A MAS (magic angle spinning) NMR (nuclear magnetic resonance) apparatus, with automatic sample supply by a supply unit (45), is characterized in that an automatic preparation station (44) for samples is provided, with a rotor storage (49) having several rotors (1) for receiving sample material (53) soaked with NMR solution (55), a cap storage (48) with several caps (3), each cap (3) being suited for closing a rotor (1), wherein each cap (3) has a central axial bore (4), several movable pins (5), each being insertable into the central axial bore (4) of a cap (3) to close the bore (4) in the inserted state, a cap handling unit (46b) which can grip a cap (3) from the cap storage (48), move it, and dispose it onto a rotor (1), a plunger (61) for inserting a pin (5) into the bore (4) of the cap (3), and a suctioning device (62) for suctioning excess NMR solution (55) that escapes from the bore (4). The apparatus provides simple and automatic preparation of sample units, in particular, closure of rotors with caps.