Abstract: A fluid assay system and a method for immobilizing magnetic particles within a fluid assay system are provided which employ a vessel for receiving magnetic particles and a solenoid actuator comprising a core component and a coil of wire wound around at least a portion of the core component. The solenoid actuator is configured such that an application of current through the coil of wire moves the core component toward the vessel. In some cases, core component includes a magnet to immobilize one or more magnetic particles disposed within the vessel. An embodiment of the solenoid actuator includes a telescoping body holding a core component and a coil of wire wound around at least a portion of the telescoping body.