The invention relates to an assembly of a cooperating capillary and cap for containing an aqueous solution in an inner volume of the capillary. The assembly is used in a high pressure freezer in which the aqueous solution is frozen at a high pressure to form an amorphous frozen sample at a cryogenic temperature. The cap forms a closure at one end of the capillary, and the part of the cap that is in contact with the inner volume of the capillary has an indent; as a result of which the cap, after freezing the aqueous solution, can be removed from the capillary and a free standing pillar of frozen aqueous material extends from the capillary.