HPLC determination of 2-hydroxy-1-methoxyaporphine, pronuciferine, nuciferine and roemerine in Nelumbo nucifera and its alkaloid fraction Separation and preparation of alkaloids in Nelumbinis Folium by pH-zone-refining counter-current chromatography Separation and preparation of alkaloids in Nelumbinis Folium by pH-zone-refining counter-current chromatography