a instrument for performing synthesis of small molecules such as oligos and peptides when using solid phase synthesis techniques to synthesize small molecules . the system and methods include a mechanism for controlling the pressure differential across the reaction vessels that contain the solid support used in solid phase synthesis . reaction vessels are held in a holder that provides a sealable chamber at the outlet ends of the reaction vessels . the rotor containing the reaction vessels is placed within a sealable chamber . the sealable chamber is fitted with a means for engaging the rotor and draining the reaction vessels to waste . the sealable chamber is also fitted with a means for engaging the rotor to drain the reaction vessels at a slower , variable rate .