An apparatus comprising an assembly and a cover. The assembly comprises a sample block configured to receive a microtiter tray having a plurality of vials, each vial comprising a cap having a surface and a perimeter. The cover comprises a platen, vertically and horizontally displaceable in relationship to the sample block, the platen having a plurality of recesses. Each recess corresponds to a respective vial and is shaped and arranged to clear the surface of its respective cap when the cover is displaced onto the sample block. The cover includes a skirt in contact with and surrounding a perimeter of the platen. The skirt is configured for mating with a perimeter of a microtiter tray when a microtiter tray is received in the sample block, and also configured such that the cover contacts a microtiter tray when a microtiter tray is received in the sample block.