a liquid input manifold comprising a sample bore for accommodating a flow of sample liquid surrounded by a frustoconical shaped chamber for accommodating a flow of sheath liquid received from a pre-injection chamber that surrounds the sample bore adjacent one end thereof; and

a cuvette including a central bore positioned for receiving from the input manifold the flow of the sample liquid surrounded by and in non-mixing contact with the flow of the sheath liquid, the section of the central bore receiving the sample liquid and the sheath liquid having a frustoconical shape, said frustoconical shaped section converging with increasing distance from where the flow of the sample liquid surrounded by the flow of the sheath liquid is received in the central bore, wherein:

the pre-injection chamber is in liquid communication with an end of the frustoconical shaped chamber opposite the cuvette; and

at an intersection between said pre-injection chamber and the end of the frustoconical shaped chamber opposite the cuvette, said pre-injection chamber has a larger cross-sectional area than the end of the frustoconical shaped chamber opposite the cuvette whereupon sheath liquid input into the pre-injection chamber gradually drains into the end of the frustoconical shaped chamber opposite the cuvette.