A cap that is securable to a vial includes a plug configured for insertion into an open end of the vial. An upper portion of the cap defines a probe recess with an open top end and configured to receive a distal end of a probe of a pipettor inserted into the open top end, to frictionally secure the cap to an end of the probe. The cap includes a radially-oriented annular surface with one or more locking members depending from a periphery of the annular surface. The locking members are spaced from the plug and each locking member includes a radial locking groove and a radial locking ridge beneath the radial locking groove. A lip of the vial snaps into the radial locking groove above the radial locking ridge to secure the cap to the vial when the plug is inserted into the open end of the vial.