Impurities of lapatinib such as N-{3-chloro-4-[(2-fluorobenzyl)oxy]phenyl}-6-[5-({[2-(methylsulfonyl)ethyl]amino}methyl)furan-2-yl]quinazoline-4-amine compound of formula (I) or a salt thereof: and analytical methods for identifying and quantifying such impurities of Lapatinib and salts thereof are provided. Also provided is Lapatinib containing less than about 0.05 percent of this and related impurities and methods for preparing such pure forms of Lapatinib.