# Plate Theory and Extensions - The Compressibility of the Mobile Phase > Page 21

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_{i}Q_{i} = P_{o}Q_{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_{f}u_{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.