According to the GE Healthcare web page about Ficoll400, it is soluble up to 50% (i.e. 50g/100ml of solution), resulting in a density of 1.2 g/ml. This is the density of the solution, not the concentration of the Ficoll. Remember, water has a density, too. (1g/ml at 4oC for pure water). If you can't find a table for how to make solutions of various densities, you could make this 1.2g/ml solution, then dilute it with water (or buffer) until it has the desired density, which you could determine by weighing 1 ml on an analytical balance.

Thanks Adam. I will try. nIf I stillhave problems I will come back to you

You may prepare solution of Ficoll in water, then withdraw 2-3 aliquots, weigh each and freeze-dry each. Weigh the dried samples (in the same tubes) and determine quantity of polymer in each aliquot. Since the weight of the aliquot is known - you determine the polymer concentration.

Good luck!