Now, the mass of mobile phase passing any point in the column must be conserved (i.e., the mass of fluid passing any point in the column per unit time  is constant), consequently, the product of pressure and flow rate will be constant at all points along the length of the column. Thus, under isothermal conditions,

                                      P_iQ_i = P_oQ_o

       where (Q_i) is the volume flow of mobile phase into the column at

                 (P_i),

           and (Q_o) is the flow of mobile phase from the column at (P_o).

Now, at a position (x) along the column,

                                        Q_x = a_fu_x

         where (a_f) is the cross-sectional area of the column available for

         gas flow and is assumed constant for a well-packed column.