I'm trying to determine the % of zinc oxide and/or titanium dioxide in sunscreens. I analyze organic sunscreen actives daily via HPLC, but I need a method to assay these two inorganic actives in a sunscreen.
I had started working on this a while back but then had to drop the project to work on other things. I was working on using an ashing/loss on ignition method...I figured that once the sunscreen was heated and then ignited to 800 C, that all that would be remaining would be the ZnO or TiO2 (all other ingredients would have burned off). All of my products except for one contain either ZnO or TiO2 (not both).
I had read that there is a spectrophotometric method for determining TiO2, but I have yet to find any details on the method.
Does anyone have any ideas or methods that would allow me to assay either one (or both) of these inorganics in sunscreens?