Sentence:  For preparative ultracentrifugation , fractions from the column were concentrated by negative pressure <WSD>dialysis</WSD> to volumes of 1 ml or less , transferred to cellulose tubes and diluted to 12 ml with isotonic saline . 
Meaning of the word marked with <WSD> tags:  separation of substances in solution by means of their unequal diffusion through semipermeable membranes