Hi Dachun:

Are you changing the glassy carbon chips too?  I have found that upper chips, which are in contact with the samples become altered (rims form, light brown to white in color), and are perhaps becoming porous.  And, this situation can lead to the tailing you are observing.  I would ask some other groups that run jus water and see if they have to change out the liner often or not.  We run solids and our carbon liner is fine after three years.

All the best,

Bill Anderson
Earth Sci. Dept./SERC
FIU