Methotrexate EP Impurity H Catalogue G101338 CAS 67022-39-3 M.W 468.47 M.F C21H24N8O5 Specifications FAQs (S)-2-(4-(((2,4-diaminopteridin-6-yl)methyl)(methyl)amino)benzamido)-5-methoxy-5-oxopentanoic acid