Sentence:  In the present work whole sera have been fractionated by chromatography on DEAE-cellulose using <WSD>single</WSD> gradients similar to those described by Sober and Peterson , and certain chemical and serological properties of the fractions containing antibodies of the ABO and Rh systems have been described . 
Meaning of the word marked with <WSD> tags:  being or characteristic of a single thing or person