I'm using the Centriprep Centrifugal Filter Unit (Millipore, 30kDa cutoff) to separate molecules of different molecular weight, where molecules lower than 30kDa would be filtered through the cellulose membrane and be found in the filtrate. But I'm wondering what happens to the very small molecules, such as 500 - 1000 Da? Will they also be affected by the centrifugal filtration and found only in the filtrate or be evenly distributed between the filtrate and concentrate?

Thanks!