A halogen-free ionic liquid, tetraoctylammonium dodecylbenzene sulfonate was used to extract Cu2+, Pb2+, Cd2+, and Hg2+ from their aqueous solutions by using dithizone as chelating agent. Inductively coupled plasma-mass spectrometry (ICP-MS) was applied, at different pH-values, to simultaneously measure the metal content in the aqueous phase, both before and after extraction. At the optimum pH value, extraction of the four metal cations from pure solution and marine water samples was successfully accomplished and their concentrations were simultaneously determined.