A device and method for separating heavier and lighter fractions of a fluid sample. The device includes a plurality of constituents comprising a container and a composite element in the container. The composite element is a separator comprising at least two components and more particularly, a bellows with a seal body, a low-density float and a high-density ballast. A fluid sample is delivered to the container and the device is subjected to centrifugation whereby the centrifugal load causes the seal body of the separator to deform so that the separator migrates through the fluid sample and then stabilizes between the heavier and lighter fractions of the fluid sample. The seal body of the separator will resiliently return to its initial configuration upon termination of the centrifugal load such that the seal body sealingly engages the container and the composite element separates the heavier and lighter fractions of the fluid sample.