The assembly includes a horizontally disposed sample transfer arm which is mounted to and extends radially outwardly from a vertically positioned cylindrical support member. A drive shaft is coupled by a drive mechanism to the cylindrical member for rotating the cylindrical member a predetermined arcuate distance to move the transfer arm between a first position over a first receptacle and a second position over a second receptacle. Locating stops associated with the cylindrical support member define and limit the arcuate movement of the arm. The drive mechanism includes a lost motion clutch assembly including a flat cam member which has a generally rectangular cross-section, which is fixed to the top of the drive shaft and which is received in a slot in the cylindrical member that extends axially inwardly from one end of the cylindrical member and that has a generally rectangular cross-section greater than the rectangular cross section of the cam member. A wide, generally U-shaped spring clip has a flat bight portion situated within the slot at the inner end thereof and has two leg portions which extend toward each other and which have opposed outer ends which are spaced apart a distance less than the thickness of the cam member. The cam member is received between and engaged by the leg portions of the spring clip within the slot in the cylindrical support member. Overrotation of the drive shaft in excess of the predetermined arcuate distance will cause the cam to turn within the slot and spread the opposed leg portions of the spring clip as movement of the arm is blocked by engagement of the arm with one of the locating stops at one end of the arcuate path of travel of the arm thereby to prevent damage to the assembly.