Sentence:  For preparative ultracentrifugation , fractions from the column were concentrated by negative pressure dialysis to volumes of 1 ml or <WSD>less</WSD> , transferred to cellulose tubes and diluted to 12 ml with isotonic saline . 
Meaning of the word marked with <WSD> tags:  (comparative of `little' usually used with mass nouns) a quantifier meaning not as great in amount or degree