Abstract: An instrument includes a first chassis and a second chassis that includes a cartridge support cradle configured to hold a cartridge with a sample chamber between the first and second chassis. An actuator coupled to one or both of the first and second chassis is configured to effect automated relative movement between the first and second chassis to vary a distance therebetween. A detector mounted within the upper chassis includes a plunger rod movable between first and second positions and a sensor for detecting when the plunger rod is in the second position. As the first and second chassis are moved relatively toward each other by the actuator, if the fluidic cartridge is situated on the cartridge support cradle, and a cap closes the sample chamber, the plunger rod will contact the cap and move from the first position to the second position.