Sentence:  For protein identification , fractions from the column were concentrated by pervaporation against a stream of air at 5 ` C or by negative pressure dialysis in an apparatus which permitted simultaneous concentration of the protein and <WSD>dialysis</WSD> against isotonic saline . 
Meaning of the word marked with <WSD> tags:  separation of substances in solution by means of their unequal diffusion through semipermeable membranes