(2R,5S)-5-(4-Amino-5-fluoro-2-oxo-1(2H)-pyrimidinyl)-1,3-oxathiolane-2-carboxylic acid ; Emtricitabine Carboxylic Acid ; (2R,5S)-Emtricitabine Carboxylic Acid ; Emtrictiabine IP Impurity A;1,3-Oxathiolane-2-carboxylicacid,5-(4-amino-5-fluoro-2-oxo-1(2H)-pyrimidinyl)-, (2R,5S)