Dear Hajar,

Welcome,

From the conceptual point of view you have to measure the optical properties of an y substance by putting it into neutral medium. That is which will not affect the performance of the specimen.  Then you have to choose a material which is transparent in the absorption range of graphine sheet.

It seems that Benzne has a large absorption coefficient in the absorption range of graphine. Then there are two solutions:

- measure exactly the Benzene alone

Then measure Benzene + graphine , and subtract the two from each other you can get the absorbance of graphine.

The other solution is to choose a solvent which is transparent in the absorption range of graphine.

The last solution is much accurate than the first solution because the subtraction process may lead to large errors if the sheet absorbance much less than Benzene in the intended wave length range.

One have to note that the subtraction valid for absorbance in decibels rather than ratios. In case of ratios one has to divide the absorbance of the combined specimen to the that of Benzene alone.

Best wishes 

 Thanks, So it's probable to have inaccurate data using the first method, the one that is common in publications. Thanks again. If you have any text refer to this point, It would be great if you could let me know.