2-amino-1,7-dihydro-6H-purin-6-one; Guanine (EP & USP) ; Valaciclovir EP Impurity A ; Valaciclovir USP Related Compound A; 2-Amino-1,9-dihydro-6H-purin-6-one; 2-Amino-hypoxanthine; 2-Amino-6-hydroxy-1H-purine; 2-Amino-6-hydroxypurine; 2-Aminohypoxanthine; 9H-Guanine; Ganciclovir Impurity F; Aciclovir Impurity B