radiation comprises the step of emitting between about 50-150 million pulses per second, wherein each pulse has a width between about 1-100 picoseconds.

18. The method of claim 11 , wherein the step of generating a timing signal comprises sensing scattered light resulting from interaction of a pulse with the fluid stream.

19. The method of claim 11 , wherein the step of generating a timing signal comprises generating a clock signal and the step of emitting a plurality of electromagnetic radiation pulses from a laser comprises using the clock signal to trigger the laser to emit a pulse.

20. The method of claim 11 , wherein the step of analyzing the digital signal further comprises extracting at least one of the following features from the signal: critical slope difference; pulse rise time; pulse peak; and pulse area.