Embodiments of the invention provide a cap for a vessel for performing a multi-stage process for analyzing a sample, such as nested PCR or RT-PCR. In one embodiment, the cap comprises a body configured to be mated to the vessel to enclose a vessel interior, a cap cavity for holding reagents, and a cap cavity control portion that is adjustable with respect to the body between a first-stage position in which the cap cavity is enclosed and fluidicly isolated from the vessel interior and a second-stage position in which the cap cavity is fluidicly coupled with the vessel interior.