Sentence:  Fractions from the column which were to be subjected to analytical ultracentrifugation were concentrated by negative pressure <WSD>dialysis</WSD> and dialyzed for 16 hr in the cold against at least 500 volumes of phosphate buffered saline , pH 7.2 , ionic strength 0.154 . 
Meaning of the word marked with <WSD> tags:  separation of substances in solution by means of their unequal diffusion through semipermeable membranes