[redzilla]

Hi Everyone,

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?

Thanks!

[asqa]

Shortcut to: http://www.archpersonalcare.co.za/personal_care_agencies/personal_care_agencies_sunjin_catalogue.htm
i hope the above link may help you.

[Dude]

     The standard way to measure for zinc and titanium in an organic matrix would be to use atomic absorption (AA) spectroscopy.  Unfortunately, that would require an acid digestion sample preparation procedure, which is quite time consuming.  Ti is also quite difficult to get into solution so you might need to resort to a literature technique for sample preparation.  An AA will cost about $20-25 K new (Perkin Elmer) and will require two lamps if done in absorption mode (most new instruments offer emissive and absorption mode standard).  ICP will allow simultaneous measurement of both elements (the AA will require switching conditions (emissive mode) or lamps (absorption mode)).  Detection limits are in the 1 ppm range for AA and a little lower for ICP.  
     From a quality control perspective, the easiest way to measure for titanium and zinc at the same time would be to use X-ray fluorescence spectroscopy.  Gravimetric standards can be made and used as calibrants after confirmation of zinc and titanium content by ICP or AA.  XRF is predominantly a surface technique so if the suspension settled over time it may overestimate the metal content (retains would have to be shaken before measured).  Detection limits are in the 1-10 ppm range for XRF.  
     A cheaper way might be to do as you initally suggested.  Combust the sample in air and then measure the weight.  Redissolve the sample in solution (might be difficult) and see if you can selectively titrate one of the metals (ie zinc portion with EDTA).  Check the ASTM (American Society of Testing and Materials) books and SciFinder.  There might already an established technique known in the suntan industry.