Sentence:  Fractions from the column which were to be subjected to analytical ultracentrifugation were concentrated by negative <WSD>pressure</WSD> dialysis 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:  the force applied to a unit area of surface; measured in pascals (SI unit) or in dynes (cgs unit)