Sentence:  Fractions from the column which were to be subjected to analytical ultracentrifugation were concentrated by negative pressure dialysis and dialyzed for 16 hr in the cold against at least <WSD>500</WSD> volumes of phosphate buffered saline , pH 7.2 , ionic strength 0.154 . 
Meaning of the word marked with <WSD> tags:  denoting a quantity consisting of 500 items or units