I am attempting to assay a raw material sample, oxybenzone, against oxybenzone, USP using UV-vis. The requirement for acceptance is NLT 99% mg C14H12O3/mg raw material sample. The method instructs a standard preparation of 10ug/mL in CH3OH, and then a sample preparation by 100 mg sample oxybenzone in 100 mL, pipette one mL into another 100 mL flask, dilute to volume with CH3OH. I used these same steps to prepare my standard solution as well. Scan and use A value to calculate mg C14H12O3 in the sample using 10(C)(At/As) with At being A of the sample and As being A of the standard, C being concentration of the standard solution in ug/mL
assuming I know my sample should pass, my results make no sense.
As=.8341 At=.6304 C= 10
I used methanol as the blank per the method.
I am sure I must have made a mistake somewhere, but I am stuck. The only thing I can think of is if I made a mistake preparing the standard solution and it was too concentrated perhaps A would go up, but I don't think I did. I weighed 100 mg oxybenzone USP standard, diluted to volume, removed 1 mL, diluted to 100 mL again, same as I did for the sample solution prep.