Abstract: The present disclosure provides a capture assembly optionally for use in an automated sample analyzer. The sample analyzer includes an optical assembly for scanning a sample well of a planar substrate, e.g., a multiwell plate that is loaded onto and secured to the capture assembly to perform an assay, e.g., detection of an analyte in the sample. The capture assembly automatically aligns the planar substrate about a rotational axis of a drive shaft and secures the substrate to the drive shaft to prevent unwanted movement or slippage of the substrate during starting, stopping and rotation of the substrate at varying rotational velocities thereby ensuring the sample well is reliably detected by the optical assembly.