Sentence:  After passing * * f through DEAE-cellulose , the titer of antibodies to WTV in the specific <WSD>fraction</WSD> was 1 : 4 of the titer before such passage ( precipitin ring tests by R. F. Whitcomb ) ; but mere dilution of the conjugate to 1 : 4 did not satisfactorily remove nonspecific staining . 
Meaning of the word marked with <WSD> tags:  a component of a mixture that has been separated by a fractional process