Rokicka K, Wójciak-Kosior M, Sowa I, Tylus J. Chromatographic separation of oleanolic, ursolic and betulinic acid in plant extracts on amiopropyl silica using aqueous mobile phases. Curr. Issues Pharm. Med. Sci. 2013;26(3):296-298. doi:10.12923/j.2084-980X/26.3/a.12