Simplest way is to dry the entire batch at 60 degrees Celsius and weigh. If you are looking for exact concentration weigh the amount of graphene in 10 ml of solution after drying with a precise analytical balance.
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In many previous papers, the amount of produced graphene is much smaller than that of the weight of graphite has been used. With mentioned method, the weight of dried materials might be similar to the weight of graphite.
If the amount of graphene produced via exfoliation process is small, then I would suggest to use UV-Vis spectra to measure the concentration of the graphene dispersion. Once you know the concentration and the total volume of the graphene dispersion and the starting graphite material, you can back calculate the amount of the exfoliatied graphene.