why can't I get my absorbance values to come out right on my oxybenzone uv-vis assay test?

Question

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.

Answer 1

The method, and your result, both make sense to me: it indicates that the test sample is 75.57% pure, although as you point out the result is not consistent with the assumption that the sample should pass the requirement for acceptance. However, why would you run the test if you knew ahead of time that the material would pass?

I would recommend that you repeat your test 2 more times from scratch - that way you would know if there was an error in your technique. If you get the same result (within experimental error), then you would have to admit that it is hard to argue with the data.