Optical detection system for particles

1. A detection system for particles, comprising:

a channel;

a light source configured to generate light;

one or more optical elements configured to focus a beam of the light on an irradiation zone within the channel;

a mask operatively disposed in an optical path between the light source and the channel, wherein the mask is configured to block a portion of the beam, thereby producing a shadow region;

an optical slit operatively disposed in the optical path between the light source and the channel at a position optically upstream of the mask; and

a detector configured to detect light deflected from the beam into the shadow region by interaction with a particle passing through the irradiation zone;

wherein the mask is a line mask including a masking region elongated parallel to the optical slit and elongated orthogonal to a flow direction in the channel through the irradiation zone.

2. The detection system of claim 1 , wherein the mask intersects a collimated region of the beam.

3. The detection system of claim 1 , wherein the optical slit is a first optical slit, further comprising a second optical slit operatively disposed in an optical path between the irradiation zone and the detector.

4. The detection system of claim 1 , wherein the detector is a first detector, further comprising a second detector configured to detect photoluminescence induced by the beam in the irradiation zone, the one or more optical elements providing an objective that collects emitted light from the irradiation zone for propagation to the second detector.

5. The detection system of claim 4 , wherein the same light source generates (i) light deflected by the particle at the irradiation zone and detected by the first detector, and (ii) excitation light that induces photoluminescence in the irradiation zone for detection by the second detector.

6. The detection system of claim 1 , wherein the particle is selected from the group consisting of a droplet, a solid-phase particle, and a biological cell.

7. The detection system of claim 1 , wherein the detector is configured to detect light near an image plane of the mask.

8. The detection system of claim 1 , further comprising a source of carrier fluid containing particles of interest and disposed in fluid communication with the channel.

9. The detection system of claim 8 , further comprising a source of dilution fluid in fluid communication with the channel and configured to increase a distance between particles in the channel at a position fluidically upstream of the irradiation zone by dilution of the carrier fluid containing particles of interest with the dilution fluid.

10. The detection system of claim 1 , further comprising at least one source of positive/negative pressure operatively connected to the channel and configured to create a pressure differential that drives fluid flow through the channel.

11. A method of detecting a particle, the method comprising:

generating a beam of light;

blocking a portion of the beam optically upstream of a channel to produce a shadow region;

focusing the beam on an irradiation zone within the channel;

passing a particle through the irradiation zone, wherein the step of passing deflects light into the shadow region by interaction with the particle; and

detecting light in the shadow region;

wherein the step of blocking includes a step of blocking a portion of the beam with a line mask having a masking region elongated orthogonal to the channel and elongated parallel to an optical slit operatively disposed optically upstream of the line mask.

12. The method of claim 11 , wherein the step of blocking is performed on a collimated region of the beam.

13. The method of claim 11 , wherein the step of detecting is performed with a detector, further comprising a step of spatially filtering light of the beam with an optical slit disposed in an optical path between the channel and the detector.

14. The method of claim 11 , further comprising a step of detecting photoluminescence induced by the beam at the irradiation zone.

15. The method of claim 11 , wherein the step of passing a particle includes a step of passing a droplet through the irradiation zone.

16. The method of claim 11 , wherein the step of detecting is performed behind an image plane of the mask.

17. The method of claim 11 , wherein the step of detecting is performed with a detector, and wherein the light detected in the shadow region is not focused between the channel and the detector.