According to an aspect of the invention, an apparatus for self-sealing of a reaction vessel, such that the reaction vessel may be repeatedly accessed by a fluid transfer mechanism and subsequently self-sealed, is detailed. The reaction vessel has an access opening, and the apparatus comprises a cap, a ball which is comprised of a magnetizable material (such as a ferrous metal), and a ring magnet. The cap is sized to fit over the reaction vessel access opening, and has an access port. The cap has an internal surface which faces the reaction vessel, and an external surface, which faces away from the reaction vessel. The ball is sized to fit over the cap access port, such that a portion of the ball seats partially in the cap access port on the cap internal surface, thus sealing the cap access port. According to an aspect of the invention, the ball may be comprised of a ferromagnetic material, or a magnetizable material that is preferably magnetized. The ring magnet produces a magnetic field, and is mounted to the cap on the cap external surface. The ring magnet is spaced from the cap such that the magnetic field is sufficient to hold the ball in the cap access port, and to reseat the ball in the cap access port after the reaction vessel is accessed.