A particle analyzing apparatus for analyzing a sample fluid comprising: an elongated capillary tube having a first end, a second opposite end and a predetermined internal cross-sectional area, a pump connected to the first end of the capillary tube for drawing a sample fluid into the capillary tube through the second end of the capillary tube and through the capillary tube to cause particles to flow along said capillary tube, a light source for illuminating a predetermined analyzing volume of the sample fluid in the capillary tube, wherein the second end of the capillary tube is adapted for immersion into the sample fluid retained in a cuvet or vial, wherein the apparatus is characterised in that it further comprises: an optical system for receiving and focusing the light from said light source to form and direct a rectangular beam through said capillary tube to define the analyzing volume in said capillary tube, and at least one first detector for detecting fluorescent light emitted by particles in said analyzing volume excited by the illumination impinging upon particles in said predetermined analyzing volume, wherein the rectangular beam has a first dimension wider than an inner dimension of the capillary tube in a direction perpendicular to a longitudinal axis of the capillary tube and a second dimension narrower than an inner dimension of the capillary tube in a direction parallel to the longitudinal axis of the capillary tube, whereby the analyzing volume is defined by the intersection of the rectangular beam and the internal walls of the capillary tube. an elongated capillary tube having a first end, a second opposite end and a predetermined internal cross-sectional area, a pump connected to the first end of the capillary tube for drawing a sample fluid into the capillary tube through the second end of the capillary tube and through the capillary tube to cause particles to flow along said capillary tube, a light source for illuminating a predetermined analyzing volume of the sample fluid in the capillary tube, wherein the second end of the capillary tube is adapted for immersion into the sample fluid retained in a cuvet or vial, wherein the apparatus is characterised in that it further comprises: an optical system for receiving and focusing the light from said light source to form and direct a rectangular beam through said capillary tube to define the analyzing volume in said capillary tube, and at least one first detector for detecting fluorescent light emitted by particles in said analyzing volume excited by the illumination impinging upon particles in said predetermined analyzing volume, wherein the rectangular beam has a first dimension wider than an inner dimension of the capillary tube in a direction perpendicular to a longitudinal axis of the capillary tube and a second dimension narrower than an inner dimension of the capillary tube in a direction parallel to the longitudinal axis of the capillary tube, whereby the analyzing volume is defined by the intersection of the rectangular beam and the internal walls of the capillary tube. an optical system for receiving and focusing the light from said light source to form and direct a rectangular beam through said capillary tube to define the analyzing volume in said capillary tube, and at least one first detector for detecting fluorescent light emitted by particles in said analyzing volume excited by the illumination impinging upon particles in said predetermined analyzing volume, wherein the rectangular beam has a first dimension wider than an inner dimension of the capillary tube in a direction perpendicular to a longitudinal axis of the capillary tube and a second dimension narrower than an inner dimension of the capillary tube in a direction parallel to the longitudinal axis of the capillary tube, whereby the analyzing volume is defined by the intersection of the rectangular beam and the internal walls of the capillary tube.