[08roselx]

I am trying to determine the SPF of various sunscreen formulations using the Mansur equation. This has been used on many papers however I am struggling to get values close to the stated SPF.

For chemical sunscreens the values are 10 lower than the actual SPF and for sunscreens containing inorganic filters they are extremely out. For inorganic sunscreens I barely get any absorbance - I am unsure if this may be an issue with the filter paper I'm using potentially?

I am following a standard protocol as used on many papers: http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1516-93322004000300014

Any advice or ideas would be greatly appreciated!