A centrifuge rotor includes a rotor body having a base, a sidewall, and a top. The top defines an opening that provides access to an annulus cavity inside the rotor body. A cover is removably attachable to the rotor body to seal the annulus cavity. A drive hub extends from a portion of the base of the rotor body and couples to a drive shaft of a centrifuge motor. The rotor body is sized to receive one or more sample containers in the annulus cavity, and to constrain the one or more sample containers inside the annulus cavity between the base, the sidewall, and the top when the one or more containers are advanced radially against the sidewall.