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 500 volumes of phosphate buffered saline , pH 7.2 , ionic <WSD>strength</WSD> 0.154 . 
Meaning of the word marked with <WSD> tags:  the property of being physically or mentally strong