Silica-based amino columns are notorious for this problem. The amine group creates a high pH in the microenvironment of the surface, leading to hydrolysis of Si-O-Si links. The first to be hydrolyzed are the links that hold the silane to the surface. Result: Steady loss of functional groups in the coating and shorter retention times, as you have noted. Solutions are as follows:
1) Use an amino column that is designed to be more durable. Some of these are made from silane oligomers, which are harder to hydrolyze off. The Shiseido CAPCell PAK NH2 column is supposed to be particularly good this way.
2) Use a polymeric amino column. Make sure it's based on a reasonably polar polymer. not polystyrene-DVD, or you may have pronounced hydrophobic interactions. There are now several on the market that are satisfactory, although the selectivity may be different from a silica-based material.
3) Use a silica-based anion-exchange material where the amine groups are not attached via a silane link. Perhaps the only example of this is our PolyWAX LP material, in which linear polyethyleneimine is adsorbed to silica and then crosslinked to itself to form a macromolecular coating that envelops the silica particle.
