An apparatus comprising at least one measuring cell (10) is disclosed. The measuring cell comprises a first cavity (16 and a second cavity (18) perpendicular to the first cavity, the first cavity and the second cavity comprising an overlap at first respective ends and a reflective surface (20) at the opposite respective ends. A beam splitter (15) is located in the overlap and an electromagnetic radiation source (12) is arranged to project a beam of electromagnetic radiation onto the beam splitter (15) such that the beam is projected into each of the cavities. A phase detector (22) for detecting a phase difference between the respective electromagnetic radiation reflected by the first and second cavity (16; 18) is also provided. In addition, the apparatus has a fluid channel (26), at least a part of which runs parallel to the first cavity (16) such that the electromagnetic radiation projected into the first cavity extends into said part of the fluid channel. This allows for the interferometric detection of particles in the fluid channel.