Sentence:  For protein identification , fractions from the column were concentrated by pervaporation against a stream of <WSD>air</WSD> at 5 ` C or by negative pressure dialysis in an apparatus which permitted simultaneous concentration of the protein and dialysis against isotonic saline . 
Meaning of the word marked with <WSD> tags:  a mixture of gases (especially oxygen) required for breathing; the stuff that the wind consists of