In this talk the general concepts of validation of an analytical method are introduced, and applied to differential FFF method (1). Chemometrics will give us the basis for defining method parameters, such as the decision, detection and quantification limits as a function of both the signal error and the blank value. FFF theory and conventional theory of signal detection will allow us to specify the different error contributions as a function of the operative parameters. Therefore by this approach the "absolute character" of SdFFF as method for mass determination is further enlarged to include the adsolute character of the analytical method. A separate experimental check of the independent sources of errors and of the total error will represent the validation of the method. Examples (2) are reported. The role and relevance of the FFF method validation is thus enphasized. 1. L. Bregola, C.Contado, M.Martin, L. Pasti, F. Dondi. Precision in Differential Field-Flow Fractionation: a chemiometric study, J. Sep. Sci., 30, 2760-2779, 2007. 2. C.Contado, L. Bregola, F. Dondi. Sedimentation field flow fractionation of immunoglobulin A coated polystyrene beads. Influence of carrier composition on complex characterization. J. Chromatogr. A, 1169(1-2), 158-174.