A fluid analyzer includes an optical source and detector defining a beam path of an optical beam, and a fluid flow cell on the beam path defining an interrogation region in a fluid channel in which the optical beam interacts with fluids. One or more flow-control devices conduct a particle in a fluid through the fluid channel. A motion system moves the interrogation region relative to the fluid channel in response to a motion signal, and a controller (1) generates the motion signal having a time-varying characteristic, (2) samples an output signal from the optical detector at respective intervals of the motion signal during which the interrogation region contains and does not contain the particle, and (3) determines from output signal samples a measurement value indicative of an optically measured characteristic of the particle.