Alternative organophosphorus compounds include trioctylphosphine oxide and bis(2,4,4-trimethyl pentyl)phosphinic acid.  Secondary, tertiary, and quaternary amines have also been used for some uranium extractions.  Compared to phosphate extractants, amines are more selective for uranium, extract the uranium faster, and are easily stripped with a wider variety of reagents.  However, the phosphates are more tolerant of solids in the feed solution and show faster phase separation.