Sentence:  For preparative ultracentrifugation , fractions from the <WSD>column</WSD> were concentrated by negative pressure dialysis 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:  a vertical glass tube used in column chromatography; a mixture is poured in the top and washed through a stationary substance where components of the mixture are adsorbed selectively to form colored bands