Tham khảo tài liệu 'hplc a praactical user's guide part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | REVERSE-PHASE ZIRCONIUM 93 ACIDIC PHASE SILICA I have discussed normal phase separations on silica and hydrated silica columns in which polar compounds retain and nonpolars elute. Separations have also appeared in the literature using an acid phase silica column for the separation of phospholipids. These represent a different type of normal phase partition separation. As much as 1-2 sulfuric or phosphoric acid is added to the mobile phase forming a protonated hydration shell around silica. The phospholipids are soluble in nonpolar solvents but differ from each other in polar functionalities such as sugars alcohols amino acids or varying numbers of phosphate groups. By going to very harshly acidic conditions even phosphate groups can be forced into their protonated form allowing them to be eluted off the silica surface by nonpolar mobile phases. With no bonded phase on the silica to be affected by acid hydrolysis pH can be kept very low. Every thing can be stripped off the column with acid water and then washed back up to nonpolar solvents. REVERSE-PHASE ZIRCONIUM A bonded-phase columns based on a zirconium backbone at first glance appears simply to be just another reverse-phase column with slightly differing separation characteristics but having the serious drawback of the strong inherent chelating nature of zirconium. It would be easy to reject these columns when doing new methods development or adapting existing methods but this would be a serious mistake. The chelating character has been largely overcome by use of covalently bonded tetraphosphonic acid chelating agents to eliminate column bleed for LC MS application. Unprotected bonded-phase zirconium columns offer fertile ground for research to provide unique methods for taking advantage of this column s strong chelating nature. There are covalently bonded phases available on zirconium that can simulate silica C18 separations with a-type peak shifting but bonded phases are available that provide truly .
