1-Oxomirtazapine;1-Oxo Mirtazapine (Mirtazapine Impurity C);3,4,10,14b-Tetrahydro-2-methylpyrazino[2,1-a]pyrido[2,3-c][2]benzazepin-1(2H)-one;Mirtazapine IMpurity C;Mirtazapine EP IMpurity C;Mirtazapine Related CoMpound C;Mirtazapine EP IMpurity C (USP RCOpen