I am trying to calculate the SPF of sunscreens using the Mansur equation. I am using a method directly from a paper.
The solutions are made up in ethanol and the UV-Vis is blanked with the ethanol prior to analysis.
I weigh 0.25g sunscreen and make up to 50ml then take 1ml and make to 25ml.
I took a spectrum of one formulation (made in triplicate) and I get breaks in the spectra where there is no absorption value?
I diluted the sample further as at some wavelengths I was getting absorption values of 4 and still the same issue. I also got negative absorption values.
Can anyone help as to what this may be as I have never seen this before?