Abstract: A sample changer for precise positioning of a selected sample from a plurality of samples stored on a removable annular sample file. The samples comprising a rectangular dimensioned object is radially removed from the annular cassette and precisely positioned at a predetermined location for testing. Upon completion of the desired testing, the sample is replaced into the cassette which is repositioned and a subsequently selected sample is accordingly extracted and positioned for testing. Furthermore, the present invention includes a combination of elements which act together to detect and dislodge samples which may become temporarily stuck or jammed in the mechanism, reducing the need for operator intervention. Furthermore, the annular cassette is removable from the sample changing mechanism and includes a concentric locking ring which selectively retains all samples within the cassette or permits the removal of all samples.