An applicator assembly (100) for closing a sample container (500) with a plug (16) is proposed. The applicator assembly comprises a magazine (102) configured to store a string (10) comprising polymer-based material, an insertion device (104) configured to insert an end piece (12) of the string at least partly into an opening (502) of the sample container (500), and a drive system (106) configured to supply at least a part of the string (10) from the magazine (102) to the insertion device (104). The applicator assembly further comprises a separator (108) configured to separate the end piece (12) from the string, such that the end piece (12) forms a plug (16) closing the sample container (500).