Emtricitabine Impurity: (2R,2'R,5S,5'S)-2,2'-Dithiobismethylenebis(5-(4-amino-5-fluoro-2-oxo-1H-pyrimidin-1-yl)-[1,3]oxathiolane)(Emtricitabine Impurity I (dimer) 1,1DISULFANEDIYLBISMETHYLENE).