Hello dear, if you employed a diluted solution you can use a concentration factor for determining the concentration of caffeine per oz. Remember that when you use your sample, you are only employing a small amount. Therefore, you can use a concentration factor account all dilutions done. 

Dear Sir. Regarding your issue about the quantification of  concentration of caffeine of your unknown sample of the beverage. Quantitative analysis of caffeine was performed by a simple and fast standard UV spectrophotometric method, using carbon tetrachloride as the extracting solvent. Results showed that the pH of the beverages were slightly acidic ranging from 5.92-6.44. The minimum caffeine level was observed in the carbonated soft drink Coca Cola (43.71±0.55 ppm), while the energy drink, Red Bull sample showed the highest caffeine content (58.31±0.35 ppm). The carbonated soft drinks showed caffeine levels in the range of 43.71 and 45.83 ppm with average concentration of 44.52 ppm, whereas in the energy drinks it ranged from 47.56 to 58.31 ppm with a mean concentration of 52.24 ppm. I think the following below links may help you in your analysis:

http://maxwellsci.com/print/ajfst/v6-155-158.pdf

http://www.jenway.com/adminimages/A09_010A_Determination_of_Caffeine_in_Beverages_using_UV_Wavelength_Spectroscopy(1).pdf

http://www.academicjournals.org/article/article1379502966_Atomssa%20and%20Gholap.pdf

Thanks

Mr. Chua,

By methods based on MS.