hi Matthew,

   Thanks for you information. The black 'beads' I observed in the thin
filmm are a few microns diameter and as the dilution ratio enhances from
1:5 to 1:50, their number reduces. I think probably submicron filtration
will work. What commercial company do you know that provides good filters?

   Could you please explain what "heating under reflux" method means, and
how to spin at temperature? Our spin-coater doesn't have a heater with it.

  Cheers

  Jiang