Abstract: Chromatograph in high performance liquid phase comprising a monochromatic light source emitting a beam which, after splitting, is divided into two parallel beams which go respectively through two cuvettes, the one containing a reference liquid, the other the measurement liquid. The two beams on coming from the cuvettes go into a detection unit. After splitting, each of beams R, M is divided into a system P with a semireflecting surface SR, a part going into one of the cuvettes CR, CM and being reflected by the rear of the cuvette forming a mirror MC to come back to semireflecting surface SR along a reverse path, the other part of the beam coming to be reflected on a mirror MR, at right angles to each beam, moved by a piezoelectric device PZ.