A structure (10) for holding sample-containing receptacles includes a cover (40) with holes formed therein through which the receptacles can be accessed with a substance transfer mechanism, such as a robotic pipettor. When the transfer mechanism is inserted into and then withdrawn from a receptacle, a string of viscous material may be suspended from the mechanism. A viscous string removal element (44) adjacent each opening (42) engages the string of viscous material and dislodges the string from the mechanism when the mechanism moves in a prescribed path with respect to the removal element. A sample rack configured to hold receptacles and to be inserted into the structure below the cover includes a sample rack having receptacle-receiving pockets, each with a resilient element and a positioning feature for holding receptacles of varying sizes in a predetermined position within the receptacle receiving pocket, and a cover including features for preventing a receptacle from being pulled out of its receptacle-receiving pocket when the transfer mechanism is withdrawn from the receptacle.