Sentence:  The chromatography was done at 6 ` C using gradient <WSD>elution</WSD> , essentially according to Sober and Peterson . 
Meaning of the word marked with <WSD> tags:  the process of extracting one material from another by washing with a solvent to remove adsorbed material from an adsorbent (as in washing of loaded ion-exchange resins to remove captured ions); used to obtain uranium ions