Which of the following methods is NOT suitable for concentrating a protein solution?

For the reaction : ${{\rm{N}}_2} + 3{{\rm{H}}_2} o 2{\rm{N}}{{\rm{H}}_3}$ ; if ${E_1}\,and\,{E_2}$ are equivalent masses of $N{H_3}$ and ${N_2}$ respectively, then ${E_1}\, - \,{E_2}$ is :

For the reaction : ${{\rm{N}}_2} + 3{{\rm{H}}_2} o 2{\rm{N}}{{\rm{H}}_3}$ ; if ${E_1}\,and\,{E_2}$ are equivalent masses of $N{H_3}$ and ${N_2}$ respectively, then ${E_1}\, - \,{E_2}$ is :