Sitagliptin Acid Impurity

Sildenafil Impurity A ; Sildenafil USP RC A ; Sildenafil Isobutyl Analog ; 5-[2-Ethoxy-5-[(4-methyl-4-oxido-1-piperazinyl)sulfonyl]phenyl]-1, 6-dihydro-1-methyl-3-(2-methylpropyl)-7H-pyrazolo[4, 3-d]pyrimidin-7-one ;