Abstract: An apparatus for delivering a plurality of liquid volumes sequentially to a reaction site is provided, the apparatus comprising a vessel having a plurality of solid spacers arranged in linear order therein so as to define a series of adjacent voids, separated by individual solid spacers, for receiving said liquid volumes, the vessel being provided in a lateral wall with a plurality of reversibly sealable openings through which the liquid volumes for delivery to the reaction site may be introduced into the voids defined therein, and an outlet port, the spacers being movable within the vessel such that, in use, liquid volumes disposed in the individual voids are deliverable sequentially through the ‘outlet port to the reaction site. Also provided are units, such as disposable cartridge units, comprising such an apparatus coupled to a reaction site and systems comprising such units together with means for interrogating the reaction site to determine the outcome of the reaction.