We are currently running ASTM D7419 on 2 x Agilent ZORBAX SIL 9.4 x 250 mm, 5 μm, Part number: 880952-201 1 x Agilent ZORBAX CN 9.4 x 250 mm, 5 μm, B12246 and with every run we are seeing the retention time for the saturates and aromatics elute later and later. We have tried several different manufacturer's columns with similar outcomes. It apears to be the silica columns that are the issues. We can regenarate them by running methylene chloride but the peaks will gradually begin eluting later and later. Has anyone else experienced this and what can we do to stop or minimize this?