a multi - pipette system of the type having sample volume determined by size of cavities into which flexible membrane portions are drawn , provides a microporous block with the cavities formed in it for drawing the flexible membrane into the cavities when pressure is reduced above the microporous block . slide - in - place location and replacement is provided for supply containers , disposable tube arrays for drawing up samples and for expelling them , and for flexible membranes . in an embodiment microporous block cavity size is adjustable .